Growth characteristics of four herbaceous legumes ( Mucuna pruriens, Lablab purpureus, Pueraria phaceoloides and Centrosema bracillanum) and a grass ( Penicetum purpureum) were evaluated on acid and non-acid soils of southern Nigeria in a greenhouse potted experiment. Shoot biomass, root distribution and nodulation determined at four months after planting showed differences between species. The nodule number and nodule dry weight of legumes grown on non-acid soil averaged 51% and 52% above those on acid soil respectively. Besides, the root diameter was 17% larger in non-acid soil than on acid soil. Whereas the average of total root length on acid soil was 37% more than in the non-acid.
Investigations on the impact of application of biosolids for land reclamation on C sequestration in soil were conducted at Fulton County, Illinois, where 41 fields (3.6-66 ha) received biosolids at a cumulative loading rate from 455 to 1654 dry Mg ha(-1) for 8 to 23 yr in rotation from 1972 to 2004. The fields were cropped with corn, wheat, and sorghum and also with soybean and grass or fallowed. Soil organic carbon (SOC) increased rapidly with the application of biosolids, whereas it fluctuated slightly in fertilizer controls. The peak SOC in the 0- to 15-cm depth of biosolids-amended fields ranged from 4 to 7% and was greater at higher rates of biosolids. In fields where biosolids application ceased for 22 yr, SOC was still much higher than the initial levels. Over the 34-yr reclamation, the mean net soil C sequestration was 1.73 (0-54-3-05) Mg C ha(-1) yr(-1) in biosolids-amended fields as compared with -0.07 to 0.17 Mg C ha(-1) yr(-1) in fertilizer controls, demonstrating a high potential of soil C sequestration by the land application of biosolids. Soil C sequestration was significantly correlated with the biosolids application rate, and the equation can be expressed as y = 0.064 x -0.11, in which y is the annual net soil C sequestration (Mg C ha(-1) yr(-1)), and x is annual biosolids application in dry weight (Mg ha(-1) yr(-1)). Our results indicate that biosolids applications can turn Midwest Corn Belt soils from current C-neutral to C-sink. A method for calculating SOC stock tinder conditions in which surface soil layer depth and mass changes is also described.
This study was carried out in Ibadan, southwestern Nigeria between 1997 and 1999 to determine changes in soil chemical properties and yields on a previously eroded Alfisol. A factorial experiment in which legumes (Vigna unguiculata (cowpea), Mucuna pruriens and Pueraria phaseoloides) and residue management (burned and mulched residues) were factors was set up in two replicates of runoff plots. Yam was planted in 1997 and 1999 whereas maize + legume intercrops were planted in 1997. Soil properties measured were particle size distribution, pH, organic C, total N, Ca, Mg, Mn, exchangeable acidity and effective cation exchange capacity. There was increased acidity at the site. Soil chemical properties were significantly improved by burning of cowpea and Pueraria residues and mulching with Mucuna. Yam tuber yields of 14-18 t ha-1 in 1997 was
In an effort to generate more information on the use of biosolids in the turf industry, a study was conducted to compare the effectiveness of biosolids with recommended organic sources in enhancing soil microbial populations and N availability in putting green sand rootzones. The treatments included four types of rootzones as follows: sand (control), sand mixed with Dakota reed-sedge peat (10%, v/v), sand mixed with yard-waste compost (10%, v/v), and sand mixed with biosolids (10%, v/v), which were established at the North Shore Country Club golf course, Glenview, IL, USA in 1997. All plots received the same fertilizers mainly in organic forms at 100 kg N ha(-1) year(-1). creeping bentgrass (Agrostis palustris) turf was established, and was managed as a practice putting green. Soil microbial biomass in the rootzone (0-30 cm) was higher either under compost (fall 2004 and summer 2005) or biosolids (summer 2003) than other treatments. Rootzone ammonium oxidizer populations were higher with biosolids (5400 g(-1) soil) than peat (1200 g(-1) soil) and compost (500 g-1 soil) amendment and in control (400 g(-1) soil) in summer 2003. The nitrite oxidizer populations were higher with biosolids (24000 g(-1) soil) than compost (8300 g(-1) soil) and peat (4700 g(-1) soil) amendment and in control (3700 g(-1) soil) in summer 2005. Soil potentially mineralizable N, also being lowest in control, was higher in the biosolids-amended than compost- and peat-amended rootzones. In most seasons, the nitrate concentrations in biosolids rootzone (3.16-19.3 mg kg(-1)) were higher than in compost (2.79-17.7 mg kg(-1)), peat (1.83-14.4 mg kg(-1)), and control rootzone (0.81-15.2 mg kg(-1)). The study indicates that while compost could restore soil microbial biomass better than peat, biosolids could maintain greater N supply than peat and compost.
Field litterbag studies were conducted during the dry season between years 2000 and 2001 in typical semi-arid and arid agroecozones of West Africa to measure the dynamics of culturable bacterial and fungal communities in the topsoils. Five different agroforestry leaf litters namely Dactyladenia, Pterocarpus, Alchonea, Senna, and Gliricidia species were decomposed, and their effects on soil microflora were studied. Bacterial densities in all the studied plots of the two agroecozones varied from the order of 10(8) to 10(10) cfu/g, while fungal densities ranged in the order of 10(3) and 10(4) cfu/g. Ecological zones impacted significantly (P
Resource-poor farmers in developing nations cultivate marginal lands, thereby exacerbating the problem of soil degradation through poor plant growth and ground coverage. An assessment of ground cover under such a practice will provide a guideline for soil conservation. Ground cover by leguminous cover crops (e.g., Mucuna pruriens, Pueraria phaseoloides and Vigna unguiculata), associated with yam, maize and rice was measured in three different experiments in southwestern Nigeria using beaded-string method while leaf area was measured using a flat-bed scanner. The leaf area was used in obtaining equivalent of ground cover fraction from the leaf area index. Ground cover by yam was
Resource-poor farmers in developing nations cultivate marginal lands, thereby exacerbating the problem of soil degradation through poor plant growth and ground coverage. An assessment of ground cover under such a practice will provide a guideline for soil conservation. Ground cover by leguminous cover crops (e.g., Mucuna pruriens, Pueraria phaseoloides and Vigna unguiculata), associated with yam, maize and rice was measured in three different experiments in southwestern Nigeria using beaded-string method while leaf area was measured using a flat-bed scanner. The leaf area was used in obtaining equivalent of ground cover fraction from the leaf area index. Ground cover by yam was
The effects of Sokoto phosphate rock ( PR) and plant residues on soil phosphorus ( P) fractions and crop performance was studied in the field on an alfisol in the derived savanna of southwestern Nigeria. The plant residues studied were leaves of Dactyladenia barteri, Flemingia macrophylla, Gliricidia sepium, Leucaena leucocephala, maize ( Zea mays) stover and rice ( Oryza sativa) straw. They were applied alone or in combination with phosphate rock. The PR was applied at a rate of 60 kg P ha(-1) and the plant residues at 5 ton dry matter ha(-1) during the onset of the major rainy season. Plots without PR and plant residues application were included as control. After four weeks incubation period of PR and plant residues, Crotalaria ochroleuca ( shrub legume) was planted in 1999 and maize ( Zea mays) was planted in 2001 as test crops. When PR was not combined with plant residues, incubation of Dactyladenia residues increased resin P from 0.13 to 0.79 mg P kg(-1) at 4 weeks after incubation ( WAI). Other plant residues did not cause appreciable changes in resin P. Application of Leucaena residues increased NaOH-extractable inorganic P ( Pi) from 14.36 to 20.47 mg P kg(-1) at 4 WAI. Total extractable P increased under all the plant residues at 4 WAI but decreased at 8 WAI. When PR was combined with plant residues, there were increases in almost all the P fractions under the plant residues at 4 WAI followed by decreases at 8 WAI. Averaged across P levels, addition of Leucaena residues resulted in higher resin P, NaOH- Pi, residual P and total extractable P compared with the other plant residue treatments. Averaged across residue treatments, resin P increased from 0.12 mg P kg(-1) to 0.75 mg P kg(-1) at 4 WAI and decreased to 0.08 mg P kg- 1 at 8 WAI. All other P pools showed similar trends but with less pronounced decreases at 8 WAI. When PR was not applied with plant residues, Crotalaria dry matter correlate positively with NaHCO3- Pi ( R-2 = 0.59), NaOH- Pi ( R-2 = 0.53) and total extractable P ( R-2 = 0.50), while Crotalaria P uptake correlate positively with NaHCO3- Pi ( R2 = 0.50) at 4 WAI. Maize grain yield and P uptake did not correlate with any of the P fractions. However, when PR was applied with plant residues, maize P uptake was positively correlated with resin P ( R-2 = 0.53) while Crotalaria dry matter ( R2 = 0.60) and P uptake ( R-2 = 0.49) correlate positively with NaOH- Po at 4 WAI. Application of PR alone did not affect yields and P uptakes of Crotalaria and maize crops. Compared with the control without residues, Crotalaria dry matter production and P uptake increased significantly when residues were applied. Interaction effects between PR and Leucaena and rice straw residues on Crotalaria dry matter production and P uptake was significant. Interaction effects between PR and plant residues on maize yield and P uptake were not significant. Our results suggest that combined application of PR with selected plant residues may enhance P dissolution from PR and improve crop performance in some cases.
Field litterbag studies were conducted in the 2000 rainy season and the 2000/2001 dry season along the transect of West African major agroecological zones (agroeco-zones) to measure the decomposition of, and N and P release from 5 plant residues (leaves of woody species) with increasing quality: Dactyladenia barteri, Pterocarpus santalinoides, Alchornea cordifolia, Senna siamea and Gliricidia sepium. The decomposition rate constant (wk(-1)) ranged from 0.034 (Dactyladenia, subhumid zone) to 0.49 (Gliricidia, humid zone) in the rainy season, and from 0.01 (Dactyladenia, subhumid zone) to 0.235 (Pterocarpus, arid zone) in the dry season. The direct correlation between the decomposition rate of plant residues and their quality was only valid in agroeco-zones where there is not moisture stress. Similarly, the direct correlation between the decomposition rate of plant residues and moisture availability was only valid for plant residues with high quality. The decomposition rate of the low quality plant residue could increase from humid to arid zone in West Africa. In the arid zone, the low quality plant residue could also decompose faster than high quality plant residue. The climate-residue quality interactive effects on plant residue decomposition in West Africa were attributed to the feedback of low quality plant residue's mulching effect, soil fauna and appreciable photodegradation in dry regions. A decomposition equation that could be used to predict the decomposition rate of plant residues with various qualities across agroeco-zones in West Africa was obtained from this study. The equation was expressed as follow: k = 0.122 -0.000747*PRQI(2)-0.0233*PRQI*CI + 0.00337*CI* PRQI(2), in which k is the decomposition rate constant (wk(-1)), PRQI the plant residue quality index, and CI the climate index (ratio of rainfall to sunshine hours cumulative during the entire decomposition). The response of N and P release from plant residues to residue quality and climate was similar to that of residue decomposition. At the late stage of the dry season decomposition, the high C/N and C/P ratio plant residue (Dactyladenia leaves) that immobilized N and P in wet zones showed a release of N and P in the dry zone.
The vast area of savanna ecology in Africa plays a significant role in food production, making a study of soils in this zone very important. Therefore, soil physical and chemical properties of 14 soil profiles were studied in a derived savanna zone of southwestern Nigeria on 2 toposequences at 2 locations (Ibadan and Alabata), which were 20 km apart. Six soil profiles were studied at Ibadan while 8 were studied at Alabata. Morphological descriptions of profiles were carried out. Data collected included particle size distribution, bulk density, clay dispersion, water retention characteristics, pH, organic carbon, exchangeable K, Ca, Mg and available P. Soil profiles along the toposequences were well developed with depths exceeding 180 cm, except for one profile at the lower slope position where an indurated plinthic layer was found at 68 cm depth. The horizons were easily distinguished with color, texture and consistency. Total sand, with the main component being coarse sand, decreased with depth from 813 to 502 g kg-1 at Ibadan and from 824 to 635 g kg-1 at Alabata. The clay content increased with depth from 54 to 356 g kg-1 at Ibadan and from 63 to 279 g kg-1 at Alabata. Gravel concentration was highest for soil horizons found between 20 and 102 cm depth. Also, bulk density increased with soil depth from 1.35 to 1.51 g cm-3 at Ibadan and from 1.38 to 1.64 g cm-3 at Alabata, indicating that subsoil horizons were more compact due to higher clay and gravel contents, and sticky consistency. The A horizon had a significantly higher water content at water potentials N2 kPa while the subsoils had higher water content at b2 kPa. Soil organic C and total N decreased with soil depth at both locations with the A horizon having significantly higher organic C (7.1012.69 g kg-1) and total N (0.841.2 g kg-1) than deeper horizons (1.94.47 g kg-1 for organic C and 0.120.58 g kg-1 for N). Particle size distribution was significantly different among the slope positions at both locations. Also, soil water retention, soil pH, total N and exchangeable K were distinguishing parameters among slope positions. The interaction of soil depth and slope position was, however, not significant suggesting that processes influencing soil horizon development acted independently in the vertical and lateral directions. Soil pH was the only attribute that distinguished the toposequences between the two locations.
A study was carried out on a previously eroded Oxic Paleustalf in Ibadan, southwestern Nigeria to determine the extent of soil degradation under mound tillage with some herbaceous legumes and residue management methods. A series of factorial experiments was carried out on 12 existing runoff plots. The study commenced in 1996 after a 5-year natural fallow. Mound tillage was introduced in 1997 till 1999. The legumes Vigna unguiculata (cowpea), Mucuna pruriens and Pueraria phaseoloides were intercropped with maize in 1996 and 1998 while yam was planted alone in 1997 and 1999. This paper covers 19971999. At the end of each year, residues were either burned or mulched on respective plots. Soil loss, runoff, variations in mound height, bulk density, soil water retention and sorptivity were measured. Cumulative runoff was similar among interactions of legume and residue management in 1997 (57151 mm) and 1999 (206397 mm). However, in 1998, cumulative runoff of 95 mm observed for Mucuna-burned residue was significantly greater than the 46 mm observed for cowpea-burned residue and the 3951 mm observed for mulched residues of cowpea, Mucuna and Pueraria. Cumulative soil loss of 7.6 Mg ha 1 observed for Mucuna-burned residue in 1997 was significantly greater than those for Pueraria-mulched (0.9 Mg ha-1) and Mucuna-mulched (1.4 Mg ha-1) residues whereas in 1999 it was similar to soil loss from cowpea treatments and Pueraria-burned residue (2.35.3 Mg ha 1). There were no significant differences in soil loss in 1998 (13.2 Mg ha 1) whereas Mucuna-burned residue had a greater soil loss (28.6 Mg ha 1) than mulched cowpea (6.9 Mg ha 1) and Pueraria (5.4 Mg ha 1). Mound heights (23 cm average) decreased non-linearly with cumulative rainfall. A cumulative rainfall of 500 mm removed 0.32.3 cm of soil from mounds in 1997, 3.56.9 cm in 1998 and 2.34.6 cm in 1999, indicating that (detached but less transported) soil from mounds was far higher than observed soil loss in each year. Soil water retention was improved at potentials ranging from 1 to 1500 kPa by Mucuna-mulched residue compared to the various burned-residue treatments. Also, mound sorptivity at 1 cm water head (14.3 cm h 1/2) was higher than furrow sorptivity (8.5 cm h 1/2), indicating differences in hydraulic characteristics between mound and furrow. Pueraria-mulched residues for mounds had the highest sorptivity of 17.24 cm h 1/2, whereas the least value of 6.96 cm h 1/2 was observed in furrow of Mucuna-burned residue. Pueraria phaseoloides was considered the best option for soil conservation on the previously eroded soil, cultivated with mound tillage.
Plant residues are being suggested as an amendment to enhance P release from rock phosphate, however, plant residue enhanced P release could depend on the residue quality, application rate and placement method. Effects of plant residue quality, application rate and placement method on solubility and P release from rock phosphate ( PR) were studied in laboratory and field experiments. Leaves of ten woody and herbaceous species were incubated in a P-deficient soil with Sokoto PR under laboratory conditions to study the effects of residue quality on P release from PR. Effects of residue application rate and placement method were investigated in a field trial with five rates (0, 1, 2, 4 and 8 t DM ha(-1)) of leaves of Dactyladenia barteri, two placement methods (incorporation and mulching), and two levels of Sokoto PR (0 and 60 kg P ha(-1)) in a split-split plot design replicated three times. The plant residues were applied 4 weeks before the main season planting. Maize (main season) followed by cowpea (minor season) was used as test plant. In the laboratory incubation study, addition of plant residues increased the soil pH. Phosphorus released at 8 weeks was greater for plant residues with high C/N ratio, and low magnesium and potassium. In the field trial, soil pH was not affected by the addition of plant residues. Residue placement method showed little effects on P availability from PR. When residues were incorporated with PR, soil Olsen P was highest at lower rates of residues (1 and 2 t DM ha(-1)) at maize planting. Maize P concentration and P uptake were highest at the incorporation of 4 t DM ha(-1) residues with PR. Incorporation at higher rate (8 t DM ha(-1)) resulted in the greatest P uptake of the second crop, cowpea. The study shows the potential of plant residues in enhancing P release from PR. However, there could be initial immobilization of P, but this could be overcome within a short period if residues of high C/N ratio are used. A lower rate of residues (1-4 t DM ha(-1)) is efficient at releasing PR-P for short-term effect especially if incorporated
A microcosm experiment was set up under laboratory conditions and verified under field conditions with the objective of investigating the interaction of soil microarthropods and earthworms in litter decomposition, nutrient release, and uptake by maize crop. The treatments included: soil alone (control), soil with leaf litter (Senna siamea leaves), soil with leaf litter and soil microarthropods, soil with leaf litter and earthworms (Hyperiodrilus africanus), and soil with litter and both of the soil faunal groups. After an 8-week incubation period, the amounts of litter decomposed and N, P, K, Ca, and Mg released followed the order: with microarthropods and earthworms > with earthworms > with microarthropods > no faunal addition. The presence of microarthropods and earthworms also increased the net level of mineral N in the incubated soil. The additive roles of soil microarthropods and earthworms were observed on decomposition and nutrient release. Such faunal interactions resulted in an increased N uptake by maize in the incubated soil. Despite their lower biomass, soil microarthropods contributed significantly to nutrient turnover in the presence of earthworms. This study emphasizes the need to quantify and devise ways of controlling and regulating the abundance and activities of soil fauna for effective nutrient cycling and, consequently, for better crop yields in low-input tropical agricultural ecosystems.
The vast area of savanna ecology in Afiica plays a significant role in food production, making a study of soils in this zone very important. Therefore, soil physical and chemical properties of 14 soil profiles were studied in a derived savanna zone of southwestern Nigeria on 2 toposequences at 2 locations (Ibadan and Alabata), which were 20 kin apart. Six soil profiles were studied at Ibadan while 8 were studied at Alabata. Morphological descriptions of profiles were carried out. Data collected included particle size distribution, bulk density, clay dispersion, water retention characteristics, pH, organic carbon, exchangeable K, Ca, Mg and available P. Soil profiles along the toposequences were well-developed with depths exceeding 180 cm, except for one profile at the lower slope position where an indurated plinthic layer was found at 68 cm depth. The horizons were easily distinguished with color, texture and consistency. Total sand, with the main component being coarse sand, decreased with depth from 813 to 502 g kg(-1) at Ibadan and from 824 to 635 g kg(-1) at Alabata. The clay content increased with depth from 54 to 3 5 6 g kg(-1) at Ibadan and from 63 to 279 g kg(-1) at Alabata. Gravel concentration was highest for soil horizons found between 20 and 102 cm depth. Also, bulk density increased with soil depth from 1.35 to 1.51 g cm(-3) at Ibadan and from 1.38 to 1.64 g cm(-3) at Alabata, indicating that subsoil horizons were more compact due to higher clay and gravel contents, and sticky consistency. The A horizon had a significantly higher water content at water potentials > 2 kPa while the subsoils had higher water content at
A study was carried out on a previously eroded Oxic Paleustalf in Ibadan, southwestern Nigeria to determine the extent of soil degradation under mound tillage with some herbaceous legumes and residue management methods. A series of factorial experiments was carried out on 12 existing runoff plots. The study commenced in 1996 after a 5-year natural fallow. Mound tillage was introduced in 1997 till 1999. The legumes - Vigna unguiculata (cowpea), Mucuna pruriens and Pueraria phaseoloides - were intercropped with maize in 1996 and 1998 while yam was planted alone in 1997 and 1999. This paper covers 1997-1999. At the end of each year, residues were either burned or mulched on respective plots. Soil loss, runoff, variations in mound height, bulk density, soil water retention and sorptivity were measured. Cumulative runoff was similar among interactions of legume and residue management in 1997 (57-151 mm) and 1999 (206-397 mm). However, in 1998, cumulative runoff of 95 mm observed for Mucuna-burned residue was significantly greater than the 46 mm observed for cowpea-burned residue and the 39-51 mm observed for mulched residues of cowpea, Mucuna and Pueraria. Cumulative soil loss of 7.6 Mg ha(-1) observed for Mucuna-burned residue in 1997 was significantly greater than those for Pueraria-mulched (0.9 Mg ha(-1)) and Mucuna-mulched (1.4 Mg ha(-1)) residues whereas in 1999 it was similar to soil loss from cowpea treatments and Pueraria-burned residue (2.3-5.3 Mg ha(-1)). There were no significant differences in soil loss in 1998 (1-3.2 Mg ha(-1)) whereas Mucuna-burned residue had a greater soil loss (28.6 Mg ha(-1)) than mulched cowpea (6.9 Mg ha(-1)) and Pueraria (5.4 Ms ha(-1)). Mound heights (23 cm average) decreased non-linearly with cumulative rainfall. A cumulative rainfall of 500 mm removed 0.3-2.3 cm of soil from mounds in 1997, 3.5-6.9 cm in 1998 and 2.3-4.6 cm in 1999, indicating that (detached but less transported) soil from mounds was far higher than observed soil loss in each year. Soil water retention was improved at potentials ranging from -1 to -1500 kPa by Mucuna-mulched residue compared to the various burned-residue treatments. Also, mound sorptivity at -1 cm water head (14.3 cm h(-1/2)) was higher than furrow sorptivity (8.5 cm h(-1/2)), indicating differences in hydraulic characteristics between mound and furrow. Pueraria-mulched residues for mounds had the highest sorptivity of 17.24 cm h(-1/2), whereas the least value of 6.96 cm h(-1/2) was observed in furrow of Mucuna-burned residue. Pueraria phas eoloides was considered the best option for soil conservation on the previously eroded soil, cultivated with mound tillage.
In the development of short fallow systems as alternatives to shifting cultivation in West Africa, a long-term trial was established at the International Institute of Tropical Agriculture (IITA) on an Alfisol in the forest-savanna transitional zone of southwestern Nigeria, comparing three fallow systems; natural regrowth fallow, cover crop fallow and alley cropping on soil productivity and crop yield sustainability. The natural fallow system consisted of natural regrowth of mainly Chromolaena odorata shrub as fallow vegetation. The cover crop fallow system consisted of Pueraria phaseoloides, a herbaceous legume as fallow vegetation. The alley cropping system consisted of woody hedgerows of Leucaena leucocephala as fallow vegetation. The fallow lengths were 0 (continuous cropping), 1, 2 and 3 years after 1 year of maize/cassava intercropping. Biomass produced from natural fallow and cover crop fallow was burnt during the land preparation. Fertilizer was not applied throughout the study. Without fertilizer application, maize yield declined from above 3.0 t ha(-1) to below 0.5 t ha(-1) during 12 years of cultivation (1989-2000) on a land cleared from a 23-year old secondary forest. Temporal change in cassava tuber yield was erratic. Mean maize grain yields from 1993-2000 except for 1999 were higher in cover crop fallow system (1.89 t ha(-1)) than in natural fallow system (1.73 t ha(-1)), while natural fallow system outperformed alley cropping system (1.46 t ha(-1)). During the above 7 years, mean cassava tuber yield in cover crop system (7.7 t ha(-1)) did not differ from natural fallow system (8.2 t ha(-1)), and both systems showed higher cassava tuber than the alley cropping system (5.7 t ha(-1)). The positive effect of fallowing on crop yields was observed for both crops in the three systems, however, insignificant effects were seen when fallow length exceeded 1 year for cover crop and alley cropping, and 2 years for natural fallow. Soil pH, organic carbon, available P and exchangeable Ca, Mg and K decreased considerably after 12 years of cultivation, even in a 3-year fallow subplot. After 12 years, soil organic carbon (SOC) within 0-5 cm depth in alley cropping (13.9 g kg(-1)) and natural fallow (13.7 g kg(-1)) was higher than in cover crop fallow (11.6 g kg(-1)). Whereas significant increase in SOC with either natural fallow or alley cropping was observed only after 2 or 3 years of fallow, the SOC in the 1-year fallow alley cropping subplot was higher than that in continuous cropping natural fallow subplot. It can be concluded from our study that in transforming shifting cultivation to a permanent cropping, fallow with natural vegetation (natural fallow), herbaceous legumes (cover crop fallow) and woody legumes (alley cropping) can contribute to the maintenance of crop production and soil fertility, however, length of fallow period does not need to exceed 2 years. When the fallow length is reduced to 1 year, a better alternative to natural regrowth fallow would be the cover crop for higher maize yield and alley cropping for higher soil organic matter. For fallow length of 2 years, West African farmers would be better off with the natural fallow system.
Sod phosphorus fractions, maize and cassava P uptakes in natural regrowth fallow and Pueraria phaseoloides fallow systems, as affected by the duration (1-3 years) of fallow, were assessed in 1998 and 1999 in a long-term fallow management trial, established in 1989 at the International Institute of Tropical Agriculture, Ibadan, Nigeria. After one and three years fallow respectively, the mean P accumulation in fallow vegetation was 4.2 and 9.4 kg ha(-1) in 1998, and 8.9 and 12.1 kg ha(-1) in 1999 for natural fallow. For Pueraria, the corresponding values were 6.3 and 7.4 kg ha(-1) in 1998 and 6.1 and 13.5 kg ha(-1) in 1999. The positive effects of fallow lengths on dry matter and P accumulation were not as pronounced under Pueraria. Continuous cropping reduced soil pH and depleted Olsen P more than crop-fallow rotations under both fallow systems. An increase in fallow duration enhanced the availability of resin P2NaHCO3-, and NaOHextractable P fractions, especially under natural fallow. Continuous cropping in a natural fallow system resulted in lower resin inorganic P (P-i) and NaHCO3 -extractable inorganic and organic P (P-o) fractions, and higher refractory P fractions (concentrated HCl-extractable P-i and P-o and residual P) as compared to crop-fallow rotations. Although continuous cropping in the Pueraria system resulted in higher refractory P fractions than the crop-fallow rotations, the effects of fallow length on P fractions were not consistent or pronounced. Maize and cassava P uptakes increased with fallow duration. Phosphorous in maize grain was positively correlated with fallow vegetation P accumulation. Total P uptake by maize was positively but weakly correlated with NaHCO3-P-o, Olsen P and fallow vegetation P accumulation.
Tree species in agroforestry systems contribute to soil improvement through the litter produced on the soil surface and below-ground modification of soil structure by tree roots. Therefore, litter production and soil characteristics under I I tree species were evaluated in the derived savanna (Ibadan) and humid tropical forest (Onne) zones of southern Nigeria between 1998 and 1999 in 7-year old arboreta. In Ibadan, the soil was a gravelly Alfisol, while in Onne it was an Ultisol. The trees were Alchornea cord folia. Baphia pubescens, Calliandra calothyrsus, Dactyladenia barteri, Dialium guineense, Grewia pubescens, Inga edulis, Irvingia gabonensis, Nauclea diderrichii, Ptero-carpus santalinoides and Treculia africana. Forest and natural regrowth (no-tree) plots were included as control. Litter-fall under the trees ranged from 5 to 14 t ha(-1) y(-1) compared to 15 t ha(-1) y(-1) for the forest at Ibadan, while it ranged from 7 to 16 t ha(-1) y(-1) compared to 20 t ha(-1) y(-1) for the forest at Onne. Litter production was high under D. barteri and I. edulis and relatively low under A. cord folia, G. pubescens, I. gabonensis and A santalinoides. There was a more drastic change in soil chemical properties in the Alfisol than the Ultisol, particularly with available P. Soil bulk density and penetrometer resistance were improved by A. cordifolia, C. calothyrsus, D. guineense, G. pubescens and T africana. This study showed that ecological differences resulted in differences in annual litter production for only 33% of the species considered. However, the effects of the trees on soil properties were strongly linked to inherent soil properties.
Phosphate rock (PR) is being considered as an important source for phosphorus (P) replenishment in sub-Saharan Africa soils. Field trials were conducted with two-season crops to compare the utilization of PR combined with or without plant residues by test crops. The plant residues studied include leaves of Dactyladenia barteri, Flemingia macrophylla, Gliricidia sepium, and Leucaena leucocephala, maize (Zea mays) stover, and rice (Oryza sativa) straw. The test plants were Crotalaria ochroleuca (legume cover crop) and maize (Zea mays) in sequential cropping systems. Where no plant residues were applied, apparent utilization efficiency (AUE) of PR was 1.0% in 1999 and 2.7% in 2001. The AUE of PR was improved in many cases with the addition of plant residues. The AUE of PR in all treatments with plant residues was on average 3.4% in 1999 and 6.2% in 2001 with a pronounced effect from Flemingia and Gliricidia. Negative values of AUE were observed in treatments with Leucaena and maize stover in 1999, and Dactyladenia in 2001. These results suggest that the choice of plant residues is an important step for using plant residues as PR amendment on the near-neutral and neutral soils of West Africa.
Few direct measurements are made to quantify the erosion from upstope to lower field boundaries by intensive tillage. We conducted 50 plowing operations over a 5-d period using a donkey-drawn mold-board-plow on steep backslope in the Chinese Loess Plateau. Topographic changes at different slope positions were quantified using differential global positioning system (DGPS). Soil organic matter (SOM), extractable P and N, and soil bulk density were measured along a downslope transect after each 10-tillage series. Fifty operations resulted in a decrease in maximum soil surface level (SSL) of 1.25 m in the upper slope position and an increase of 1.33 m at the bottom of the slope. Slope gradients decreased from 37 to 14degrees at the upper position and from 18 to 0degrees at the lower position. Surface soil bulk density increased from 1.14 to 1.28 Mg m(-1) in the upper slope and decreased from 1.10 to 1.03 Mg m(-3) in the middle slope. Mean SOM concentrations in the upper and middle positions of the slope decreased from 8.3 to 3.6 g kg(-1), mineral N from 43.4 to 17.4 mg kg(-1), and Olsen-P from 4.5 to 1.0 mg kg(-1). Intensive tillage resulted in a short-term increase in SOM and available nutrients in the lower portion during the tillage operations. Geomorphologic evolution and landscape variability of dissected hillslopes are attributable to soil movement and resulting physical and fertility degradation induced by intensive tillage.
The effects of burning, incorporation and surface mulching after one year of natural regrowth (NF) and Pueraria cover crop fallows on soil P fractions and maize and cassava yields were assessed. Data were collected between 1998 and 2000 as part of an on-going long-term fallow management trial established in 1989 at the International Institute of Tropical Agriculture (IITA), Ibadan, in the derived savanna zone of southwestern Nigeria. After NF in 1998, burning temporarily increased resin- (63%) and NaOH- (19%) extractable inorganic P fractions when compared with mulching and incorporation. Organic P and total extractable P decreased as the season progressed. Fallow systems and residue management options had no consistent and significant effect on P fractions. For both years after W and in 1998, after Pueraria fallow, residue management options had no significant effect on cassava yield. However, in 1999 (wetter year), compared with burning and surface mulching, incorporation of Pueraria fallow residues improved maize and cassava yields. Similarly, maize yield was improved in 1999 with incorporation of NF residues. Maize yield increases due to incorporation of NF residues were 23% compared with burning and 49% compared with mulching. Maize yield increases due to incorporation of Pueraria residues were 37% compared with burning and 47% compared with mulching. Cassava yield was increased by 31% compared with burning and by 26% compared with mulching. Longer period of time may be required to assess whether differences in soil P fractions and crop yields will occur between fallow systems.
Trees play a significant role in improving soils for crop production through root activities. This study was carried out on four farmers' fields in Mokwa, central Nigeria in May 1999 to quantify root distributions of 2-year-old Gliricidia sepium (G. sepium) and Acacia leptocarpa (A. leptocarpa). Root samples were taken up to 300-cm soil depth. Root length, surface area, and volume densities were evaluated by using Delta-T Scan device. Mass density was also evaluated. Root length densities of G. sepium ranged from 0.56-11mm mL(-1) while those of A. leptocarpa ranged from 0.135.93mmmL(-1), declining with soil depth. Potential contribution of roots to soil total porosity was less than 3%. Soil bulk density ranged from 1.28-1.51 g cm(-3) under G. sepium and from 1.29-1.61 g cm(-3) under A. leptocarpa. Although G. sepium had relatively more roots deeper in the soil than A. leptocarpa, both trees demonstrated deep rooting, suggesting that the trees would improve soil productivity if sufficient fallow length were allowed.
A study was conducted in long-term alley cropping plots established in 1989 at International Institute of Tropical Agriculture (IITA), Ibadan, southwestern Nigeria to determine the seasonal and yearly dynamics of dry matter and nutrient concentrations of hedgerow pruning under continuous cropping and response to fallow of 1-3 years. At one year after Leucaena leucocephala transplanting, pruning was started at the 75 cm height. The pruning frequency increased from one to five times per year as trees grew. In continuous alley cropping without fertilizer application, pruning dry matter decreased gradually from a peak of 5 t ha(-1) at 3 years after Leucaena transplanting to about 2.5 t ha(-1) at 11 years after transplanting, while the ratio of dry season to wet season biomass increased. Within 5 years after establishment, nitrogen (N) and phosphor-us (P) concentrations in pruning decreased and Mg increased with the continuous cropping. Fallow clearly increased pruning biomass and nutrient concentrations. At the completion of the first cycle (1993), the pruning N yield was 170 kg ha(-1) for the continuous cropping plot, and 269 for plots with 1 yr fallow, 344 for plots with 2 yr fallow and 241 kg ha(-1) for plots with 3 yr fallow. In 1993 pruning P yield increased from 6.5 to 10.9 in the continuous alley cropping and in 1995, it increased from 4.5 to 7.0 kg ha(-1) in one-year fallow plots. Fallow effect on potassium (K), calcium (Ca), and magnesium (Mg) yield in pruning was also evident. These results suggest alley cropping with one-year fallow could be an effective rotation system for nutrient cycling in Alfisols of West Africa.
A study was conducted in 1998 and 1999 on a long-term fallow management trial, established in 1989 at the International Institute of Tropical Agriculture (IITA), Ibadan, in the derived savanna of southwestern Nigeria, to quantify P fractions under natural fallow (NF) and a Pueraria cover crop fallow. Plots with previous 1: 1, 1: 2, and 1: 3 crop/fallow ratios before reverting to fallow in either 1998 or 1999 were considered. Biomass accumulation under the two fallow types was comparable and increased linearly with fallow age, reaching slightly above 7 t dry matter ha(-1) after 8 months of fallow. Phosphorus accumulation in the fallow vegetation ranged from 2.1 - 9.1 kg ha(-1) for natural fallow (NF) and from 1.5 - 6.6 kg ha(-1) for Pueraria. Magnesium was also higher under NF (9.1 - 21 kg ha(-1)) than under Pueraria (4.4 - 13 kg ha(-1)), whereas N, Ca, and K contents were higher in Pueraria biomass than under NF at 1 year after fallow. Pueraria fallow tended to lower soil pH compared with NF. However, plots with less frequent cropping ( 1: 3 crop/fallow ratio) did not have significantly different pH irrespective of the fallow vegetation type. Olsen extractable soil P increased as fallow length increased irrespective of the fallow system and previous crop/fallow ratio. For example, under NF ( 0 - 5 cm depth, 1: 1 crop/fallow ratio in 1998) Olsen P increased from 12 mg kg(-1) to 17 mg kg(-1) after 1 year of fallow and under Pueraria, it increased from 8 mg kg(-1) to 15 mg kg(-1). Fallow type and previous crop/fallow ratio had no significant and consistent effects on soil P fractions. However, NaOH- and concentrated HCl- extractable organic P fractions increased with fallow length. In 1998, under NF, NaOH- extractable organic P increased from 12 to 21 mg kg(-1) (1: 1 crop/fallow ratio) and from 10 to 19 mg kg(-1) for both 1: 2 and 1: 3 crop/ fallow ratio. HCl- extractable organic P increased from 11 to 30 mg kg(-1) ( 1: 1 crop/ fallow ratio), from 13 to 27 mg kg(-1) ( 1: 2 crop/ fallow ratio) and from 18 to 35 mg kg(-1) ( 1: 3 crop/ fallow ratio). Similar trend was observed under Pueraria fallow. These results suggest that P was reallocated to non-readily available organic P fractions irrespective of fallow type and previous land use. These organic P fractions, which are usually more stable, reflect the overall change in soil organic P levels when the soil was stressed by cultivation and then reverted to fallow. These pools may thus represent an active reservoir ( source and sink) of P in shifting cultivation under tropical conditions without inorganic fertilizer application.
In 1989, 15 fallow species were planted on a degraded Alfisol in southwestern Nigeria, but only Pueraria phaseoloides, Senna siamea, Leucaena leucocephala, Acacia leptocarpa and Acacia auriculiformis survived beyond 1993. After clearing different subplots of the fallowed plots in 1993 and 1995, intercropping of cassava + maize was practiced with level (minimum) and mound tillage. Nitrogen fertilizer, recommended while in the third year, mound tillage with 60 kg N ha(-1) was recommended
Leguminous cover crops have the potential of making cropping systems in the tropics sustainable if they would not deplete resources such as soil water and nutrients to the detriment of companion crops. Therefore, a study was carried out at Alabata, Ibadan, southwestern Nigeria, to evaluate the effects of leguminous cover crops on soil water suctions in 1993 and 1994 in order to assess the possibility of integrating them into the farming systems of the savanna zone of West Africa. In 1993, 13 leguminous cover crops (Aeschynomene histrix, Centrosema brasilianum, Centrosema pascuorum, Chamaecrista rotundifolia, Cajanus cajan, Crotalaria verrucosa, Crotalaria ochroleuca, Lablab purpureus, Mucuna pruriens, Psophocarpus palustris, Pseudovigna argentea, Pueraria phaseoloides and Stylosanthes hamata) were planted in a randomized complete block design with four replications. Maize and natural fallow (mainly Chromolaena odorata and Imperata cylindrica) were included as comparisons. Only six of the legumes (A. histrix, C. pascuorum, C. cajan, C. ochroleuca, M. pruriens, and P phaseoloides) were included in the measurements in the 1994 new plots. Soil water suctions at various stages of legume growth were measured at daily or weekly intervals (depending on the frequency of rainfall events) using tensiometers installed at 0-15 and 15-30 cm soil depths. Soil water suctions exceeding 10 kPa (theoretical field capacity) were observed mainly between 6 and 12 weeks after planting (WAP), and by 20 WAP when cover crops had matured and rainfall frequency was very low. Soil water suctions were significantly related (r(2) > 0.80) to dry matter between 8 and 10 WAR The studied cover crops were classified in three groups which can be used as a guide for choosing the legumes in tropical farming systems. Soil water depletion was markedly influenced by growth characteristics of legumes and distribution of rainfall during the rainy season. Leguminous cover crops conserved soil water after their growth needs were satisfied.
This study was carried Out in 1994 in the derived (DS) and northern Guinean savanna (NGS) zones of Nigeria at about 17-18 weeks after planting forage legumes. The forage legumes, which included Cajanus cajan, Crotalaria ochroleuca, Stylosanthes hamata, Centrosema pascuorum, Lablab purpureus, Mucuna pruriens (black and white) and Pueraria phaseoloides, were treatments in the randomised complete block design. Root morphology and length were evaluated for the 2 zones, but light received under canopies, soil temperature and soil water potential were measured only in the DS. Roots on soil profile walls were traced, and in the laboratory, computer software Was used to evaluate root length after scanning. Canopy coverage was evaluated using a light meter. In the DS, the presence of gravel in-the subsoil modified the morphology and distribution of roots even for the same species compared with the root architecture in the NGS zone. A hardpan in the shallow soil layers in the NGS caused a proliferation of roots in the topsoil, and a tendency for increased diameter (visual observation) of the primary roots. Total root length ranged from 834-1861 turn in the DS, and 939-1277 mm in the NGS, and was similar in both locations. Legume canopies intercepted 81-99% of sunlight, suggesting that they had the potential to maximise sunlight for their growth under adequate soil moisture and nutrient in the moist savanna. Legumes such as C. cajan, L. purpureus, M, pruriens and P phaseoloides with potential to grow roots beyond 60 cm depth and which intercepted greater than or equal to95% of incident Sunlight were recommended for sustained growth and rapid regeneration after cutting or browsing. Furthermore, the legumes can be combined with shallow-rooted crops for enhanced crop production in a crop-livestock farming system or in pasture, based oil the differences in their root and canopy growth.
The benefit of planted fallow with legume cover crops may be limited on P deficient soil. A trial was conducted at two P deficient sites in northern Nigeria to test the hypothesis that application of P to legume cover crop fallow can substitute for N application to subsequent maize. Mainplots consisted of leguminous fallows followed by unfertilized maize, or native (mostly grass) fallows followed by maize with 0 or 40 kg N ha(-1) (Kaduna) and 0, 30 or 60 kg N ha(-1) (Bauchi). Three rates of P (0, 9, and 18 kg ha(-1)) were applied to fallow sub-plots as single superphosphate. In the first year, dry matter accumulation of lablab (Lablab purpureus) responded to P application, while mucuna (Mucuna cochinchinensis) dry matter did not. Lablab mulch dry matter during the dry season was significantly increased by previous season P application while mucuna was not. Previous fallow vegetation was a significant factor for maize growth in the second year but the interaction with P applied to the fallow was not significant at P
The capacity of vegetation fallow to replenish carbon and nutrients in degraded soil is related largely to the nature of the fallow vegetation, particularly the chemical composition. Therefore, a study was conducted at Ibadan, southwestern Nigeria (humid tropics), to look into these relationships using, fallow species with varying chemical compositions. The treatments include three woody species, Senna siamea, Acacia leptocarpa and Leucaena leucocephala, planted in 1989, and the natural shrub Chromolaena odorata. A continuous cropping of maize/cassava was maintained as a control (no fallow). Composite surface soil (0-15 cm) at three distances from a tree hedgerow (0.5, 2.0, and 3.5 m) was sampled in 1996 for the determination of soil C and nutrient stocks. Maize and cassava were planted as a test crop of soil productivity after fallowing. Litterbags were placed to determine the N immobilization during the decomposition of fallow leaves. Soil. organic C (SOC) within 0-15 cm ranged from 19,100 (continuous cropping) to 26,400 kg ha(-1) (Leucaena fallow), whereas total N ranged from 1820 (continuous cropping) to 3110 kg ha(-1) (Leucaena fallow). High polyphenols in fallow leaves favored SOC and N accumulation. The amount of potentially mineralizable N was increased from 41 kg ha(-1) in continuous cropping to 159 under Leucaena fallow and 176 under der Acacia fallow. The percentage increase in mineralizable N stock under fallow was, on average, 5.5 times greater than that in total N, implying that fallow causes change in soil organic matter quality.. The available P (Olsen) was 8.6 kg ha(-1) in the continuous cropping and ranged from 14.1, kg ha(-1). (natural fallow) to 29.2 kg ha(-1) (Leucaena fallow). Greater maize and cassava yields were obtained in the fallow plots than in the control during, the subsequent cropping. The maize grain yield after fallow could be predicted by the potentially mineralizable N in surface soil (0-15 cm). Leucaena and Acacia are promising species for planted fallows for soil regeneration in the humid tropics. The binding of protein by polyphenols during leaf decomposition, as confirmed by higher N immobilization with the increase in leaf polyphenols, could be the main mechanism in the contribution of polyphenols to SOC and N replenish, ment in the degraded soil. The study suggests the possibility of stabilizing C and N in tropical ecosystems by manipulating polyphenols in vegetation.
To rehabilitate a degraded Alfisol at Ibadan, southwestern Nigeria, Senna siamea (non-N-fixing legume tree), Leucaena leucocephala, and Acacia leptocarpa (N-fixing legume trees) were planted in 1989, and Acacia auriculiformis (N-fixing legume tree) in 1990. Pueraria phaseoloides (a cover crop) and natural fallow were included as treatments. Litterfall and climatic variables were measured in 1992/1993 and 1996/1997 while biomass production and nutrient concentrations were measured in 1993 and 1995. Total litter production from the natural and planted fallows was similar, with means ranging from 10.0 (L. leucocephala) to 13.6 t ha(-1) y(-1) (natural fallow) during the 1996/1997 collection. Leaves constituted 73% (L. leucocephala) to 96% (A. auriculiformis) of total litterfall. Acacia auriculiformis grew most quickly but S. siamea produced the highest aboveground biomass which was 127 t ha(-1) accumulated over four years, and 156 t ha(-1) accumulated over six years of establishment. The aboveground biomass of P. phaseoloides and natural fallow was only 6 to 9 t ha(-1) at six years after planting. Nitrogen concentration in the leaves/twigs of was 2.5% for L. leucocephala, and 2% for other planted species and natural fallow. Pueraria phaseoloides had concentrations of P, K, Ca and Mg comparable to levels in the leaves/twigs of the tree species. Through PATH analysis, it was found that maximum temperature and minimum relative humidity had pronounced direct and indirect effects on litterfall. The effects of these climatic variables in triggering litterfall were enhanced by other variables, such as evaporation, wind, radiation, and minimum temperature. Improvement in chemical properties by fallows was observed in the degraded soil
Topsoil (0-15 cm) hulk density, aggregate stability soil dispersibility, water retention and infiltration were measured between 1989 and 1996 on an Alfisol under rehabilitation in southwestern Nigeria. The planted leguminous species were Pueraria phaseoloides, Senna siilnrrir, Leucaena leucocephala,, Acacia leptocarpa and A. auriculiformis. plots with natural fallow and maize/cassava intercropping were included. Level (minimum) and mound tillage with hoes was adopted for the cultivated areas under study after 4 and 6 year fallow periods. Under fallow, the soil bulk density decreased from 1.56 to 1.11 t m(-3). The continuously cropped treatment (level tillage) had significantly higher bulk density than the fallowed subplots after 6 years. Mean soil penetrometer resistance ranged from 75 to 157 kPa for fallowed plots and from 192 to 295 kPa for the continuously cropped (level tillage) subplot. Surface soil water contents were similar for all the treatments during the soil strength measurements. Although soil aggregates were generally of low stability and not well formed, they were improved by fallowing. Soil structural improvement by planted fallows was similar to that by natural fallow, but the trees were more promising for long-term fallow (>6 years) than the herbaceous P. phaseoloides. However, the improvement in soil structure after 4 or 6 rear falloff could not be maintained in subsequent cropping. Furthermore, the significant improvement in soil bulk density caused by A. auriculiformis and natural fallow was more rapidly lost on the cultivated subplots compared with other fallow treatments. Thus, soil structure recovery under a fallow does not imply a sustained improvement when stress is applied to this soil. Post-fallow soil management options such as residue incorporation and tillage to ameliorate compaction or soil strength mill be necessary to enhance the improvements by fallow species.
Legume cover crops are a potential means for overcoming N depletion in the derived savanna of West Africa. A 3-year trial was, therefore, conducted near Ibadan, southwestern Nigeria to measure the N contribution of 13 legume cover crops as compared to urea N, using a N fertilizer replacement index for a maize test crop. Two series of trials involved the following legume cover crop species: Aeschynomene histrix, Centrosema brasilianum, Centrosema pascuorum, Chamaecrista rotundifolia, Cajanus cajan, Crotalaria verrucosa, Crotalaria ochroleuca, Lablab purpureus, Mucuna pruriens, Psophocarpus palustris, Pseudovigna argentea, Pueraria phaseoloides and Stylosanthes hamata. Trials were undertaken using a complete block design. Cover crops were planted in 1994 (Series 1) and 1995 (Series 2) in separate sites and each series was subsequently slashed and planted for one season with maize (Zea mays) in 1995 and 1996. At the 50% flowering stage, N concentration of above-ground vegetation of cover crops ranged from 21 to 38 g N kg??1. Nitrogen accumulated by 4.5-month old cover crops ranged from 14 to 240 kg N ha??1, depending on species and year. Cover crops increased grain yield of the subsequent maize crop by 25136% over the control without N application. Nitrogen uptake by the maize crop was higher following cover crops than after maize or natural grass. The N fertilizer replacement index of cover crops for maize ranged from 11 (A. histrix) to 96 kg N ha??1 (C. cajan) in Series 2. Perennial (C. brasilianum, S. hamata, C. cajan, P. phaseoloides and C. verrucosa) and annual (C. rotundifolia, M. pruriens, C. ochroleuca and L. purpureus) species could potentially save 50 to 100 kg N ha??1 for maize crops. The cover crops accumulated more N in the wetter than in the drier year. However, the N fertilizer replacement index was higher for subsequent maize grown in the drier year. The cover crop-N recovery in maize was also higher than the urea-N uptake in the drier year. The N fertilizer replacement indexes can be predicted using the above-ground biomass amount of cover crops at 20 weeks after planting (drier year) or the N concentration at that stage (wetter year).
Soil degradation is a major problem in west African land use environments. Although burning of crop residue is known to enhance soil degradation, it is still the most commonly used practice for land preparation. Intercropping with herbaceous legumes is a promising method to combat fertility decline and minimise resource degradation. A study was conducted from 1996 to 1999 in southwestern Nigeria to investigate the interaction of residue burning and legume intercropping on soil degradation as an improved and practical approach toward rehabilitating and conserving an Alfisol. Leguminous cover crops, [Mucuna pruriens (mucuna), and Pueraria phaseoloides (pueraria) with grain legumes [Vigna unguiculata (cowpea)] as control were intercropped with maize in 1996 and 1998; yam was cultivated in 1997 and 1999. Following local practices, mound tillage with hoe was adopted since 1997 for yam production. Subtreatments were burning and mulching of residues from the cover crops, cowpea and maize. Soil erosion and yam mound stability were used as sustainability indicators. The weighted annual mean erosivity was 6.5 103 MJ mm ha-1h-1 (EI30 index), 422 cm2 h-1 for the AIm index. Mean runoff amounts ranged from 20 to 32 mm in 1996, from 94 to 158 mm in 1997, and from 37 to 103 mm in 1998. Mean soil erosion ranged from 0.9 to 2.1 Mg ha-1 in 1996, from 3.8 to 7.8 Mg ha-1 in 1997, and from 1.8 to 3.7 Mg ha-1 in 1998. The burned residue plots had higher runoff and soil erosion than the unburned residue plots. Soil particles were eroded in proportions similar to plot particle size distribution. Mounding in 1997 accentuated soil erosion although transportation of detached soil by runoff was impeded by the mounds. Variations in mound heights were due to accretion as well as removal of soil from the furrows. There was no significant difference in soil erosion between the cover crops and cowpea. Yam mound erodibility (slumping) was largely determined by the length of exposure to rain in the 1998 cropping season. Mounds on the burned residue plot of each legume, except cowpea, slumped more than mounds on the unburned residue plot between April and August 1998 . The combined yields (1996-98) of maize, grain and yam tuber were higher in the unburned residue treatment compared to the burned residue treatment. Cover crop intercropping resulted in higher crop yield than the cowpea. We concluded that the Alfisol is best managed for sustainable crop production by no tillage, residue mulching and cover cropping. Residue burning should be avoided even if cover crops are used.
Earthworm populations (predominantly Hyperiodrilus africanus and Eudrilus eugeniae) were sampled monthly for 1 yr during 1994 and 1995 in natural regrowth vegetation fallow (dominated by the natural fallow shrub Chromolaena odorata L.), planted fallow (the woody species Senna siamea Lam., Leucaena leucocephala Lam., and Acacia leptocarpa), and intercropped maize (Zea mays L.)-cassava (Manihot esculenta Crantz) established in 1989 in a degraded Alfisol (Oxic paleustalf) in southwestern Nigeria. Compared to leaves of Chromolaena (3.3% N), N concentrations were lower in those of Senna and Acacia, and higher in Leucaena. Acacia and Leucaena had higher polyphenol relative to the natural fallow (2%), The lignin was lower in Leucaena than the natural fallow leaves (14%). For 65% of the sampling dates, earthworm numbers under all fallows were significantly higher than under continuous maize-cassava, The mean earthworm numbers (no. m(-2)) during the rainy season (April-October) decreased in the following order: Chromolaena (147), Senna (131), Leucaena (92), Acacia (80), and maize-cassava (14). Earthworm fresh weights in fallow plots were higher than in the maize-cassava plot, though this was significant for only 4 out of 11 sampling dates. Higher earthworm numbers and biomass in fallow plots were attributed to higher litterfall, lower soil temperature, and higher soil moisture. The mean earthworm numbers were directly correlated with the mean soil moistures (r(2) = 0.80, P
Legume cover crops are a potential means for overcoming N depletion in the derived savanna of West Africa. A 3-year trial was, therefore, conducted near Ibadan, southwestern Nigeria to measure the N contribution of 13 legume cover crops as compared to urea -N, using a N fertilizer replacement index for a maize test crop. Two series of trials involved the following legume cover crop species: Aeschynomene histrix, Centrosema brasilianum, Centrosema pascuorum, Chamaecrista rotundifolia, Cajanus cajan, Crotalaria verrucosa, Crotalaria ochroleuca, Lablab purpureus, Mucuna pruriens, Psophocarpus palustris, Pseudovigna argentea, Pueraria phaseoloides and Stylosanthes hamata. Trials were undertaken using a complete block design. Cover crops were planted in 1994 (Series 1) and 1995 (Series 2) in separate sites and each series was subsequently slashed and planted for one season with maize (Zea mays) in 1995 and 1996. At the 50% flowering stage, N concentration of above-ground vegetation of cover crops ranged from 21 to 38 g N kg(-1). Nitrogen accumulated by 4.5-month old cover crops ranged from 14 to 240 kg N ha(-1), depending on species and year. Cover crops increased grain yield of the subsequent maize crop by 25-136% over the control without N application. Nitrogen uptake by the maize crop was higher following cover crops than after maize or natural grass. The N fertilizer replacement index of cover crops for maize ranged from 11 (A. histrix) to 96 kg N ha(-1) (C. cajan) in Series 2. Perennial ( C. brasilianum, S. hamata, C. cajan, P. phaseoloides and C. verrucosa) and annual (C. rotundifolia, M. pruriens, C. ochroleuca and L. purpureus) species could potentially save 50 to 100 kg N ha(-1) for maize crops. The cover crops accumulated more N in the wetter than in the drier year. However, the N fertilizer replacement index was higher for subsequent maize grown in the drier year. The cover crop-N recovery in maize was also higher than the urea-N uptake in the drier year. The N fertilizer replacement indexes can be predicted using the above-ground biomass amount of cover crops at 20 weeks after planting (drier year) or the N concentration at that stage (wetter year).
A laboratory experiment was carried out to assess nitrogen (N) mineralization of roots of four woody agroforestry species (Dactyladenia barteri, Senna siamea, Gliricidia sepium, and Leucaena leucocephala) and maize (Zea mays). Addition of Gliricidia and Leucaena roots with high N contents and low carbon (C)-to-N ratio (less than or equal to 14.0) resulted in net N mineralization, while the addition of maize roots with low N and high C-to-N ratio (40.3) immobilized N. Addition of Dactyladenia and Senna roots with intermediate N content and C-to-N ratio immobilized N to a lesser extent. Nitrogen mineralization was negatively and significantly correlated with C-to-N ratio of the added roots. Addition of roots of the five species also differentially affected soil pH during the first 12 weeks of incubation. Soil pH was positively and significantly correlated with the roots initial C-to-N ratio and negatively correlated with their initial N content. At 24 weeks of incubation, the pH of all treatments reverted to initial value.
The aerobic heterotrophic bacteria, fungal and springtail populations of four monoculture agroforestry plots, a regrowth forest and a grass plot were investigated in September October, and November 1994 so as to throw more light on the relationships between them in a tropical soil. Soil properties, litter composition and temperature and moisture conditions were also investigated. The mean of four replicates was recorded For each parameter. Bacteria and fungi were estimated using a standard pour plate technique while springtails were extracted in a modified Berlese-Tullgren funnel extractor. Bacterial counts were of the order of 10(5) to 10(6) cfu/g while fungal counts were of the order of 10(3) to 10(4) in the experimental plots. There was a predominance of aerobic heterotrophic bacteria over fungi in all the experimental plots. The dynamics of the populations of bacteria. fungi and springtails were different from one another and between experimental plots. Springtail populations declined gradually from September to November in all experimental plots. This trend did not occur fur bacteria and fungi. All experimental plots supported similar bacteria and springtail populations during the period of study but the species composition and diversity of fungal populations differed between plots. Results of correlation studies suggest that there is a negative relationship between springtails and bacteria.
A longterm alley cropping trial was undertaken on an eroded Oxic paleustalf in the forest-savanna transition zone of southwestern Nigeria from 1981-1993. Two nitrogen fixing hedgerow species (Gliricidia sepium and Leucaena leucocephala) and two non legume hedgerow species (Alchornea cordifolia and Dactyladenia barteri) were used in the trial compared to a control (with no hedgerow) treatment. Plots were sequentially cropped with maize (main season) followed by cowpea (minor season). With 4 m interhedgerow spacing and pruning at 0.75 m height, the mean annual pruning biomass yields were observed in the following order: Leucaena (7.1 t ha(-1)) > Gliricidia (4.9 t ha(-1)) > Alchornea (3.7 t ha(-1)) > Dactyladenia (3.0 t ha(-1) ). Alley cropping with the four woody species greatly enhanced the total plot (woody species + crop) biomass yield/ha as follows; Leucaena (21.8 t ha(-1)) > Gliricidia (17.7 t ha(-1)) > Alchornea (11.7 t ha(-1)) > Dactyladenia (9.5 t ha(-1)). Total biomass yield of crops in control plot was 5.3 t ha(-1). Higher biomass yields with alley cropping also increased nutrient yield and cycling. Gliricidia and Leucaena showed higher nutrient yields than Alchornea and Dactyladenia. Alley cropping with Gliricidia and Leucaena could sustain maize yield at moderate level (>2 t ha(-1)), which would require a N-rate of 45 kg N ha(-1) with sole cropping. Application of N in Gliricidia and Leucaena alley cropping still improved maize yield. Higher nitrogen rates are required for alley cropping with Alchornea and Dactyladenia hedgerows. A low rate of phosphorus application is needed for sustaining crop yields with all treatments. Occasional tillage is recommended to increase maize yield. Alley cropping and tillage showed little effect on cowpea seed yield. Surface soil properties declined with time with continuous cultivation. Alley cropping with woody species maintained higher soil organic carbon, phosphorus and potassium levels. Plots alley cropped with Gliricidia and Leucaena showed lower pH and extractable calcium level. Leucaena alley cropped plot also showed lower magnesium level. The decline in soil pH and extractable cations may be due to increased cation leaching with application of high rates of Gliricidia and Leucaena prunings. Alley cropping with the four woody species showed no effect on population of parasitic nematodes.
Integrated soil management with leguminous cover crops was studied at two sites in the northern Guinea savanna zone of northern Nigeria, Kaduna (190 day growing season) and Bauchi (150 days). One-year planted fallows of mucuna, lablab, and crotalaria were compared with natural grass fallow and cowpea controls. All treatments were followed by a maize test crop in the second year with 0, 30, or 60 kg N ha(-1) as urea. Above ground legume residues were not incorporated into the soil and most residues were burned early in the dry season at the Kaduna site. Legume rotation increased soil total N, maize growth in greenhouse pots, and dry matter and N accumulation of maize. Response of maize grain yield to 30 kg N ha(-1) as urea was highly significant at both sites and much greater than the response to legume rotation. The mean N fertilizer replacement value from legume rotation was 14 kg N ha(-1) at Kaduna and 6 kg N ha(-1) at Bauchi. With no N applied to the maize test crop, maize grain yield following legume fallow was 365 kg ha(-1) higher than natural fallow at Bauchi and 235 kg ha(-1) higher at Kaduna. The benefit of specific legume fallows to subsequent maize was mostly related to above ground N of the previous legume at Bauchi, where residues were protected from fire and grazing. At Kaduna, where fallow vegetation was burned, maize yield was related to estimated below ground N. The results show that legume rotation alone results in small maize yield increases in the dry savanna zone.
Populations of soil-dwelling mites were monitored in monoculture plots of four agroforestry tree species, Gliricidia sepium, Leucaena leucocephala, Dactyladenia barteri and Treculia africana, and compared to those in grass and secondary forest plots in the dry season (December 1993 to January 1994) and in the wet season (April to June 1994) in southwest Nigeria. Mite populations were very low in all plots during the dry season (500-3000 m(-2)), compared to those during the wet season (10 000-30 000 m(-2)). The highest mite population was observed in Gliricidia plots (3 044 m(-2)) for the dry season and Leucaena plots (30 240 m(-2)) for the wet season. Mite genera that were dominant in all the experimental plots were Annectacarus, Haplozetes, Machadobelba, Scheloribates and members of the Galumnidae, Dermanyssidae and Parasitidae. The community structure of mites was similar in the soil for Treculia and Gliricidia plots and for Leucaena and Dactyladenia plots. There were more taxonomic groups of mites under Leucaena than in the other agroforestry plots. Based on the density, diversity and complexity of the mite communities, Leucaena was considered to be better than other agroforestry species in encouraging the growth of mite populations.
The potential of planted leguminous cover crop fallow as an alternative to the natural regrowth fallow for sustaining the productivity of low activity clay (LAC) soils in the tropics as the fallow period shortens was tested at Ibadan in the forest-savanna zone of southwestern Nigeria. The study, initiated in 1990, compared an improved system (cover crop-fallow) and a traditional system (natural fallow), with the aim of developing an improved cover crop-fallow system for sustainable management of LAC soils of the tropics. Pueuraria phaseoloides, an herbaceous, N-fixing, perennial legume with a slow initial growth habit, was selected as a test cover crop. Pueraria was sown with a maize-cassava intercrop at the same season. In a 2-year cycle, 1 year of cropping was followed by 1 year fallow with pueraria fdr the cover crop-fallow system or with natural regrowth (mainly Chromolaena odorata) for the natural fallow system. Maize-cassava intercropping without a fallow period (continuous cropping) was included as a control. No fertilizer was applied throughout the experimental period. Pueraria produced 253 kg N ha(-1) after the fallow period, compared with 109 kg N ha(-1) for chromolaena; pueraria was also better in recycling P (11 kg P ha(-1)) than was chromolaena (9 kg P ha(-1)), Natural fallow for 1 year increased the maize yield from 75 to 350% from 1992 to 1996, and the cassava yield from 9 to 130% compared with continuous cropping. Cover crop-fallow resulted in a 22 to 72% higher maize yield than the natural fallow from 1992 to 1996. Although in 1992 and 1994 cassava tuber yield was lower with the cover crop-fallow than with the natural fallow, with better pueraria husbandry in 1996, cover crop-fallow increased the cassava tuber yield by 41%. With residue burning, the N balance was 27 kg N ha(-1) yr(-1) for the pueraria cover crop-fallow system and -15 kg N ha(-1) yr(-1) for the chromolaena natural fallow system. Cover crop-fallow maintained soil organic carbon status better than the natural fallow. Integration of a pueraria legume cover crop did not affect soil pH. Results indicate cover crop-fallow with pueraria could be a better alternative to traditional natural fallow under shortened fallow periods for raising or maintaining productivity of LAC soils of the humid tropics
The biomass and chemical compositions of selected leguminous cover clops Aeschynomene histrix, Cajanus cajan, Centrosema brasilianum, Centrosema pascuorum, Chamaercrista rotundifolia, Crotalaria verrucosa, Lablab purpureus Psophocarpus palustris, Pseudovigna argentea, Mucuna pruriens, Pueraria phaseoloides, and Stylosanthes I?hamata were studied in a pot experiment, using soil with two fertility levels. Biomass yield responded to soil fertility levels and fertilizer application. The highest response to soil fertility was observed with C. brasilianum (340%, 200% and 310% more shoots, roots and nodules in high fertility soil than low fertility soil without fertilizer application). The highest response to fertilizer application was with P. palustris (500% more shoots with fertilizer than without in high fertility soil), with C. rotundifolia (410% more roots with fertilizer than without in low fertility soil), and with L. purpureus (1330% more nodules with fertilizer than without on high fertility soil). Legumes grown without fertilizer application allocated more biomass to roots than those with fertilizer application. Chamaercrista rotundifolia, L. purpureus, P. argentea, M. pruriens and C. cajan showed higher N content in roots than in shoots. Soil fertility levels did not affect N content of shoot, while fertilizer application increased it by 30%. Except for C. cajan (only shrub species), all the herbaceous legumes had lower lignin content (6-10%) in the shoots. The mean lignin content in roots was above 20% with no fertilizer: and decreased with fertilizer application compared to the no fertilizer treatment. The polyphenol concentration in shoots was higher than in roots.
The role of microarthropods in leaf decomposition and N release was studied in various land-use practices: secondary forest, bush regrowth, Pueraria phaseoloides relay cropping, Leucaena leucocephala alley cropping, and cropping without cover crop or tree hedgerows (control treatment), using litterbags with mesh-size of 0.5 mm and 0.08 mm for 98 days. Litterbags with 0.5 mm mesh-size allowed access by microarthropods, whereas those with 0.08 mm mesh-size excluded microarthropods. Microarthropod-mediated leaf decomposition and N release varied with the land-use practices, particularly the last named. Inclusion of microarthropods in most cases increased leaf decomposition and N release rates, with the highest value 38 % for decomposition in control treatment and 29 % for N release in relay cropping. A decrease in leaf decomposition rate (10 %) and N release (32 %) was observed in the secondary forest. Microarthropod-mediated leaf decomposition and N release, which mainly occurred in later stages of decomposition, increased as microbial decomposition decreased. The results suggest that microarthropods have a "buffering effect" in regulating leaf decomposition and N release, which is mediated by land-use history.
Characterization of degradation of an Alfisol caused by clearing and long-term cultivation and the effect of the degradation on leaf (Cajanus cajan) decomposition and nutrient (N and P) release were studied in the humid zone of southwest Nigeria. Degradation of the Alfisol as a result of land clearing and 17-year continuous cropping led to loss of soil biota and deterioration of soil physical and chemical properties. Compared with nondegraded soil, the degraded soil was 38% lower in soil organic carbon, 55% lower fn exchangeable bases, 56% lower in microbial biomass, and had 44% lower microarthropod density, 34% less silt, and 88% less water infiltration; it also had 13% higher bulk density acid 14% more sand. Soil pH was lower in the degraded than in the nondegraded soil. The macrofauna community in the degraded soil was termite-dominant (Ancistratermes cavithorax), whereas it was earthworm-dominant (Hyperiodrilus africanus) in the nondegraded soil. Leaf decomposition and nutrient release were lower irt the degraded soil than in the nondegraded soil during the 150-day experiment. Higher decreases in leaf decomposition and nutrient release in the degraded soil were observed when macrofauna were excluded. Macrofauna-mediated leaf decomposition and nutrient release in the degraded soil were higher than in the nondegraded soil. Irrespective of soil degradation, macrofauna-mediated decomposition and nutrient release resulted in net nutrient mineralization.
The soil-and litter-dwelling Collembola under four agroforestry tree species (Treculia africana, Dactyladenia Acioa) barteri, Gliricidia sepium and Leucaena leucocephala) were monitored monthly for a period of 12 months and results were compared with those of a secondary forest and a grass plot. Treculia and Dactyladenia produced lower quality litter, leading to lower soil temperature and higher soil moisture under those species, relative to Gliricidia and Leucaena. The agroforestry tree plots contained more soil-and litter-dwelling Collembola than the secondary forest and grass plots. The slowly decomposing litter under Treculia and Dactyladenia supported more litter Collembola than the quickly decomposing litter under Gliricidia and Leucaena. Soil moisture and temperature were, respectively, positively and negatively correlated with populations of soil Collembola. Based on the results of this study, it is suggested that the use of prunings of agroforestry tree species as mulch in agroecosystems would encourage the growth of Collembola populations and enhance their role in decomposition processes.
An experiment was conducted in 1990 and 1991 at the International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria to study the role of earthworms in the decomposition of plant residue mulches with different qualities. Five mulches of Dactyladenia barteri, Gliricidia sepium, Leucaena leucocephala prunings, maize (Zea mays) stover and rice (Oryza sativa) straw, which had a wide range of C-to-N ratio, lignin and polyphenol concentrations were studied. Based on their chemical compositions, Dactyladenia prunings were defined as low-quality mulch, Leucaena and Gliricidia prunings as high quality-mulches, and maize stover and rice straw as intermediate-quality mulches. The mean density of earthworms (Hyperiodrilus africanus and Eudrilus eugeniae) in the experimental plots decreased in the following order: high quality > intermediate quality > low quality mulches. High quality mulch (Leucaena and Gliricidia prunings) supported 54% higher earthworm populations than the (no mulch) control, whereas low-quality mulch (Dactyladenia prunings) only increased earthworm density by 15%, compared to the control in 1990. Plots with Leucaena and Gliricidia prunings had the highest earthworm populations at the initial stage of the experiment, while the other treatments showed increased earthworm numbers at a later stage. Effects of earthworms on mulch decomposition were examined in the field in large pots with or without earthworms (Eudrilus eugeniae). The effects of earthworms were more pronounced for Dactyladenia prunings (low quality), than for Leucaena and Gliricidia prunings (high quality). The results indicate that manipulation of earthworm activity with application of high- and low-quality mulches may improve the synchronization of soil nutrient supply and crop nutrient demand.
Effects of five types of plant residues [Acioa, presently Dactyladenia barteri, Gliricidia sepium, and Leucaena lecocephala prunings, maize (Zea mays) stover and rice (Oryza sativa) straw] applied as mulch on soil organic matter (SOM) content and effective cation exchange capacity (ECEC) were studied on an Alfisol in the humid tropics. Plant residue mulch resulted in a decline in SOM and ECEC during two years of cropping following six years of grass fallow. Rice straw mulch resulted in less and maize stover mulch in a greater decrease of SOM and ECEC than the other mulches. Decrease in SOM and ECEC is attributed to the mulching effect on the soil micro-climate which enhanced the decomposition of SOM accumulated during the grass fallow prior to the initiation of the experiment. In order to maintain SOM for a tropical soil, plant residues with high lignin, polyphenols, and silica will have to be among residue species when applied in continuous cropping systems.
Agricultural soils, having been depicted of much of their native carbon stocks, have a significant CO2 sink capacity. Global estimates of this sink capacity are in the order of 20-30 Pg C over the next 50-100 years. Management practices to build hip soil C must increase the input of organic matter to soil and/or decrease soil organic matter decomposition rates. The most appropriate management practices to increase soil C vary regionally, dependent on both environmental and socioeconomic factors. In temperate regions, key strategies involve increasing cropping frequency and reducing bare fallow, increasing the use of perennial forages (including N-fixing species) in crop rotations, retaining crop residues and reducing or eliminating tillage (i.e. no-till). In North America and Europe, conversion of marginal arable land to permanent perennial vegetation, to protect fragile soils and landscapes and/or reduce agricultural surpluses, provides additional opportunities for C sequestration. In the tropics, increasing C inputs to soil through improving the fertility and productivity of cropland and pastures is essential. In exclusive systems with vegetated fallow periods (e.g. shifting cultivation), planted fallows and cover crops can increase C levels over the cropping cycle. Use of no-till, green manures and agroforestry are other beneficial practices. Overall, improving the productivity and sustainability of existing agricultural lands is crucial to help reduce the rate of new land clearing, from which large amounts of CO2 from biomass and soil are emitted to the atmosphere, Some regional analyses of soil C sequestration and sequestration potential have been performed, mainly for temperate industrialized countries. More are needed, especially for the tropics, to capture region-specific interactions between climate, soil and management resources that are lost in global level assessments. By itself C sequestration in agricultural soils can make only modest contributions (e.g. 3-6% of total fossil C emissions) to mitigating greenhouse gas emissions. However, effective mitigation policies will not be based on any single 'magic bullet' solutions, but rather on many modest reductions which are economically efficient and which confer additional benefits to society. In this context, soil C sequestration is a significant mitigation option. Additional advantages of pursuing strategies to increase soil C are the added benefits of improved soil quality for improving agricultural productivity and sustainability.
Intensification of agriculture in the tropics has resulted from a shortage of farmland and insufficient food production to satisfy the needs of an expanding population. Many tropical farmers are challenged by the prospect of intensifying their production while sustaining or improving the fertility and productivity of soils with only locally available natural resources. The waste products of plant and animal production represent some of the most abundant natural resources available for use by tropical farmers to achieve these goals. The efficient use and management of these resources depends on understanding the role that decomposer biota play in regulating the structure and function of agricultural ecosystems. Furthermore, the development of agricultural management practices which promote the beneficial attributes of these organisms will be essential to sustaining the productivity and environmental integrity of tropical agriculture. Finally, understanding the role of biodiversity among decomposer biota in maintaining the functional properties of tropical agricultural ecosystems is critical to achieving this goal. The objective of this review is to further that understanding by describing the taxonomic and functional diversity of decomposer biota in the tropics and evaluating known links between their diversity and the function of agricultural ecosystems. We further describe the effects of changing land-use and agricultural intensification on the structure and diversity of decomposer communities in the tropics and suggest some priorities for future research
Field litterbag decomposition study, through widely accepted for examining decomposition of plant residues, is time consuming, as it requires several months to complete. An experiment was conducted to explore the possibility to introduce ruminant nylonbag assay which only takes 3-4 days, to plant residue decomposition studies for agricultural purpose. Results showed decomposability of plant residues, measured using the ruminant nylonbag significantly correlated with those using the litterbags, indicating that the ruminant nylonbag assay, widely used for studying herbage breakdown in animal nutrition research, can be used for rapidly estimating the decomposability of plant residues in the field.
A simplified Shaw's wet-combustion technique for carbon (C) determination in plant materials is described which requires less reagent and operation time. The simplified technique gave high recovery (99.4%) of the C in analytical pure grade sucrose and citric acid, and high reproducibility of C measured in 18 different plant materials.
Twelve herbaceous and shrub legume species were grown in pot and field experiments in five sites representing three agroecological zones in moist savanna in Nigeria. The objectives were to: (1) assess natural nodulation of the legumes and characterize their indigenous rhizobia, (2) determine their need for rhizobia inoculation and (3) estimate the amount of N-2 fixed by each of these legumes. At 4 weeks after planting (WAP), Crotolaria verucosa was not nodulated at any of the sites while Centrosema pascuorum had the highest number of nodules in all sites. At 8 WAP, all legumes were nodulated, with Mucuna pruriens having the least number of nodules and Stylosanthes hamata the highest. The number of nodules, however, was inversely correlated to the mass of nodules. Significant differences in nodulation of the legume species grown in the field also occurred between and within sites. Mucuna pruriens and Lablab purpureus produced more shoot and nodule biomass than the other legumes in all sites. Growth of most of these legumes responded to fertilizer application, except for C. verrucosa and Aeschynomene histrix. Except for C. verrucosa, average proportion of N-2 fixed was about 80% and this was reduced by about 20% with N fertilizer application. The majority of rhizobia isolates (60%) were slow growing, belonging to the Bradyrhizobia spp. group. Selected rhizobia isolates evaluated on Cajanus cajan, C. pascuorum, M. pruriens and Psophocarpus palustris varied from ineffective to highly effective in Leonard jar conditions. However, only growth of M. pruriens responded to inoculation in potted soils, whereas it was lower than that obtained with N fertilizer application. This indicated the need to screen more rhizobia in order to improve N-2 fixation and growth of legume species such as M. pruriens when it is introduced in soils deficient in N.
The effects of tropical earthworms (Eudrilus eugeniae) and millipedes (Spirostreptidae) on the breakdown of plant residues [Acioa (presently, Dactyladenia) barteri, Gliricidia sepium and Leucaena leucocephala prunings, maize (Zea mays) stover and rice (Orysa sativa) straw], with contrasting chemical compositions, were studied in the field under humid tropical circumstances. Addition of earthworms significantly increased the breakdown of maize stover. Addition of millipedes significantly increased the breakdown of maize stover and rice straw. Combined addition of earthworms and millipedes generally resulted in greater plant residue breakdown, compared to that of a single group of fauna. During 10 weeks of exposure, earthworms and millipedes, on average, accounted for the breakdown of all plant residues by 10.4 and 28.4%, respectively. Millipedes and earthworms contributed more to the breakdown of plant residues with low quality (high C-to-N ratio, lignin and polyphenol contents) than to the degradation of those with high quality. It is concluded that fauna-enhanced breakdown of plant residues will have different effects on soil nutrient supply, depending on residue quality.
An equation was developed for calculating a plant residue quality index (PRQI) in the (sub-)humid tropics using the C/N ratio and lignin and polyphenol concentration of plant residues. Among 18 plant species tested, there was a large variation in PRQI. The PRQI was correlated with the decomposition rate of plant residues, soil microclimate, soil fauna density and maize crop performance in the field. Soil moisture and termite density increased with decreases in PRQI, whereas decomposition rate constants of plant residues, soil temperature and ant density increased with increase in PRQI. Improvement in crop performance, such as grain yield, by plant residue mulching was lowest in the case of intermediate PRQI. It is concluded that PRQI can be used for selecting plant residues and projecting their agronomic value in the tropics.
Effects of application of prunings of three woody species (Acioa barteri, Gliricidia sepium and Leucaena leucocephala), maize (Zea mays L.) stover and rice (Oryza sativa L.) straw as mulch on maize were studied on an Alfisol in southern Nigeria in 1990 and 1991. Maize dry matter and grain yield were higher with applications of plant residues and N fertilizer in both years. Addition of Leucaena prunings gave the highest maize grain yield in both years. Compared to the 1990 results, Acioa showed the least grain yield decline among the mulch treatments in 1991. Nutrient uptake was enhanced by applications of plant residues. Leucaena prunings had the highest effect in both years and increased the mean N, P, and Mg uptake by 96%, 84%, and 50%, respectively, over the control. Addition of Acioa prunings increased K and Ca uptake by 59% and 92%, respectively, over the control. 'High quality' (low CIN ratio and lignin level) plant residues enhance crop performance through direct nutritional contributions, whereas 'low quality' (high C/N ratio and lignin level) plant residues do so through mulching effects on the microclimate. 'Intermediate quality' plant residues have no clear effects on crop performance.
Effects of application of five types of plant residues [Acioa barteri, Gliricidia sepium and Leucaena leucocephala prunings, maize (Zea mays) stover and rice (Oryza sativa) straw] as mulch on soil fauna were examined under field conditions in the humid tropics in 1990 and 1991. Earthworm mean population over 2 years was higher under any type of plant residues by 41% compared to control. Leucaena prunings supported the highest earthworm population. Mulched plots also showed 177% higher mean termite population over 2 years than control. Highest termite population was observed in plots mulched with Acioa prunings followed by maize stover > rice straw > Leucaena prunings > Gliricidia prunings. The mean ant populations were 36% higher with Leucaena and Gliricidia prunings, and were not affected by Acioa prunings, maize stover and rice straw as compared to control. Millipede populations were not significantly affected by mulching. Earthworm populations were negatively correlated with the ratio of lignin: N of plant residues. Ant populations were significantly related to the N content of plant residues (R2 = 0.87 and 0.84 for 1990 and 1991 respectively). The results imply that chemical plant composition, particularly N and lignin contents, play a critical role in faunal abundance in the soil through their effect on palatability and decomposibility. Indirect microclimatic and mulching effects may also be important.
Decomposition and nutrient release patterns of prunings of three woody agroforestry plant species (Acioa barteri, Gliricidia sepium and Leucaena leucocephala), maize (Zea mays) stover and rice (Oryza sativa) straw, were investigated under field conditions in the humid tropics, using litterbags of three mesh sizes (0.5, 2 and 7 mm) which allowed differential access of soil fauna. The decomposition rate constants ranged from 0.01 to 0.26 week-1, decreasing in the following order; Gliricidia prunings > Leucaena prunings > rice straw > maize stover > Acioa prunings. Negative correlations were observed between decomposition rate constants and C:N ratio (P