Nitrogen fertilizer in leucaena alley cropping. I. Maize response to nitrogen fertilizer and fate of fertilizer-15N

Field microplot experiments were conducted in the semi-arid tropics of northern Australia to evaluate the response of maize (Zea mays L.) growth to addition of N fertilizer and plant residues and to examine the fate of fertilizer¹⁵N in a leucaena (Leucaena leucocephala) alley cropping system, in which supplemental irrigation was used. Leucaena prunings, maize residues and N fertilizer were applied to alley-cropped maize grown in microplots which were installed in the alleys formed by leucaena hedgerows spaced 4.5 metres apart. The¹⁵N-labelled fertilizer was used to examine the fate of fertilizer N applied in the presence of mulched leucaena prunings and maize residues.Application of leucaena prunings increased maize yield while addition of N fertilizer in the presence of the prunings produced a further increase in maize production. There was a significant positive interaction between N fertilizer and leucaena prunings in increasing maize production. The addition of maize residues in the presence of N fertilizer and leucaena prunings decreased maize yield and N uptake and increased fertilizer¹⁵N loss from 38% to 47%. Maize recovered 24–79% of fertilizer¹⁵N in one cropping season, depending on application rate of N fertilizer and field management of plant residues. About 20–34% of fertilizer¹⁵N remained in the soil. More than 37% of fertilizer¹⁵N was apparently lost from the soil and plant system largely through denitrification when N fertilizer was applied at 40 kg N ha^–1 or more in the presence or absence of plant residues. Application of N fertilizer improved maize yield and increased the contribution of mulched leucaena prunings to crop production in the alley cropping system.     

Nitrogen use efficiency and N-competition betweenLeucaena hedgerows and maize in an alley cropping system

A two-year field experiment was undertaken on an Alfisol in the forest-savanna transition zone of southwestern Nigeria, to study the effect of alley cropping, root barrier, application of N fertilizer andLeucaena leucocephala (Lam.) de Wit. prunings, on N utilization by maize andLeucaena hedgerows. The trial was conducted using a split-plot design with three replications. Mainplot treatments were alley cropping withLeucaena leucocephala planted at 4m interhedgerow spacing and control (no hedgerows). Four subplot treatments were with and without addition of hedgerow prunings, and with and without presence of root barriers. There were no significant effects of presence ofLeucaena hedgerows and root barriers on maize grain yield.Leucaena hedgerows recovered about 7% of the 30 kgN/ha applied as (¹⁵NH₄)₂SO₄ to maize during one year. Application of hedgerow prunings increased maize yield by 82% and N-uptake in the grain by 50% over the treatment without prunings. Recoveries of¹⁵N-labelled prunings and fertilizer N by maize plants were about 10 and 16% respectively. Prunings addition increased¹⁵N-fertilizer recovery in main season maize and the residual value during the minor season by about 36% over the treatment without prunings. Pruning N-use efficiency was higher for plants grown adjacent to the hedgerows than in the middle of alleys during main season and the reverse was observed during the minor season. Absence of a root barrier increased N-use from prunings. Large amounts of applied N in the system were unaccounted for. Results of the trial showed, that without root barrier there was no measurable below-ground N competition betweenLeucaena hedgerows and maize.     

Longterm alley cropping with four hedgerow species on an Alfisol in southwestern Nigeria – effect on crop performance, soil chemical properties and nematode population

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.     

Hedgerow species and environmental conditions effects on soil total C and N and C and N mineralization patterns of soils amended with their prunings

An important criterium in selecting species for alley cropping is themineralization pattern of their prunings. This study determined effects of 5years of hedgerow pruning applications on soil organic C and total N at threelocations in Haiti and mineralization patterns from soil amended with theprunings during an incubation using micro-lysimeters. Soils (0–5cm) under 5 hedgerows were collected at each site and analyzed fororganic C and total N. In the laboratory, ground leaves and stems (<1cm diameter) of the hedgerow species were mixed with soil at ratesof 3 and 1.5 Mg ha^–1, respectively, andaerobically incubated in the dark at 25 °C. A non-amended soilwas used as control. Soils were leached to determine mineral N at 1, 3, 7, 14,28, 42, 84 and 120 days of incubation. Evolved CO₂ was measuredfollowing each leaching procedure. At the calcareous site, application ofprunings from Leucaena leucocephala (Lam.) De Wit andDelonix regia (Boj. ex Hook. Raf.) resulted in 23 and 13%higher soil N than the control, respectively, after 5 years. There were nodifferences in total N at the other sites but soil N was highest underLeucaena hybrid and Acaciaangustissima (Mill.) Kuntze, respectively at the basaltic and highelevation sites. Soils under D. regia (calcareous) andA. angustissima and Leucaena hybrid(high elevation) had higher organic C than the respective controls. Carbon andNmineralization and C turnover were highest when soils were amended with leavesof Leucaena diversifolia (Schlecht.) Benth (calcareous andbasaltic soils) and A. angustissima (high elevation) andlowest in non-amended control soils. Stem-amended soils showed differences in Cmineralization for calcareous and high elevation soils whereas N mineralizationwas similar among treatments within sites. Carbon and N mineralization (highelevation soil) correlated positively with N concentrations of leaf prunings.Amendments with leaf prunings increased soil C and N mineralization andturnoverrates, suggesting greater nutrient availability for the crop during a shortperiod than in non-amended control soils.     

Patterns of decomposition and nutrient release by fresh Gliricidia (Gliricidia sepium) leaves in an ultisol

Legume leaves used as green manure are a potential alternative to the use of commercial N fertilizers for non-legume crop production. Thus it is important to understand the rate of legume leaf decomposition and its release of nutrients. This paper reports the results of a litter bag experiment using fresh Gliricidia sepium leaves under field conditions. The leaves were confined in litterbags (5.3 mm mesh) and buried 10 cm in the soil. Dry matter, C, N, P, K, Ca, and Mg remaining after decomposition was determined at 0, 5, 10, 20, 30, 40, 50, 60 and 70 days. A rapid initial phase followed by a much slower one was identified in the decay and nutrient release patterns. The duration of the former ranged from 21 to nearly 30 days for dry matter and nutrients. Potassium and Ca were the most rapidly released nutrients, with the early phase lasting 28 and 6 days respectively. Nitrogen and P showed similar patterns of release and initial phase duration (21 and 22 days). Their rate constants were also 10 and 8 times that of their respective slow phases. C:N and C:P ratios increased initially and then decreased in the subsequent phase of decomposition. Magnesium also gave an identical pattern and rate of release as N in the early phase. No influence of rainfall was observed on the parameters studied.     

Green leaf manuring with prunings of Leucaena leucocephala for nitrogen economy and improved productivity of maize (Zea mays)–wheat (Triticum aestivum) cropping system

Green leaf manuring with prunings of Leucaena leucocephala is regarded as a useful source of N to plants but the actual substitution of N fertilizer, release and recovery of N as well as effects on soil fertility are not adequately studied. The present studies investigated the effect of sole and combined use of Leucaena prunings and urea N fertilizer in different proportions on productivity, profitability, N uptake and balance in maize (Zea mays)–wheat (Triticum aestivum) cropping system at New Delhi during 2002–2003 and 2003–2004. Varying quantities of Leucaena green leaf biomass containing 3.83–4.25% N (18.2–20.5 C:N ratio) were applied to provide 0, 25, 50, 75 and 100% of recommended N (120 kg ha⁻¹) to both maize and wheat before sowing. In general, direct application of urea N increased the productivity of both crops more than Leucaena green leaf manure, but the reverse was true for the residual effect of these sources. The productivity of maize increased progressively with increasing proportions of N through urea fertilizer and was 2.41–2.52 t ha⁻¹ with 60 kg N ha⁻¹ each applied through Leucaena and urea, which was at par with that obtained with 120 kg N ha⁻¹ through urea alone (2.56–2.74 t ha⁻¹). Similarly, wheat yield was also near maximum (4.46–5.11 t ha⁻¹) when equal amounts of N were substituted through Leucaena and urea. Residual effects were obtained on the following crops and were significant when greater quantity of N (>50%) was substituted through Leucaena. Nitrogen uptake and recovery were also maximum with urea N alone, and N recovery was higher in maize (33.4–42.1%) than in wheat (27.3–29.8%). However, recovery of residual N in the following crop was more from Leucaena N alone (8.5–10.3%) than from urea fertilizer (1.7–3.8%). Residual soil fertility in terms of organic C and KMnO₄ oxidizable N showed improvement with addition of Leucaena prunings, which led to a positive N balance at the end of second cropping cycle. Net returns were considerably higher with wheat than with maize, and were comparatively lower with greater proportion of Leucaena because of its higher cost. Nonetheless, it was beneficial to apply Leucaena and urea on equal N basis for higher productivity and sustainability of this cereal-based cropping system.     

Effects of cropping history and phosphorus source on yield and nitrogen fixation in sole and intercropped cowpea–maize systems

Symbiotic N₂-fixation, N uptake efficiency, biomass- and crop production of cowpea and maize as affected by P source, sole- and intercropped, and introduction of break crops were studied on a farmer’s fields in semi-arid Tanzania. Cowpea fixed around 60% of its N from the atmosphere amounting to 70 kg N ha⁻¹ under sole and 36 kg N ha⁻¹ under intercropping as estimated by the ¹⁵N isotope dilution method around peak biomass production. The amount of N₂-fixed was 30–40% higher when P was applied as either TSP or MRP whereas cowpea yield were unaffected. Intercropped maize with 19,000 plant ha⁻¹ accumulated the same amount of N as 38,000 sole cropped maize plants although intercropping reduced the dry matter accumulation by 25%. The N uptake efficiency of the applied ¹⁵N labelled fertiliser was 26%, which equal a total pool of early available plant N of 158 kg N ha⁻¹. Under the N deficient conditions, P application did not increase the grain yield of maize. The LER indicate that sole cropping required 18% more area than intercropping in order to produce the same grain yield, and 35% more land when LER was based on N uptakes. Introduction of break crops in the maize systems, more than doubled accumulation of dry matter and N in the grain compared to continuous maize cropping. During maturation sole crop cowpea shedded leaves containing 41 kg N ha⁻¹. The current findings underline the importance of crop diversity in Sub Saharan Africa agriculture and emphasise the need for including all residues, including shedded leaves, in nutrient balance studies.     

Nitrogen contributions from decomposing cover crop residues to maize in a tropical derived savanna

In cover cropping systems in the tropics with herbaceous legumes, plant residues are expected to supply nitrogen (N) to non-legume crops during decomposition. Field experiments were carried out to (i) determine the effects of residue quality on decomposition and N release patterns of selected plants in cover cropping systems, (ii) relate the pattern of residue N release to N uptake by maize in cover cropping systems. To study decomposition, litter bags were used and monitored over two maize growing seasons. The residues studied were mucuna (Mucuna pruriens (L.) DC. var. utilis (Wright) Bruck), lablab (Lablab purpureus (L.) Sweet), and leaves and rhizomes of imperata (Imperata cylindrica (L.) Raueschel). Mucuna and lablab decomposed rapidly losing more than 60% of their dry weight within 28 days. In contrast, imperata decomposed slowly with only 25% of its dry matter lost in 56 days. At 28 days, mucuna had released 154 kg N ha^-1 in in-situ mulch systems and 87 kg N ha^-1 in live- mulch systems representing more than 50% of its N. More than 64% of N in lablab was released within 28 days amounting to 21 to 174 kg N ha^-1. Imperata rhizomes mineralized 4 to 14 kg N ha^-1 within 14 days, and subsequently immobilized N until 112 days whereas imperata leaves immobilized N throughout the study period. Decomposition and N release rates from the plant residues were most strongly correlated with the (lignin+polyphenol)/N ratio, N content, lignin/N ratio, polyphenol/N ratio, C/N ratio and lignin content of the residues. Relative to the controls, herbaceous legume residues increased maize dry matter yield and N uptake during the two cropping seasons. At 84 days, the maize crop had utilized 13 to 63 kg N ha^-1from mucuna representing 13 to 36% of N released, whereas 16 to 25% of N released from mucuna was recovered by the maize crop at 168 days. The first maize crop recovered 9 to 62 kg N ha^-1 or 28 to 35% of N released from lablab. However, at 168 days, N uptake by maize in antecedent live-mulched lablab was 32% higher than the quantity of N released, whereas imperata residues generally, resulted in net reduction of maize N uptake.     

Nutrient release dynamics from decomposing organic materials and their mulching-effect on pearl millet yields in a low-input Sahelian cropping system

Organic material inputs for increased crop yields are insufficient in the Sahelian West Africa. There is a need for diversifying organic amendment sources for improved nutrient supply in low-input cropping system. The 2-year study aimed to (1) explore the rates of mass losses and nutrient release dynamics from Acacia tumida prunings (AT) and millet straw (MS) under field conditions, (2) assess termite’s contribution to the decomposition of AT and MS, and (3) ascertain the mulching-effect of these organic materials on pearl millet yields. The study was conducted in Niger using field experiment and litterbag methodology and the data modelled using single exponential decay equations. Under field conditions, mulching with AT and MS increased millet grain yield by 35 and 33%, respectively compared to control. The harvest index (HI) in 2014 increased by 21% compared to that obtained in 2013 with the highest HI being recorded for the AT mulched treatment. The results from litterbag experiment indicated a greater dry mass losses from MS decomposition in 2013 whereas relatively higher mass losses were recorded from AT decomposition in 2014. The differences in mass losses among the organic materials could be related to the interaction of soil moisture dynamics and termites’ population which are positively correlated with mass losses. The contribution of termites to the decomposition was estimated to be 36% for MS and 8% for AT. In 2013, at 126 days after litterbags placement, the amounts of N, P, and K released from MS were 16, 1, and 25 kg ha^?1 of initial nutrient applied, respectively compared with the 22, 1, and 23 kg ha^?1 recorded from AT treatment. During the same period in 2014, the total amounts of N, P and K released from MS were 15, 0.6, and 29 kg ha^?1, respectively compared to the 32 kg ha^?1 of N, 1 kg ha^?1 of P, and 29 kg ha^?1 of K released from the AT treatment. The intrinsic organic material quality could explain markedly the variation in nutrient released among the organic material. These results indicate that Acacia tumida prunings have a potential to provide nutrient through mineralization for enhanced crop yield in the Sahel.     

Leucaena (Leucaena leucocephala (Lam) de Wit) prunings as nitrogen source for maize (Zea mays L.)

The effectiveness ofLeucaena leucocephala (Lam.) de Wit, prunings as N source for maize (Zea mays L.) was evaluated in field and pot trials at Ibadan, southern Nigeria. An N deficient, sandy Apomu soil (Psammentic Usthorthent) was used. The prunings significantly increased N uptake of seedlings and N percentage in ear leaves of maize. High maize gain yield was obtained with application of 10 tons fresh prunings or a combination of 5 tons fresh prunings and N at 50 kg ha^–1. The prunings as N source, appeared to be more effective when incorporated in the soil than when applied as mulch. In the pot trial, prunings applied two weeks before planting was more effective than when applied at time of planting maize. Under screen house conditions, the apparent N recovery from prunings with early incorporation about equals that of fertilizer N.     

Nitrogen rhizodeposition from soybean (<Emphasis Type="Italic">Glycine max</Emphasis>) and its impact on nutrient budgets in two contrasting environments of the Guinean savannah zone of Nigeria

Nitrogen (N) rhizodeposition by grain legumes such as soybean is potentially a large but neglected source of N in cropping systems of Sub-Saharan Africa. Field studies were conducted to measure soybean N rhizodeposition in two environments of the Guinean savannah of Nigeria using ¹⁵N leaf labelling techniques. The first site was located in Ibadan in the humid derived savannah. The second site was in Zaria in the drier Northern Guinean savannah. Soybean N rhizodeposition in the top 0.30 m of soil varied from 7.5 kg ha⁻¹ on a diseased crop in Ibadan to 33 kg ha⁻¹ in Zaria. More than two-thirds of soybean belowground N was contained in the rhizodeposits at crop physiological maturity, while the rest was found in the recoverable roots. Belowground plant-derived N was found to constitute 16–23% of the total soybean N. Taking rhizodeposited pools into account led to N budgets close to zero when all residues were removed. If residues were left in the field or recycled as manure after being fed to steers, soybean cultivation led to positive N budgets of up to +95 kg N ha⁻¹. The role and potential of grain legumes as N purveyors have been underestimated in the past by neglecting the N contained in their rhizodeposits.     

Soil and leaf nutrient interactions following application of calcium silicate slag to sugarcane

In certain areas of the Everglades Agricultural Area, plant and ratoon sugarcane (Saccharum L.) yields are increased by application of Si from calcium silicate slag. The greatest yield responses are obtained in the plant crop the first year after application of slag and when plant uptake of Si is increased. Magnesium deficiencies have been reported after slag application. The objective of this study was to quantify interactions of soil and leaf nutrients on sugarcane grown on a Terra Ceia muck (Euic, hyperthermic Typic Medisaprist) that had previously received calcium silicate slag. Slag was applied at five rates, and yields were evaluated from plant, first-ratoon, and second-ratoon (stubble) crops at two locations. Soil and leaf from each crop were sampled for nutrient analysis and the results were used to interpret the yield data. Although slag increased cane yield by as much as 39% and sugar yield by 50%, for each 100 mg L^–1 drop in extractable soil Mg, cane yields declined by 5.3 Mg ha^–1 and sugar yields by 0.9 Mg ha^–1. At leaf Si concentrations exceeding 10 g kg^–1, optimum cane and sugar yields were observed, while leaf Mg concentrations approached critical leaf concentrations below 1.5 g kg^–1. Estimates of total leaf nutrient uptake during each crop indicated that uptake of Mg did not meet nutrient demands at high biomass production. Nutrient antagonism between Si and Mg is suggested. Low soil Mg may contribute to the marked crop responses to slag and for the decline in stubble production. Application of a magnesium fertilizer may be necessary to maintain high nutrient availability.Contribution from the University of Florida, Institute of Food and Agricultural Sciences. Florida Agric. Exp. Stn., Journal Series no. R-00859.     

Moisture conservation and nitrogen recycling through legume mulching in rainfed maize (<Emphasis Type="Italic">Zea mays</Emphasis>)–wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis>) cropping system

Mulching with vegetative materials is a highly beneficial and widely-investigated agro-technique in rainfed areas but the adoption of this practice has been constrained due to non-availability of mulch biomass locally. Live mulching with fast-growing annual green manure legumes like sunnhemp (Crotalaria juncea) or prunings of Leucaena leucocephala grown as hedge rows can be done for moisture conservation as well as nutrient cycling in the maize–wheat cropping system, which is predominantly followed in the high rainfall sub-mountainous region of north-western India. A field experiment was conducted at Selakui, Dehradun during 2000–2004 to study the effect of legume mulching, viz. in situ grown sunnhemp and Leucaena prunings, along with varying N levels, viz. 0, 30, 60 and 90 kg N ha⁻¹ (to maize), and 0, 40 and 80 kg N ha⁻¹ (to wheat) on productivity, soil moisture conservation and soil physico-chemical properties. Intercropped sunnhemp added 0.75–1.45 t dry matter and 21.6–41.3 kg N ha⁻¹ at 30–35 days, while Leucaena twigs added 1.89–4.15 t dry matter and 75.2–161.3 kg N ha⁻¹ at 60–65 days of maize growth. Live mulching with sunnhemp or Leucaena biomass improved soil moisture content at maize harvest (+1.15–1.57%) and crop productivity by 6.8–8.8% over no mulching. Combined use of both the mulching materials was more effective in improving the soil moisture content (+2.08–2.29%) and grain yield (15.1%) over their single application. Response of maize to N fertilizer application was significant up to 90 kg N ha⁻¹, and it was relatively more pronounced under the mulching treatments. Residual effect of mulching on wheat showed an increase in yield of 10.2% with sunnhemp or Leucaena, and 27.9% with sunnhemp + Leucaena. There was an improvement in organic C and total N status of soil, and a decrease in bulk density associated with an increase in infiltration rate due to mulching at the end of 4 cropping cycles. It was concluded that legume mulching is a highly beneficial practice for enhanced moisture and nutrient conservation, leading to increased productivity and soil health of maize–wheat cropping system under Doon valley conditions of north-western India.     

Leaf and root litter of a legume tree as nitrogen sources for cacaos with different root colonisation by arbuscular mycorrhizae

Traditionally cacao (Theobroma cacao L.) is cultivated under legume shade trees, which produce N-rich litter that improves soil organic matter content, microbial activity, and recycles N to the crop. Arbuscular mycorrhiza forming fungi (AMF) are known to play an important role in plant nutrient uptake, yet their role in plant N uptake from organic residues in tropical agroforestry systems is not clear. We studied root and leaf litter of the legume shade tree Inga edulis Mart. as a source of N for cacao and the importance of AMF colonisation in the uptake of litter N under controlled conditions. Leaf and root litter of I. edulis enriched with ¹⁵N was added to cacao pots filled with field soil. Half of the cacao saplings were AMF-inoculated and the soil of non-inoculated saplings was treated with fungicide to suppress AMF. During the 10-week experiment, young cacao leaves were sampled for ¹⁵N analyses and at the end of the experiment whole plants were harvested. Microbial populations in the soil were determined using phospholipid fatty acid (PLFA) analysis, and AMF structures in the roots were quantified. Fungicide treatment decreased AMF structures in roots and increased bacterial populations, but did not affect the decomposition rate of either litter type. Inoculated and non-inoculated cacao saplings used 2.6 and 2.1%, respectively, of N added to the pots in leaf litter and 12.1 and 7.1% of N available in root litter indicating that root litter of I. edulis may be a more efficient N source than leaf litter for cacao. Although the fungicide treatment did not completely suppress AMF in non-inoculated pots, it created sufficient contrast in root AMF colonisation for concluding that AMF significantly enhanced cacao N use from both litter types. The role of root litter of shade trees as a N source in agroforestry should not be neglected.     

Recovery of N applied as 15N-manure or 15N-gliricidia biomass by maize,cotton and cowpea

Proper management of N applied in fertilizers is important to optimize crop production and to avoid negative environmental impacts. The best way to study N dynamics in the soil plant system is to use fertilizers labeled with ¹⁵N. Recoveries of nitrogen following fertilization with ¹⁵N-labeled goat (Capra hircus L.) manure and gliricidia (Gliricidia sepium Jacq. Walp) biomass were evaluated in a greenhouse experiment with three successive planting cycles of three crops: maize (Zea mays L.), cotton (Gossypium hirsutum L.), and cowpea (Vigna unguiculata (L.) Walp.). Each 1 kg soil pot received 8 g (equivalent to 20 Mg ha^?1) of either manure (12.3 mg g^?1 of N) or gliricidia (37.8 mg g^?1 of N). Plants were harvested 50 days after germination and real (¹⁵N) and apparent recoveries of the applied N were determined. Biomass and N amounts in the cotton and maize crops in all three cycles were higher with gliricidia application than with manure, except for cotton in the first cycle. The biomass of cowpea was also higher with gliricídia in the first and second cycles but the amount of N was significantly higher only in the second cycle. In the first cycle, the largest recoveries of ¹⁵N were obtained with gliricidia, for all three crops, but in the second and third cycles recoveries were greater with manure, so that the real recoveries of gliricidia and manure were similar (cotton, 35 and 37 %; maize, 27 and 26 %; and cowpea, 41 and 38 % of the applied N, respectively). Estimates of apparent recoveries were different from the real ones and therefore inadequate for cotton and cowpea. The fast release of N from gliricidia prunings and, on the other hand, the strong residual effect of goat manure-N to subsequent cropping cycles should be considered by farmers in their fertilization strategies.     

Pruning management effects on soil carbon and nitrogen in contour-hedgerow cropping with Leucaena leucocephala (Lam.) De Wit on sloping land in Haiti

Application of hedgerow prunings to the soil in alley cropping ispracticed to sustain crop yield but information is lacking on effects ofhedgerow management on soil C and N dynamics under continuous cropping.Cumulative effects of 3.5 years of leucaena [Leucaenaleucocephala (Lam.) De Wit] hedgerow management on soil organic CandN and potential C and N mineralization were determined in an alley croppingexperiment in Haiti. Treatments were combinations of pruning uses and pruningregimes in a 3 × 3 factorial with a no-tree control (rock walls) in arandomized complete block design with 3 replicates. A hedgerow + fertilizertreatment in an adjacent trial was included for comparison. Soil samplescollected at 0–5, 5–10 and 10–20 cm depths weresealed in mason jars and incubated at 25 °C for 30 days.Organic C and N and inorganic N were measured before the incubation. InorganicNand respired CO₂-C were measured after the incubation. After 3.5years, surface soil samples with prunings applied as mulch or incorporated atplanting had, respectively, 20 and 16% higher organic C, 34 and 18% higherorganic N concentrations, higher potential C and N mineralization and higherrelative N mineralization than with prunings removed. Soil C and N dynamicsweresimilar between pruning application methods. The two-cut regime with a longerregrowth period after cutting (0–40 DAP) had highest organic C in the0–20 cm soil layer whereas two-cut with a shorter regrowthperiod (0–30 DAP) had highest C turnover. Within soil layers, pruningapplication had higher organic C and N and potential C and N mineralizationthanthe no-tree control and alley plots with prunings removed in the 0–5cm layer whereas the latter treatments had similar C and Ndynamicsat all depths. Addition of N-P-K fertilizer in presence of fresh prunings didnot increase soil organic C and N but enhanced N dynamics in the surface soil.     

Phosphorus,Potassium and Leucaena Pruning Interactions on Corn in Haiti

Few P and K fertility studies have been conducted on Haitian soils. Alley cropping is promoted in Haiti but has not been studied in relation to P and K fertility. The effects of P and K fertilizer and leucaena (Leucaena leucocephala (Lam) De Witt) pruning mulch applications on corn (Zea mays L.) were assessed in field experiments on Rhodudalfs over limestone at Salagnac, Lithic Hapludolls over limestone at Bergeau and Typic Hapludalfs over basalt at St. Georges, Haiti. Complete block designs arranged in incomplete factorials of P and K rates were carried out during three seasons. Additional treatments consisted of 30 Mg ha⁻¹ of leucaena prunings applied with and without 60 kg P and 40 kg K ha⁻¹. Grain yield and net income were analyzed and a surface response curve was fitted on treatments without pruning applications. The soils differed markedly in response to P and K with significant PXK interactions. Without leucaena prunings, response surface analysis predicted maximum yields with applications of 95–117 kg P ha⁻¹ and 51–72 kg K ha⁻¹ and maximum net returns with applications of 0.22–89 kg P ha⁻¹ and 43–79 kg K ha⁻¹. Leucaena pruning applications increased yield at all sites. Highest net income was obtained at Salagnac and St. Georges with leucaena prunings applied with P and K, at Bergeau with prunings applied without P and K, suggesting that alley cropping with judicial fertilizer use is a good economic alternative for low resource farmers. Soil parent material and mineralogy should be considered in making fertilizer recommendations.     

Effects of N management on growth, N2 fixation and yield of soybean

Soybean (Glycine max) is one of the most importantfood and cash crops in China. Although soybean has the capacity to obtain alarge proportion of its N from N₂ fixation, it is commonfarmer's practice to apply an N top dressing to maximize grain yield. Afield experiment was conducted to study the effects of N application as urea atvarious stages during the vegetative and reproductive phases on crop biomass,N₂ fixation and yield of two soybean genotypes, Luyuebao and Jufeng.Starter N at 25 kg ha^–1 resulted in minimumbiomass and pod yield while starter N at 75 kgha^–1 had no significant effect and N top dressing, ateither the R1 or R5 stage, resulted in increased biomass and pod yield. Maximumbiomass and pod yield were obtained when a top dressing of 50 kgha^–1 was applied at the flowering stage. The effects oftop dressing on the capacity to fix N₂ were complex. The proportionof plant N derived from N₂ fixation (Pfix) was highest when onlystarter N at 25 kg ha^–1 was applied. Any topdressing reduced nodulation and Pfix, but increased biomass, so that totalN₂ fixed increased for top dressing at the flowering or pod fillingstage. Common farmer's practice of applying 75 kg Nha^–1 at the V4 stage, resulted in a significantreduction in N₂ fixation. To evaluate the application of Nfertilization at various stages ofdevelopment on growth, nodulation and N₂ fixation in more detail, anexperiment in nutrient solution with or without 20 mMNO₃ ^– was conducted with genotype Tidar. The N-freetreatment gave the lowest biomass and total N accumulation, as in the fieldexperiment. A continuous nitrate supply resulted in the highest biomass,associated with an increase in total leaf area per plant, maximum individualleaf area, branch and node number per plant, shoot/root ratio and leaf arearatio, compared to the N-free treatment. R1 was the most responsive stage fornitrate supply as well as for interruption of the nitrate supply. Since theresults from the field experiment were in agreement with thosefrom the experiment in nutrient solution in a greenhouse, the latter can beusedto predict crop performance in the field.     

Nitrogen balances of smallholder farms in major cropping systems in a peri-urban area of Beijing,China

An in-depth understanding of nutrient management variability on the regional scale is urgently required due to rapid changes in cropping patterns and farmers’ resource use in peri-urban areas of China. The soil surface nitrogen (N) balances of cereal, orchard and vegetable systems were studied over a 2-year period on smallholder fields in a representative peri-urban area of Beijing. Positive soil surface N balances were obtained across all three cropping systems. The mean annual N surplus of the vegetable system was 1,575 kg N ha⁻¹ year⁻¹, or approximately 3 times the corresponding values in the cereal (531 kg N ha⁻¹ year⁻¹) and orchard systems (519 kg N ha⁻¹ year⁻¹). In the vegetable system, animal manure (1,443 kg N ha⁻¹ year⁻¹ on average) was the major source of N input (65 % of the total N input) and the factor with strongest impact on the N surplus. In the cereal system, however, about 74 % of the total N input originated from mineral fertilizer application which was the major contributor to the N surplus, while in the orchard system, the N surplus was strongly and positively correlated with both mineral fertilizer and animal manure applications. Furthermore, within each cropping system, N fertilization, crop yields and N balances showed large variations among different smallholder fields, especially in orchard and vegetable systems. This study highlights that differences in farming practices within or among cropping systems should be taken into account when calculating nutrient balances and designing strategies of integrated nutrient management on a regional scale.     

Effects of conservation tillage,crop residue and cropping systems on changes in soil organic matter and maize–legume production: a case study in Teso District

The effects of conservation tillage, crop residue and cropping systems on the changes in soil organic matter (SOM) and overall maize–legume production were investigated in western Kenya. The experiment was a split-split plot design with three replicates with crop residue management as main plots, cropping systems as sub-plots and nutrient levels as sub-sub plots. Nitrogen was applied in each treatment at two rates (0 and 60 kg N ha⁻¹). Phosphorus was applied at 60 kg P/ha in all plots except two intercropped plots. Inorganic fertilizer (N and P) showed significant effects on yields with plots receiving 60 kg P ha⁻¹ + 60 kg N ha⁻¹ giving higher yields of 5.23 t ha⁻¹ compared to control plots whose yields were as low as 1.8 t ha⁻¹ during the third season. Crop residues had an additive effect on crop production, soil organic carbon and soil total nitrogen. Crop rotation gave higher yields hence an attractive option to farmers. Long-term studies are needed to show the effects of crop residue, cropping systems and nutrient input on sustainability of SOM and crop productivity.