Nitrogen fertilizer in leucaena alley cropping. II. residual value of nitrogen fertilizer and leucaena residues

Legume residues have been credited with supplying mineral nitrogen (N) to the associated cereal crop and improving soil fertility in the long term. Few studies using¹⁵N have reported the fate of legume N and fertilizer N in the presence of legume residues in soil-plant systems over periods of two years or longer. A field experiment was conducted in microplots to evaluate: (1) the residual value of the¹⁵N added in leucaena residues; (2) the residual value of fertilizer¹⁵N applied in the presence of unlabelled leucaena residues in the first year to maize over three subsequent years; and (3) the long-term fate of residual fertilizer and leucaena¹⁵N in a leucaena alley cropping system.There was a significant increase in maize production over three subsequent years after addition of leucaena residues. The residual effect of fertilizer N increased maize yield in the second year when N fertilizer was applied at 36 kg N ha^–1 in the first year in the presence of leucaena residues. Of the leucaena¹⁵N applied in the first year, the second, third and fourth maize crop recovered 2.6%, 1.8% and 1.4%, respectively. The corresponding values for the residual fertilizer¹⁵N were 0.7%, 0.4% and 0.3%. About 12–14% of the fertilizer¹⁵N added in the first year was found in the 200 cm soil profile over the following three years. This differed from the 38–41% of leucaena¹⁵N detected in the soil over the same period. Most of the residual fertilizer and leucaena¹⁵N in the soil was immobilized in the top 25 cm with less than 1% leached below 100 cm. More than 36% of the leucaena¹⁵N and fertilizer¹⁵N added in the first year was apparently lost from the soil-plant system in the first two years. No further loss of the residual leucaena and fertilizer¹⁵N was detected after two years.     

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.     

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.     

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.     

Recovery of 15N-labelled fertilizer applied to winter wheat and perennial ryegrass crops and residual 15N recovery by succeeding wheat crops under different crop residue management practices

The recovery of ¹⁵N-labelled fertilizer applied to a winter wheat (120 kg N ha^–1) and also a perennial ryegrass (60 kg N ha^–1) crop grown for seed for 1 year in the Canterbury region of New Zealand in the 1993/94 season was studied in the field. After harvests, ryegrass and wheat residues were subjected to four different residue management practices (i.e. ploughed, rotary hoed, mulched and burned) and three subsequent wheat crops were grown, the first succeeding wheat crop sown in 1994/95 to examine the effects of different crop residue management practices on the residual ¹⁵N recovery by succeeding wheat crops. Total ¹⁵N recoveries by the winter wheat and ryegrass (seed, roots and tops) were 52% and 41%, respectively. Corresponding losses of ¹⁵N from the crop-soil systems represented by un-recovered ¹⁵N in crop and soil were 12% and 35%, respectively. These losses were attributed to leaching and denitrification. The proportions of ¹⁵N retained in the soil (0-400 mm depth) at the time of harvest of winter wheat and ryegrass were 36% and 24%, respectively. Although the soil functioned as a substantial sink for fertilizer N, the recovery of this residual fertilizer by subsequent three winter wheat crops was low (1-5%) and this was not affected by different crop residue management practices.     

Uptake and balance of labelled fertilizer nitrogen by potatoes

Potatoes have a shallow rooting system. This can seriously affect the efficient use of fertilizer N. During two consecutive years, 1985 and 1986, a study was conducted on a commercial field to investigate the uptake of labelled N by potatoes under the recommended N rate and existing agricultural practices. The fertilizer efficiency, fertilizer distribution within the plant and soil and the total fertilizer balance were made using¹⁵NH₄ ¹⁵NO₃ 3.63 At. %¹⁵N excess. The recovery of the applied N-fertilizer in the whole plant was 25 and 56% for 1985 and 1986, respectively. The % Ndff and % Ndfs ranged between 30–40% and 60–70% respectively in both years. An important amount of fertilizer N was left in the soil after harvest. It reached 44 and 34% in 1985 and 1986, respectively.The total balance of the applied fertilizer N showed that up to 31 and 10% of the fertilizer N was lost during 1985 and 1986, respectively. The differences between the two growing seasons were mainly related to the method and timing of fertilizer N application and to the amount of rainfall.     

Influence of delaying flood and preflood nitrogen application on dry-seeded rice

In the southern U.S. rice belt it is recommended that rice (Oryza sativa L.) grown in the dry-seeded, delayed flood cultural system have the preflood N fertilizer applied and the field flooded at the fourth to fifth leaf stage of plant development. The objective of this field study was to determine if delaying the flood and preflood N application past the fifth leaf stage was detrimental to rice total N and fertilizer¹⁵N uptake, total dry matter, and grain yield. This study was conducted on a Crowley silt loam (Typic Albaqualfs) and a Perry clay (Vertic Haplaquepts). The preflood N fertilizer and flood were delayed 0, 7, 14, or 21 d past the fourth to fifth leaf stage, after which time a permanent flood was established and maintained until maturity. All treatments received 20.5 g N m^–2 as¹⁵N-labeled urea in three topdress applications. All plant and soil samples were taken at maturity. Harvest index increased as the preflood N and flood were delayed past the 4 to 5 leaf stage. Total N in the grain + straw either decreased or showed a decreasing trend as the N and flood were delayed. Similarly, uptake of native soil N decreased as flood was delayed. Conversely, percent recovery of fertilizer N in the rice plant and the plant-soil system increased as the preflood N and flood were delayed. Rice grain yield was not significantly affected by delaying the preflood N and flood up to 21 d.Received....... . Published with permission of the Director of the Arkansas Agric. Exp. Stn. Project ARK01386. Supported in part by the Tennessee Valley Authority National Fertilizer and Environmental Research Center and the Arkansas Rice Research and Promotion Board.     

Effect of K on nitrogen fixation of intercrop groundnut and the competition between intercrop groundnut and maize

Application of adequate level of K has shown to improve the competitive ability of the legume in legume/grass mixtures. However, the effect of K on the competitive ability of grain legumes in legume/cereal intercropping systems has not been adequately studied. Hence, studies were made to ascertain if the effects of K could be exploited in improving the performance of groundnut (Arachis hypogaea L.) cv. No. 45 when intercropped with maize (Zea mays L.) cv. Badra. The study was conducted at the Faculty of Agriculture, University of Ruhuna, Kamburupitiya, Sri Lanka in 1988 in basins filled with 36 kg of soil. It involved establishing maize and groundnut as monocrops and as intercrops at three K levels viz. 0, 20 and 40 mg of K kg^–1 of soil. Monocrop maize and groundnut had 2 and 5 plants/basin, respectively while the intercrop had 1 maize plant and 3 groundnut plants/basin. The soil used was Red Yellow Podzolic which was tagged by incorporating¹⁵N-labelled plant material. When grown as a monocrop, K had no effect on the percent N derived from atmosphere, amount of N₂ fixed, dry matter production, pod yield and total N content of groundnut. However, when intercropped with maize lack of K application affected the above parameters significantly which was overcome by improving K level. Thus, the optimum level of K for groundnut was greater when intercropped than monocropped. A significant interaction between K level and cropping system was evident with regard to N₂ fixation, pod yield and total dry matter production of groundnut. Intercrop maize derived 30–35% of its N content from the associated groundnut plants which amounted to 13–22 mg N/plant. The amount of N supplied by groundnut to associated maize plant was not affected by K level. It appears that there is scope for alleviating growth depression of the legume component in legume/cereal intercropping systems by developing appropriate K fertilizer practices.     

Sugarcane residue management and grain legume crop effects on N dynamics,N losses and growth of sugarcane

To reduce greenhouse gas emissions farmers are being encouraged not to burn sugarcane residues. An experiment was set up in NE Thailand, where sugarcane residues of the last ratoon crop were either burned, surface mulched or incorporated and subsequently the field left fallow or planted to groundnut or soybean. The objectives of the current experiment were to evaluate the residual effects of these treatments during the following new sugarcane crop on (i) microbial and mineral N dynamics, (ii) performance of sugarcane and (iii) effectiveness of recycled legume residues compared to mineral N fertilizer on N use efficiencies, ¹⁵N recovery in the system and in soil particle size and density fractions (using ¹⁵N labelled legume residues and fertilizer). The millable cane and sugar yield were positively affected by sugarcane residue mulching and incorporation compared to burning suggesting microbial remobilization of previously immobilized N. Residual effects of legumes increased sugarcane tillering and yield (127 and 116 Mg ha⁻¹ for groundnut and soybean, respectively) compared to the fallow treatment without N fertilizer (112 Mg ha⁻¹). Soybean residues of higher C:N ratio (33:1) and lignin content (13%) compared to groundnut residues (21:1 C:N, 5% lignin) decomposed slower and improved N synchrony with cane N demand. This led to a better conservation of residue N in the system with proportionally less ¹⁵N losses (15–17%) compared to the large losses from groundnut residues (50–57%) or from mineral N fertilizer (50–63%). ¹⁵N recoveries in soil were larger from residues (41–80%) than from fertilizer (30%) at final harvest. Recycled legume residues were able to substitute basal fertilizer N application but not topdressing after 6 months.     

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.     

Detectability of15N-depleted fertilizer N in soil and plant tissue during a corn-rye crop rotation

Use of¹⁵N-depleted fertilizer materials have been primarily limited to fertilizer recovery studies of short duration. The objective of this study was to determine if¹⁵N-depleted fertilizer N could be satisfactorily used as a tracer of residual fertilizer N in plant tissue and various soil N fractions through a corn (Zea mays L.) -winter rye (Secale cereale L.) crop rotation. Nitrogen as¹⁵N-depleted (NH₄)₂SO₄ was applied at five rates (0, 84, 168, 252, and 336 kg N ha^–1) to corn. Immediately following corn harvest a winter rye cover crop treatment was initiated. Residual fertilizer N was easily detected in the soil NO ₃ ^- -N fraction following corn harvest (140-d after application). Low levels of exchangeable NH ₄ ⁺ -N (<2.5 mg kg^–1) did not permit accurate isotope-ratio analysis. Fertilizer-derived N recovered in the soil total N fraction following corn harvest was detectable in the 0 to 30-cm depth at each N rate and in the 30 to 60 and 60 to 90-cm depths at the 336 kg ha^–1 N rate. Atom %¹⁵N concentrations in the nonexchangeable NH ₄ ⁺ -N fraction did not differ from the control at each N rate. Nitrogen recovery by the winter rye cover crop reduced residual soil NO ₃ ^- -N levels below the 10 kg ha^–1 level needed for accurate isotope-ratio analysis. Atom %¹⁵N concentrations in the soil total N fraction (approximately one yr after application) were indistinguishable from the control plots below the 168, 252, and 336 kg ha^–1 N rate at the 0 to 30, 30 to 60, and 60 to 90-cm depths, respectively. Recovery of residual fertilizer N by the winter rye cover crop was verified by measuring significant decreases in atom %¹⁵N concentrations in rye tissue with increasing N rates. The greatest limitation to the use of¹⁵N-depleted fertilizer N as a tracer of residual fertilizer N in a corn-rye crop rotation appears to be its detectibility from native soil N in the total N pool.Research partially supported by grants from the National Fertilizer and Environmental Research Center/TVA and the Virginia Division of Soil and Water Conservation.     

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.     

The effect of plant residues on plant uptake and leaching of soil and fertilizer nitrogen in a tropical red earth soil

Two field experiments, in which differing amounts and types of plant residues were incorporated into a red earth soil, were conducted at Katherine, N.T., Australia. The aim of the work was to evaluate the effect of the residues on uptake of soil and fertilizer N by a subsequent sorghum crop, on the accumulation and leaching of nitrate, and on losses of N.Stubble of grain sorghum applied at an exceptionally high rate (~ 18 000 kg ha^–1) reduced uptake of N by sorghum by 13% and depressed the accumulation of nitrate under a crop and particularly under a fallow.Loss of fertilizer N, movement of nitrate down the profile, and uptake by the crop was studied in another experiment after application of N as¹⁵NH₄ ¹⁵NO₃ to field microplots. By four weeks after fertilizer application 14% had been lost from the soil-plant system and by crop maturity 36 per cent had been lost. The pattern of¹⁵N distribution in the profile suggested that losses below 150 cm had occurred during crop growth. The recovery of¹⁵N by the crop alone ranged from 16 to 32 per cent. There was an apparent loss of N from the crop between anthesis and maturity. Residue levels common to sorghum crops in the region (~ 2000 kg ha^–1) did not significantly affect uptake by a subsequent sorghum crop, N losses, or distribution of nitrate in the profile.     

Nitrogen fixation of and N transfer from cowpea, mungbean and groundnut when intercropped with maize

Grain legumes are used widely in intercropping systems. However, quantitative and comparative data available as to their N₂ fixation and N beneficial effect on the companion crop in intercropping systems are scarce. Hence, studies were conducted to ascertain the above when cowpea (Vigna unguiculata L.), mungbean (Vigna radiata L.) and groundnut (Arachis hypogaea L.) were intercropped with maize. The study was¹⁵N-aided and made outdoors in basins (30 L) filled with 38 kg of soil.¹⁵N labelling was effected by incorporating¹⁵N-tagged plant material or applying¹⁵N-labelled fertilizer along with sucrose to stabilize¹⁵N enrichment in the soil during the experimental period. Intercropped groundnut fixed the highest amount of nitrogen from the atmosphere (i.e. 552 mg plant^–1), deriving 85% of its N from the atmosphere. Intercropped cowpea and mungbean fixed 161 and 197 mg N plant^–1, obtaining 81% and 78% of their N content from the atmosphere, respectively. The proportion of N derived by maize from the associated legume varied from 7-11% for mungbean, 11–20% for cowpea and 12–26% for groundnut which amounted to about 19–22, 29–45 and 33–60 mg N maize plant^–1, respectively. The high nitrogen fixation potential of groundnut in dual stands and its relatively low harvest index for N have apparently contributed to greater N-benefical effect on the associated crop.     

Fate of Fertilizer15N applied as urea and ammonium sulphate in opium poppy (Papaver somniferum L.) grown under greenhouse conditions

Laboratory incubation and greenhouse experiments were conducted to investigate the comparative effectiveness of urea and ammonium sulphate in opium poppy (Papaver somniferum L.) using¹⁵N dilution techniques. Fertilizer treatments were control (no N), 600 mg N pot^–1 and 1200 mg N pot^–1 (12 kg oven dry soil) applied as aqueous solution of urea or ammonium sulphate. Fertilizer rates, under laboratory incubation study were similar to that under greenhouse conditions. A fertilizer¹⁵N balance sheet reveals that N recovery by plants was 28–39% with urea and 35–45% with ammonium sulphate. Total recovery of¹⁵N in soil-plant system was 77–82% in urea. The corresponding estimates for ammonium sulphate were 89–91%. Consequently the unaccounted fertilizer N was higher under urea (18–23%) as compared to that in ammonium sulphate (9–11%). The soil pH increased from 8.2 to 9.4 with urea whereas in ammonium sulphate treated soil pH decreased to 7.3 during 30 days after fertilizer application. The rate of NH₃ volatilization, measured under laboratory conditions, was higher with urea as compared to the same level of ammonium sulphate. The changes in pH of soil followed the identical trend both under laboratory and greenhouse conditions.     

Nitrogen utilization by lowland rice as affected by fertilization with urea and green manure

Field studies were conducted during two consecutive wet seasons in flooded rice (Oryza sativa L.) to determine the effect of green manure on urea utilization in a rice-fallow-rice cropping sequence. Replicated plots were fertilized with 60 to 120 kg of urea N ha^–1 in three split applications (50, 25 and 25%) with or without incorporation of dhaincha (Sesbania aculeata L.) (100 kg N ha^–1). During the first crop only 31 to 44% of the urea added was used by the rice. Incorporatingin situ grown dhaincha (GM) into the soil at transplanting had little effect on urea utilization. Forty-four to 54% of the N added was not recovered in the soil, rice crop, or as nitrate leachate during the first cropping season. Incorporation of GM had no effect on fertilizer N recovery. Only about 2% of the urea N added to the first rice crop was taken up by the second rice crop and, as in the first crop, the GM had little effect on residual N, either in amount or utilization.     

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.     

The effect of fertilizer placement on nitrogen uptake and yield of wheat and maize in Chinese loess soils

Field trials were carried out to study the fate of¹⁵N-labelled urea applied to summer maize and winter wheat in loess soils in Shaanxi Province, north-west China. In the maize experiment, nitrogen was applied at rates of 0 or 210 kg N ha^–1, either as a surface application, mixed uniformly with the top 0.15 m of soil, or placed in holes 0.1 m deep adjacent to each plant and then covered with soil. In the wheat experiment, nitrogen was applied at rates of 0, 75 or 150 kg N ha^–1, either to the surface, or incorporated by mixing with the top 0.15 m, or placed in a band at 0.15 m depth. Measurements were made of crop N uptake, residual fertilizer N and soil mineral N. The total above-ground dry matter yield of maize varied between 7.6 and 11.9 t ha^–1. The crop recovery of fertilizer N following point placement was 25% of that applied, which was higher than that from the surface application (18%) or incorporation by mixing (18%). The total grain yield of wheat varied between 4.3 and 4.7 t ha^–1. In the surface applications, the recovery of fertilizer-derived nitrogen (25%) was considerably lower than that from the mixing treatments and banded placements (33 and 36%). The fertilizer N application rate had a significant effect on grain and total dry matter yield, as well as on total N uptake and grain N contents. The main mechanism for loss of N appeared to be by ammonia volatilization, rather than leaching. High mineral N concentrations remained in the soil at harvest, following both crops, demonstrating a potential for significant reductions in N application rates without associated loss in yield.     

Management of Gliricidia (Gliricidia sepium) residues for improved sweet corn yield in an ultisol

The nitrogen contribution from Gliricidia (G. sepium) leaves and roots in a Gliricidia alley cropping with sweet corn was quantified by the 15N isotope dilution technique over 3 cropping seasons. Below-ground competition in the third crop was assessed using ³²P radioisotope. Leaf prunings, roots and their mixture were compared in the presence or absence of hedgerows in a randomized complete block design. The combination of leaf prunings and roots resulted in highest N uptake, plant N concentration, and dry matter yield of corn. The Gliricidia leaf prunings decomposed fast and provided nutrients to the corn crop when applied at 21 and 45 days after planting. By contrast, roots decomposed slowly and did not become an important nutrient source to the corn crop. Below-ground competition was observed between hedgerows and corn at 1.7 m distance under conditions of low nutrient availability. However, the competition was masked when the supply of plant available nutrient increased.     

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.