Effect of crop residue management on nitrogen dynamics and balance in a lowland rice cropping system

Two field experiments were conducted in a rice–fallow–rice cropping sequence during consecutive dry and wet seasons of 1997 on a Fluvic Tropaquept to determine the fate and efficiency of broadcast urea in combination with three residue management practices (no residue, burned residue and untreated rice crop residue). Ammonia volatilization losses from urea (70 kg N ha^–1) broadcast into floodwater shortly after transplanting for 11 d were 7, 12 and 8% of the applied N from no residue, burned residue and residue treated plots, respectively. During that time, the cumulative percent of N₂ + N₂O emission due to urea addition corresponded to 10, 4.3 and nil, respectively. The ¹⁵N balance study showed that at maturity of the dry season crop, fertilizer N recovery by the grain was low, only 9 to 11% of the N applied. Fifty to 53% of the applied ¹⁵N remained in the soil after rice harvest, mainly in the upper 0–5 cm layer. The unaccounted for ¹⁵N ranged from 27 to 33% of the applied N and was unaffected by residue treatments. Only 4 to 5% of the initial ¹⁵N-labeled urea applied to the dry season rice crop was taken up by the succeeding rice crop, to which no additional N fertilizer was applied. Grain yield and N uptake were significantly increased (P=0.05) by N application in the dry season, but not significantly affected by residue treatments in either season.     

Effect of urea management on yield and N use efficiency of lowland rice on an acid sulfate soil

Field experiments were conducted during 1988–1989 at two adjacent sites on an acid sulfate soil (Sulfic Tropaquept) in Thailand to determine the influence of urea fertilization practices on lowland rice yield and N use efficiency. Almost all the unhydrolyzed urea completely disappeared from the floodwater within 8 to 10 d following urea application. A maximum partial pressure of ammonia (pNH₃) value of 0.14 Pa and an elevation in floodwater pH to about 7.5 following urea application suggest that appreciable loss of NH₃ could occur from this soil if wind speeds were favorable. Grain yields and N uptake were significantly increased with applied N over the control and affected by urea fertilization practices (4.7–5.7 Mg ha^–1 in dry season and 3.0–4.1 Mg ha^–1 in wet season). In terms of both grain yield and N uptake, incorporation treatments of urea as well as urea broadcasting onto drained soil followed by flooding 2 d later were more effective than the treatments in which the same fertilizer was broadcast directly into the floodwater either shortly or 10 d after transplanting (DT). The¹⁵N balance studies conducted in the wet season showed that N losses could be reduced to 31% of applied N by broadcasting of urea onto drained soil and flooding 2 d later compared with 52% loss by broadcasting of urea into floodwater at 10 DT. Gaseous N loss via NH₃ volatilization was probably responsible for the poor efficiency of broadcast urea in this study.     

Nitrogen use efficiency,floodwater properties,and nitrogen-15 balance in transplanted lowland rice as affected by liquid urea band placement

Alternative N fertilizer management practices are needed to increase productivity and N use efficiency in lowland rice (Oryza sativa L.). In 1986 dry season, a field study using¹⁵N-labeled urea evaluated the effect of time and method of fertilizer N application on grain yield and N use efficiency. Conventional fertilizer application was compared with band placement of liquid urea and point placement of urea supergranules (USG). Grain yields were significantly higher with either band or point placement than with broadcast and incorporation or surface application. Partial pressure of NH₃ (NH₃) was significantly reduced when N was deep-placed.¹⁵N balance data show that fertilizer N applied basally and incorporated gave a total¹⁵N recovery of 52% and crop (grain + straw) recovery of 30%. Band placement of liquid urea N resulted in 82–90% total and 57–65% crop¹⁵N recovery. USG point placement gave 94% total and 70% crop¹⁵N recovery. Deep placement of second N application gave only slightly higher (98%)¹⁵N recovery compared with broadcast application (89%).     

Fate of nitrogen fertilizer applied to lowland rice on a Sulfic Tropaquept

A field experiment was conducted on an acid sulfate soil in Thailand to determine the effect of N fertilization practices on the fate of fertilizer-N and yield of lowland rice (Oryza sativa L.). A delayed broadcast application of ammonium phosphate sulfate (16-20-0) or urea was compared with basal incorporation of urea, deep placement of urea as urea supergranules (USG), and amendment of urea with a urease inhibitor. Deep placement of urea as USG significantly reduced floodwater urea- and ammoniacal-N concentrations following N application but did not reduce N loss, as determined from an¹⁵N balance, in this experiment where runoff loss was prevented. The urease inhibitor, phenyl phosphorodiamidate (PPD), had little effect on floodwater urea- and ammoniacal-N, and it did not reduce N loss. The floodwater pH never exceeded 4.5 in the 7 days following the first N applications, and application of 16-20-0 reduced floodwater pH by 0.1 to 0.3 units below the no-N control. The low floodwater pH indicated that ammonia volatilization was unimportant for all the N fertilization practices. Floodwater ammoniacal-N concentrations following application of urea or 16-20-0 were greater on this Sulfic Tropaquept than on an Andaqueptic Haplaquoll with near neutral pH and alkaline floodwater. The prolonged, high floodwater N concentrations on this Sulfic Tropaquept suggested that runoff loss of applied N might be a potentially serious problem when heavy rainfall or poor water control follow N fertilization. The unaccounted-for¹⁵N in the¹⁵N balances, which presumably represented gaseous N losses, ranged from 20 to 26% of the applied N and was unaffected by urea fertilization practice. Grain yield and N uptake were significantly increased with applied N, but grain yield was not significantly affected by urea fertilization practice. Yield was significantly lower (P = 0.05) for 16-20-0 than for urea; however, this difference in yield might be due to later application of P and hence delayed availability of P in the 16-20-0 treatment.     

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.     

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.     

The influences of organic residues on utilization of urea N by rice

The organic materials of vetch straw, isotopically labeled with¹⁵N and unlabeled, rice straw and¹⁵N-enriched urea were applied to rice in a greenhouse experiment to evaluate the release of available N during the decomposition of vetch material and its uptake by rice, and to measure the effect of organic materials on the efficiency of urea-N utilization by rice. Measurements were made at three sampling stages during the growth period. As expected, vetch material decomposed readily and furnished a continuous supply of N for the growth of rice, although only 18.1% of vetch-N was utilized by the rice crop. However, this was not sufficient to support the survival of all tillers until harvest. After harvest, 70% of vetch-N still remained in paddy soil. The influence of organic materials on urea-N absorption by rice became apparent at about the stage of panicle initiation. The highest urea-N uptake by rice was 42.2% in vetch straw-mixed soil. Otherwise, rice straw retarded urea-N uptake by rice. Nitrogen distribution data indicated that the vetch material would stimulate urea-N uptake by rice plants.The residual effect of vetch material was evaluated by planting Sudan grass immediately after rice was harvested. Only 4.4% of residual vetch-N was utilized in 20 weeks. This low percentage of N uptake and its low availability ratio demonstrated the poor residual effects of this leguminous material.     

Fate and efficiency of nitrogen fertilizers applied to wetland rice. II. Punjab,India

Two modified urea products (urea supergranules [USG] and sulfur-coated urea [SCU]) were compared with conventional urea and ammonium sulfate as sources of nitrogen (N), applied at 58 kg N ha^–1 and 116 kg N ha^–1, for lowland rice grown in an alkaline soil of low organic matter and light texture (Typic Ustipsamment) having a water percolation rate of 109 mm day^–1. The SCU and USG were applied at transplanting, and the whole dose of nitrogen was¹⁵N-labeled; the SCU was prepared in the laboratory and was not completely representative of commercial SCU. The SCU was broadcast and incorporated, whereas the USG was point-placed at a depth of 7–8 cm. The urea and ammonium sulfate applications were split: two-thirds was broadcast and incorporated at transplanting, and one-third was broadcast at panicle initiation. All fertilizers except the last one-third of the urea and ammonium sulfate were labeled with¹⁵N so that a fertilizer-N balance at flowering and maturity stages of the crop could be constructed and the magnitude of N loss assessed.At all harvests and N rates, rice recovered more¹⁵N from SCU than from the other sources. At maturity, the crop recovered 38 to 42% of the¹⁵N from SCU and only 23 to 31% of the¹⁵N from the conventional fertilizers, urea and ammonium sulfate, whose recovery rates were not significantly different. In contrast, less than 9% of the USG-N was utilized. Fertilizer nitrogen uptake was directly related to the yield response from the different sources. Most of the fertilizer N was taken up by the time the plants were flowering although recovery did increase up to maturity in some treatments.Analysis of the soil plus roots revealed that less than 1% of the added¹⁵N was in the mineral form. Between 20 and 30% of the¹⁵N applied as urea, SCU, and ammonium sulfate was recovered in the soil plus roots, mainly in the 0–15 cm soil layer. Only 16% of the¹⁵N applied as USG was recovered in the soil, and this¹⁵N was distributed throughout the soil profile to a depth of 70 cm, which was the lowest depth of sampling.Calculations of the¹⁵N balance showed that 46 to 50% of the urea and ammonium sulfate was unaccounted for and considered lost from the system. Only 27 to 38% of the¹⁵N applied as SCU was not recovered at maturity, but 78% of the USG application was unaccounted for. The extensive losses and poor plant recovery of USG at this site are discussed in relation to the high percolation rate, which is atypical of many ricegrowing areas.     

Nitrogen-15 balances and nitrogen fertilizer use efficiency in upland rice

Field experiments were conducted in the 1984 and 1985 wet seasons to determine the effect of N fertilizer application method on¹⁵N balances and yield for upland rice (Oryza sativa L.) on an Udic Arguistoll in the Philippines. The test cultivars were IR43 and UPLRi-5 in 1984 and IR43 in 1985. Unrecovered¹⁵N in¹⁵N balances for 70 kg applied urea-N ha^–1, which represented N fertilizer losses as gases and movement below 0.5 m soil depth, ranged from 11–58% of the applied N. It was lowest (11–13%) for urea split applied at 30 days after seeding (DS) and at panicle initiation (PI), and highest (27–58%) for treatments receiving basal urea in the seed furrows. In all treatments with basal-applied urea, most N losses occurred before 50 DS.Heavy rainfall in 1985 before rice emergence resulted in large losses of native soil N and fertilizer N by leaching and possibly by denitrification. During the week of seeding, when rainfall was 492 mm, 91 kg nitrate-N ha^–1 disappeared from the 0.3-m soil layer in unfertilized plots. Although rainfall following the basal N application was less in 1984 than in 1985, the losses from basal applied urea-N were comparable in the two years. Daily rainfall of 20–25 mm on 3 of the 6 days following basal N application in 1984 may have created a moist soil environment favorable for ammonia volatilization.In both years, highest grain yield was obtained for urea split-applied at 30 DS and at PI. Delayed rather than basal application of N reduced losses of fertilizer N and minimized uptake of fertilizer N by weeds.     

The effect of the urease inhibitor N-(n-butyl) thiophosphoric triamide on the efficiency of urea application in a flooded rice field trial in Thailand

The compound N-(n-butyl) thiophosphoric triamide (nBTPT) was tested for its ability to reduce the rate of urea hydrolysis in applications of urea at 10 d after transplanting flooded rice. The rates of urea hydrolysis were relatively slow, and nBTPT caused a 1-d reduction in the rate of disappearance of urea from the floodwater. Despite this, the vapor pressures of ammonia in the floodwater were significantly lower in the plots with nBTPT than without for the first 5 d following the N application. The vapor pressures of ammonia measured in the afternoons indicate that ammonia volatilization losses were considerable from the treatments without nBTPT and low from the treatments with nBTPT. There was no nitrogen response in this wet-season crop, apparently because of the high availability of N in the soil. N conserved from ammonia volatilization losses by use of the inhibitor was apparently susceptible to denitrification loss, and 50% of the fertilizer was lost in the 37 d following the application of¹⁵N-labeled urea both with and without the inhibitor.     

Use of chlorophyll meter for nitrogen management in lowland rice

The chlorophyll meter (SPAD-502) can be used to diagnose the nitrogen (N) status of rice (Oryza sativa L.) to decide for fertilizer-N side-dressing. The objective of this study was to determine the relationships between SPAD meter readings, unadjusted and adjusted for specific leaf weight (SLW) and N on dry weight basis (Ndw), N on leaf area basis (Na) and yield. The correlation coefficients (r) between SPAD and Ndw ranged from 0.82 to 0.90; between SPAD and yield, from 0.75 to 0.90; and between SPAD/SLW and yield from 0.77 to 0.85 on pooling the data experiment-wise. The corresponding r values over all experiments were 0.87, 0.81 and 0.80, respectively. The results of better relationship between SPAD and Ndw; and between SPAD and yield in the late reproductive and early ripening phases allows prediction of plant N status based on direct SPAD meter readings. It is quick, simple and non-destructive unlike N prediction based on Kjeldahl procedure or SPAD/SLW.     

7. The efficacy and loss of fertilizer N in lowland rice

Nitrogen fertilization is a key input in increasing rice production in East, South, and Southeast Asia. The introduction of high-yielding varieties has greatly increased the prospect of increasing yields, but this goal will not be reached without great increases in the use and efficiency of N on rice. Nitrogen enters a unique environment in flooded soils, in which losses of fertilizer N and mechanisms of losses vary greatly from those in upland situations. Whereas upland crops frequently use 40–60% of the applied N, flooded rice crops typically use only 20–40%. There is a great potential for increasing the efficiency of N uptake on this very responsive crop to help alleviate food deficits in the developing world.This article reviews current use of N fertilizers (particularly urea) on rice, the problems associated with rice fertilization, and recent research results that aid understanding of problems associated with N fertilization of rice and possible avenues to increase the efficiency of N use by rice.     

1. The chemistry and biology of flooded soils in relation to the nitrogen economy in rice fields

Response of lowland rice to sources and methods of nitrogen fertilizer application were summarized for more than 100 experiments. In about 2/3 of the experiments, the yield increase per kg of fertilizer N was judged to be relatively poor with best split applications of urea. Based on frequency distribution, sulfur coated urea and urea briquets or urea supergranules deep placed more often produced satisfactory yield increases than best split urea, but even with these sources/methods the yield increases were judged to be relatively poor in about 1/2 of the experiments. There is an enormous potential to increase rice production with no further increases in inputs of fertilizer N if we could learn to match the best method/source of fertilizer with the soil-crop management complex.About 60% of the yields with no fertilizer N were in the range of 2 to 4 t/ha. Based on the average yield response to urea, this is equivalent to about 100 kg of urea N. It would appear worthwhile to study ways to improve utilization of soil nitrogen since it is already in place on the land and apparently in fairly abundant amounts in many soils.About 50 experiments with¹⁵N fertilizers were summarized. In almost all cases, the uptake of tagged fertilizer was less than the net increase in N in the above ground matter. In about 2/3 of the experiments, the addition of fertilizer N increased soil N uptake more than 20% and in 1/3 of the experiments the uptake of soil N was increased more than 40%. These results lead to much uncertainty about practical interpretation and use of¹⁵N data.     

Partial balance of nitrogen in a maize cropping system in humic nitisol of Central Kenya

The application of nitrogen in a soil under agricultural production is subject to several pathways including de-nitrification, leaching and recovery by an annual crop. This is as well greatly influenced by the management practices, nitrogen source and soil conditions. The main objective of this study was to investigate the loss of nitrogen (N) through nitrous oxide (N₂O) emissions and mineral N leaching and uptake by annual crop as influenced by the N source. The study was carried out at Kabete in Central Kenya. Measurements were taken during the second season after two seasons of repeated application of N as urea and Tithonia diversifolia (tithonia) leaves. Results obtained indicated that nitrous oxide (N₂O) emissions at 4 weeks after planting were as high as 12.3 μg N m ⁻² h⁻¹ for tithonia treatment and 2.9 μg N m⁻² h⁻¹ for urea treatment. Tithonia green biomass treatment was found to emit N₂O at relatively higher rate compared to urea treatment. This was only evident during the fourth week after treatment application.Soil mineral N content at the end of the season increased down the profile. This was evident in the three treatments (urea, tithonia and control) investigated in the study. Urea treatment exhibited significantly higher mineral N content down the soil profile (9% of the applied N) compared to tithonia (0.6% of the applied N). This was attributed to the washing down of the nitrate-N from the topsoil accumulating in the lower layers of the soil profile. However, there was no significant difference in N content down the soil profile between tithonia treatment and the control. It could be concluded that there was no nitrate leaching in the tithonia treatment. Nitrogen recovery by the maize crop was higher in the urea treatment (76% of the applied N) as compared to tithonia treatment (55.5% of the applied N). This was also true for the residual mineral N in the soil at the end of the season which was about 7.8% of the applied N in the urea treatment and 5.2% in the tithonia treatment.From this study, it was therefore evident that although there is relatively lower N recovery by maize supplied with tithonia green biomass compared to maize supplied with urea, more nitrogen is being lost (through leaching) from the soil–plant system in the urea applied plots than in tithonia applied plots. However, a greater percentage (37.8%) of the tithonia-applied N could not be accounted for and might have been entrapped in the soil organic matter unlike urea-applied N whose greater percentage (92%) could be accounted for.     

Effect of phenyl phosphorodiamidate on the fate of urea applied to wetland rice fields

The effect of phenyl phosphorodiamidate (PPD) on floodwater properties, N uptake,¹⁵N recovery, and grain yield of wetland rice (Oryza sativa L.) was evaluated in a series of field studies conducted at Muñoz and Los Baños, Philippines. Prilled urea and PPD-amended urea were applied to soil and incorporated immediately prior to transplanting or applied to floodwater after transplanting. Urea was also deep-placed or added in a coated form in two studies.The addition of PPD with urea retarded urea hydrolysis by 1–3 days, depending on the time and method of application. Significant reductions in the concentration of ammoniacal-N in floodwater resulted when PPD-amended urea was applied between 18 and 26 days after transplanting (DT). In contrast, PPD did not appreciably affect the concentration of ammoniacal-N in floodwater when applied with urea either immediately before or after transplanting of the seedlings.Plant N uptake and grain yield were not significantly affected by the addition of PPD with urea in three of the four experiments conducted, even though PPD substantially reduced the concentration of ammoniacal-N in the floodwater in several treatments in these studies. The¹⁵N balance studies conducted at both field locations showed PPD to increase total¹⁵N recovery by between 10% and 14% of the¹⁵N applied, 14 days after the application of urea. No further loss of¹⁵N occurred between the initial sampling (40 DT) and grain harvest at Los Baños. An increase in¹⁵N recovery occurred at grain harvest at Muñoz because¹⁵N-labeled urea was applied at 50 DT in the study. PPD increased the amount of¹⁵N in the plant and nonexchangeable soil N fraction at all harvests at Los Baños. In contrast, at Muñoz, PPD increased the quantity of¹⁵N in the KCL-extractable pool 14 days after urea was applied. Reasons for the discrepancies in results between experiments and the overall failure of PPD to increase grain yield are discussed.     

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.     

Nitrogen use efficiency of <Superscript>15</Superscript>N-labelled sheep manure and mineral fertiliser applied to microplots in long-term organic and conventional cropping systems

Nitrogen (N) utilisation by crops has to be improved to minimize losses to the environment. We investigated N use efficiency of animal manure and mineral fertiliser and fate of fertiliser N not taken up by crops in a conventional (CONMIN) and a bio-organic (BIOORG) cropping system of a long-term field experiment over three vegetation periods (winter wheat–soybean–maize). Microplots planted with wheat received a single application of ¹⁵N-labelled slurries (either urine or faeces labelled) or mineral fertiliser. At the end of each vegetation period we tested whether higher microbial activity and larger microbial biomass in BIOORG than CONMIN soils, and lower long-term N input level in BIOORG, affected use efficiency and fate of fertiliser N not taken up by crops. Recovery of ¹⁵N in wheat was 37%, 10% and 47% from urine, faeces and mineral fertiliser, respectively, and decreased strongly in the residual years. In total 41%, 15% and 50% of ¹⁵N applied as urine, faeces and mineral fertiliser was recovered by the three crops. ¹⁵N recovered from originally applied urine, faeces and mineral fertiliser in the topsoil (0–18 cm) at the end of the third vegetation period was 19%, 25% and 20%, respectively. Of urine-, faeces- and mineral fertiliser-¹⁵N, 40%, 61% and 29%, respectively, was not recovered by the three crops and in topsoil suggesting significant transport of ¹⁵N-labelled components to deeper soil layers. CONMIN and BIOORG differed neither in fertiliser N use efficiency by crops nor in ¹⁵N recovery in soil indicating insignificant difference in the turnover and utilization of the applied manure nitrogen in the conventional and the bio-organic cropping systems.     

Efficiency of urea and ammonium sulfate for wetland rice grown on an acid sulfate soil as affected by rate and time of application

A pot experiment was conducted in a greenhouse to assess the effect of rate and time of N application on yield and N uptake of wetland rice grown on a Rangsit acid sulfate soil (Sulfic Tropaquepts). Response of rice at N rates of 800, 1600 and 2400 mg N/pot (5 kg of soil) was compared between urea and ammonium sulfate when applied at two times: (i) full-rate basal at transplanting and (ii) one half at transplanting and one half at the PI stage. In addition, labelled¹⁵N sources were applied either at transplanting or at the PI stage to determine the nitrogen balance sheet in the soil/plant system.No significant difference in grain and straw yields between urea and ammonium sulfate at low rate was observed. At the higher N rates, urea produced higher yields than did ammonium sulfate regardless of timing. The highest yields were obtained when urea at the high N rate was applied either in a single dose or a split dose while lowest yields were observed particularly when ammonium sulfate at the same rate was applied. Split application of N fertilizer was shown to be no better than a single basal application. The occurrence of nutritional disorder, a symptom likely reflected by high concentration of Fe (II) in combination with soluble Al, was induced with high rate of ammonium sulfate.In terms of fertilizer N recovery by using¹⁵N-labelling, ammonium sulfate was more efficient than urea when both were applied at transplanting. In contrast, application at the PI stage resulted in higher utilization of urea than of ammonium sulfate. The recovery of labelled N in the soil was higher with urea than with ammonium sulfate when the two sources were applied at transplanting, while the opposite result was obtained when the same fertilizers were applied at the PI stage. The losses from urea and ammonium sulfate were not different when these fertilizers were applied at transplanting but loss from urea was higher than that from ammonium sulfate when both were applied at the PI stage.     

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.     

Determination of dinitrogen emission and retention in floodwater and porewater of a lowland rice field fertilized with15N-urea

This paper reports a study on the distribution of dinitrogen between the atmosphere, floodwater and porewater of the soil in a flooded rice field after addition of¹⁵N-labelled urea into the floodwater.Microplots (0.086 m²) were established in a rice field near Griffith, N.S.W., and labelled urea (80 kg N ha^–1 containing 79.25 atoms %¹⁵N) was added to the floodwater when the rice was at the panicle initiation stage. Emission of nitrous oxide and dinitrogen was measured directly during the day and overnight, using a cover collection method and gas chromatographic and mass spectrometric analytical methods. Ammonia volatilization was calculated with a bulk aerodynamic method from measurements of wind speed and floodwater pH, temperature and ammoniacal nitrogen concentration. Seven days after urea application the¹⁵N₂ content of the floodwater and soil porewater was determined and total fertilizer nitrogen loss was calculated from an isotopic balance.Throughout the experimental period gas fluxes were low; nitrous oxide, ammonia and dinitrogen flux densities were less than 5, 170 and 720 g N ha^–1 d^–1, respectively. The greatest dinitrogen flux density was observed two days after urea addition and this declined to ~ 100 g ha^–1 d^–1 after seven days.The data indicate that, of the urea nitrogen added, 0.02% was lost to the atmosphere as nitrous oxide, 0.9% was lost by ammonia volatilization, and 3.6% was lost as dinitrogen gas during the 7 days of measurement. At the end of this period 0.028% and 0.002% of the added nitrogen was retained as dinitrogen gas in the floodwater and soil porewater respectively. Recovery of the¹⁵N applied as nitrogen gases, plant uptake, and soil and floodwater constituents totaled about 94% of the nitrogen added.