B. T. Kang

Biological effects of plant residues with contrasting chemical compositions under humid tropical conditions decomposition and nutrient release.

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 < 0.004), percent lignin (P < 0.014) and polyphenol content (P < 0.053) of plant residues. A positive correlation was observed between decomposition rate constant and mesh-size of litterbag (P < 0.057). These results indicate that both the chemical composition of plant residues and nature of the decomposer played an important role in plant residue decomposition. Nutrient release differed with quality of plant residues and litterbag mesh-size. Total N, P, Ca and Mg contents of plant residues decreased with time for Gliricidia and Leucaena prunings, maize stover, and rice straw, and increased with time for Acioa prunings. There was some indication of N immobilization in maize stover and rice straw; P immobilization in Leucaena prunings and rice straw; and Ca immobilization in maize stover, rice straw and Gliricidia and Leucaena prunings. Acioa prunings immobilized N, P, Ca and Mg. All plant residues released K rapidly. Nutrient release increased with increasing mesh-size of litterbags, suggesting that soil faunal activities enhanced nutrient mobilization.

Alley cropping maize (Zea mays L.) and leucaena (Leucaena leucocephala Lam) in southern Nigeria.

SummaryA maize-leucaena alley cropping system was studied on a N-deficient sandy Apomu soil series (Psammentic Usthorthent) at Ibadan in the forest zone of southern Nigeria from 1976 to 1980. In this system maize was grown in 4 m width alleys between Leucaena hedge rows.Five to six annual prunings of the leucaena hedge rows yielded between 5 to 8 tons of dry tops/ha/year with N-yield of between 180 to 250 kg N/ha/year. Though efficiency of N utilization from leucaena prunings was low, the maize-leucaena alley cropping system can be utilized as a low N-input system. Addition of leucaena prunings from the full grown hedge rows, was able to sustain maize grain yield at about 3.8 tons/ha/year for two consecutive years with no N addition, while with no addition of prunings, yield declined. Higher maize grain yields were obtained with supplementation with low N rates of 20 to 80 kg N/ha depending on variety and season.The maize-leucaena alley cropping system also has the potential for being developed as a stable alternative for the traditional bush fallow system. Leucaena plants which have unique properties suitable for growing in alley cropping, still retain important functions of a traditional bush fallow: for nutrient recycling, source of green manure, firewood, and staking material. Addition of leucaena prunings increased total soil N and organic C level.

Alleu cropping sequentially cropped maize and cowpea with Leucaena on a sandy soil in Southern Nigeria

SummaryThe potential of alley cropping maize and cowpea with the giant Leucaena (Leucaena leucocephala (Lam) de Wit) cultivar K-28 was studied on an Entisol (Psammentic Ustorthent) in Southern Nigeria. In this trial the crops were grown in 4 m wide alleys formed by periodically pruned leucaena hedgerows. The effect of application of leucaena prunings, nitrogen fertilizer and tillage was studied.Despite the very intensive pruning regime (five prunings/year) for a six-year period, the leucaena hedgerows continue to produce substantial amounts of prunings, nitrogen yield and stakes. Application of nitrogen to the maize crop increased dry matter and nitrogen yield from the leucaena prunings. Although high nitrogen yield was obtained from the prunings, the application of low nitrogen rates was still needed for obtaining a high maize yield. Maize grain yield can be sustained at about 2.0 t/ha with continuous application of leucacna prunings only. Without application of leucaena prunings and nitrogen maize yield continued to decline with subsequent croppings. Cowpea grain yield was not affected either by leucaena prunings or by residual nitrogen. Tillage (rototilling) resulted in either higher or the same maize and cowpea yields as compared with no-tillage.Application of leucaena prunings resulted in higher soil moisture retention, organic matter, exchangeable K, Ca, Mg, and also nitrate levels in the soil solution. Leucaena and maize appear to extract soil moisture from different zones in the soil. Timely pruning of hedgerows is necessary to minimize shading.

Biological effects of plant residues with contrasting chemical compositions under humid tropical conditions: Effects on soil fauna

Abstract 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.

Effects of chemical composition on N, Ca, and Mg release during incubation of leaves from selected agroforestry and fallow plant species

Nitrogen, Ca and Mg release from leaves of ten selected plant residues with varying chemical compositions was studied under laboratory conditions. Three patterns of N-release were observed over a seven week incubation period: (a)Gliricidia sepium, Leucaena leucocephala, Mucuna pruriens andCentrosema pubescens leaves showed rapid N release, (b)Acioa barteri andDialium guineense leaves immobilized N, and (c)Alchornea cordifolia, Anthonata macrophylla, Cassia siamea andPterocarpus santalinoides leaves initially showed N immobilization which gradually changes to net mineralization after about four weeks of incubation. Nitrogen mineralization rate constant (k) ranged from −0.0018 (A. barteri) to 0.0064 day−1 (G. sepium). Statistical analysis of data showed that N mineralization rate constants are significantly correlated with initial N, polyphenol and lignin contents of leaves. Nitrogen release increased with increasing N content and decreased with increasing contents of polyphenols and lignin.Addition of leaves from all species significantly increased soil exchangeable Ca and Mg levels.L. leucocephala, G. sepium, C. pubescens andM. pruriens showed relatively high Ca and Mg release rates. Calcium release rate was related to N release rate rather than to initial Ca content.

An index for assessing the quality of plant residues and evaluating their effects on soil and crop in the (sub-) humid tropics

Abstract 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.

Mulching effect of plant residues with chemically contrasting compositions on maize growth and nutrients accumulation

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 C/N 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.

Breakdown of plant residues with contrasting chemical compositions under humid tropical conditions: Effects of earthworms and millipedes

Abstract 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 ( Oryza sativa ) straw], with contrating 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.

Effect of heating on properties of some soils from southern Nigeria and growth of rice.

SummaryThe effect of heating on the properties of Apomu (Psammentic Usthorthent), Egbeda (Oxic Paleustalf) and Gambari (Typic Plinthustalf) surface soils were studied under laboratory conditions. Heating at low temperatures (≤100°C) have no detrimental effects on soil properties, on the contrary it increased the soil extractable P, Mg, Fe, Mn and Zn levels. Pronounced reductions in total N, Org. C, Org. P and extractable Ca and Mg levels and marked increases in extractable P, Zn, Mn and Fe were observed by heating to 200°C. Heating to ≥500° had an adverse effect on soil chemical and physical properties.Plant height and dry matter yeild of rice plants were higher when grown on Egbeda soil previously heated to 100°C. With addition of N, P and K there was no observed beneficial effect of the heating treatment. Rice plants grown on Egbeda soil previously heated to 200°C showed high uptake of Mn. Plants grew badly in soil previously heated to 500°C.

Effect of mulch quality on earthworm activity and nutrient supply in the humid tropics.

Abstract 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.