Nitrate leaching and soil moisture prediction with the LEACHM model

The LEACHM model developed by Wagenet and Hutson [1989] was used to predict the mineral nitrogen and water content in the soil under a winter wheat crop from February to April in two years and three locations. The model grossly overestimated soil water content, probably due to the bad fitting of the assumed water retentivity function to the experimental data at high water contents, and to the presence of a relatively shallow water table (1.0–1.5m). Measured soil hydraulic conductivity varied with water content in a different manner than predicted by the model. By assuming a sandy or gravelly soil layer between the bottom of the measured soil profile and the water table, prediction of soil water content improved considerably. Simulation showed that, under the experimental conditions studied, soil mineral nitrogen varied mainly due to the fertilizer additions, mineralization and denitrification. Nitrogen uptake by plants and leaching were small. Low values of nitrate leaching were predicted by the model because of low drainage. Large differences between predicted and observed values in the mineral nitrogen in the soil occurred in some cases, both in the total amount and its profile distribution.     

Measured and simulated nitrogen dynamics in winter wheat and a clay soil subjected to drought stress or daily irrigation and fertilization

The temporal dynamics of N in above- and below-ground parts of winter wheat and the dynamics of soil mineral-N were measured in the field in four treatments in wheat and a grass ley (L). The wheat treatments were: control (C), drought (D), daily irrigation (I), and daily irrigation and fertilization (IF). Nitrogen (20 g m^–2) was supplied as single doses in spring in C, D, and I, and according to a logistic N uptake function in IF. L, which was under establishment, was irrigated and fertilized in the same way as IF, but the total amount applied was only 5.6 g N m^–2. A soil nitrogen simulation model, SOILN, was used to combine crop and soil N data with measured litter decomposition rates and other major parts of the nitrogen cycle to calculate annual N budgets, based on daily model calculations. The dynamic patterns of crop N uptake and soil mineral N were similar in C, D, and I, although different in magnitude, but different in IF. Plant N uptake in C, D, and I was almost nil after anthesis, whereas it continued in IF until harvest. Generally, simulated soil mineral N levels (0–90 cm) agreed reasonably well with measurements on a yearly time scale, whereas their short-term dynamics were less well described by the simulations. We tested the hypothesis that the short-term variations were due to processes not included in the model,i.e., the loss of recently taken up plant N via roots during the growing season, and microbial N immobilization and remineralization processes induced by root-derived carbon. A simulated input to the soil of 150 g C m^–2 in IF, mimicking root-derived C, resulted in an improved agreement between simulated and measured short-term mineral N dynamics. Because of irrigation, net N mineralization of soil organic material in I and IF was about twice that in C and D, while that in L was about three times higher due to irrigation and high soil temperatures. Simulated N leaching during the following winter was highest in L, followed by I, IF, C and D. Measurements and simulations of N amounts in the system indicated that daily fertilization decreased N leaching compared with single-dose fertilization.     

Simulation of nitrogen in soil and winter wheat crops: Modelling nitrogen turnover through organic matter

A computer model is described that simulates leaching, organic matter turnover and nitrogen uptake by a winter wheat crop. The model is assessed against a data set from the Netherlands where winter wheat was grown in two seasons (1982–3 and 1983–4) on three different soils in two different parts of the country. The model satisfactorily simulated the growth, N uptake and production of grain. It also simulated the dynamics of indigenous soil N well but it did not always account for the fate of applied fertilizer N. Some possible reasons for this and ways of improving the model are discussed.     

Response of dryland wheat to fertilizer nitrogen in relation to stored water,rainfall and residual farm yard manure

Yield response of dryland wheat to fertilizer N application in relation to components of seasonal water (available soil moisture and rainfall) and residual farm yard manure (FYM) was studied for five years (1983–84 to 1987–88) on a maize-wheat sequence on sandy loam soils in Hoshiarpur district of Punjab, India. Four rates of N viz. 0, 40, 60 and 80 kg ha^–1 in wheat were superimposed on two residual FYM treatments viz. no FYM (F₀) and 15 t ha^–1 (F₁₅) to preceding maize. FYM application to maize increased the residual NO₃-N content by 19–30 kg ha^–1 in the 180 cm soil profile. For a given moisture distribution, F₁₅ increased attainable yields. Over the years, F₁₅ increased wheat yield by 230 to 520 kg ha^–1. Response to fertilizer N was lower in FYM amended plots than in unamended plots. Available soil moisture at wheat seeding and amount and distribution of rainfall during the vegetative and the reproductive phases of crop development affected N use efficiency by wheat. Available soil moisture at seeding alone accounted for 50% variation in yield. The residual effect of FYM on wheat yield could be accounted for by considering NO₃-N in 180 cm soil profile at seeding. The NO₃-N and available soil moisture at wheat seeding along with split rainfall for two main phases of crop development and fertilizer N accounted for 96% variation in wheat yield across years and FYM treatments.     

Simulating water and nitrogen behaviour in soils cropped with winter wheat

The SWATNIT model [26], predicting water and nitrogen transport in cropped soils, was evaluated on experimental data of winter wheat for different N treatments. The experiments were monitored at three different locations on different soil types in the Netherlands. Crop growth was simulated using the SUCROS model [11] which was integrated in the SWATNIT model. Both water and nitrogen stress were incorporated. Except for the soil hydraulic properties, all model parameters were taken from literature. The model performance was evaluated on its capability to predict soil moisture profiles, nitrate and ammonia profiles, the time course of simulated total dry matter production and LAI; and crop N-uptake. Results for the simulations of the soil moisture profile indicate that the soil hydraulic properties did not reflect the actual physical behaviour of the soil with respect to soil moisture. Good agreement is found between the measured and simulated nitrate and ammonia profiles. The simulation of the nitrate content of the top layer at Bouwing was improved by increasing the NH ₄ ⁺ -N-distribution coefficient thereby improving the simulation of the NH ₄ ⁺ -N-content in this layer. Deviations between simulated and measured nitrate concentrations also occurred in the bottom layers (60–100 cm) of the soil profile. The phreatic ground water might influence the nitrate concentrations in the bottom layers. Concerning crop growth modelling, improvements are needed with respect to the partitioning of total dry matter production over the different plant organs in function of the stress, the calculation of the nitrogen stress and the total nitrogen uptake of the crop through a better estimate of the N-demand of the different plant organs.     

Nitrogen uptake by autumn sown sugar beet

The influence of N fertilizer rate on uptake and distribution of N in the plant,¹⁵N labelled fertilizer uptake and sugar yield were studied for 3 years on autumn sown sugar beet (Beta vulgaris L.) under Mediterranean (Southern Spain) rain-fed and irrigated conditions. Available average soil N prior to sowing was 69 kg N ha^–1, and net mineralisation in the soil during the growth period was 130 kg N ha^–1. Maximum N uptake occurred in the spring and increased with increasing fertilizer rates in the irrigated crop. There was no increase in N uptake in the sugar beet cropped under rain-fed conditions because of water shortage. Maximum average N uptake both by roots and tops was between 200 and 250 kg N ha^–1. When N fertilizer was not applied, average uptake from the soil was between 130 and 140 kg N ha^–1. At the end of the growth period there was a marked translocation of N from the leaves to the root which increased with the N fertilizer rate. The N ratio top/roots at harvest was 0.45–0.5 and 0.8- - 1 in the irrigated and rain-fed sugar beet, respectively. Maximum¹⁵N labelled fertilizer uptake took place in May-June, being larger in irrigated sugar beet or when spring rainfall was more abundant. Fertilizer use efficiency varied between 30% and 68%. Sugar yield response to N fertilizer rates depended on the N available in the soil and on the total water input to the crop, particularly in spring. The response was more constant in the irrigated crop, where optimum yield was obtained with a fertilizer rate of 160 kg N ha^–1. In the rain-fed crop, the optimum dose proved more erratic, with an estimated mean of 100 kg N ha^–1. The amount of N required to produce 1 t of root and of sugar ranged between 1.5 and 3.8 kg N and between 11.1 and 22.4 kg N respectively, and varied according to the N fertilizer rates applied.     

Comparing measured and Expert-N predicted N2O emissions from conventional till and no till corn treatments

Agricultural soils are a major source of the greenhouse gas nitrous oxide (N₂O). Nitrous oxide emission models can be used to predict the effectiveness of N₂O mitigation strategies; however, these models require rigorous testing before they can be used with confidence. Expert-N, a modular process based N₂O emission model, was tested to determine its ability at predicting nitrogen (N) cycling in the soil–plant–atmosphere system under Canadian agroclimatic conditions. Ancillary data and N₂O emissions were collected/measured from a corn cultivated clay-loam soil that was under different tillage and red clover treatments. The treatments were conventional till (CT) with and without red clover (rc) underseeded in the previous year's wheat crop (CT-Crc and CT-C, respectively), and no till (NT) with and without red clover underseeded in the previous year's wheat crop (NT-Crc and NT-C, respectively). Expert-N provided good estimates of N₂O emissions, and predictions correlated well (positive) with the measured emissions (r ² 0.55–0.83). There was no statistically significant difference between measured and predicted daily emissions. The predicted emissions, integrated over the growing season (25 May–4 October, 1995), were 0.56, 0.57, 0.62, and 0.62 kg N₂O-N ha^–1 for CT-C, CT-Crc, NT-C, and NT-Crc, respectively. The measured emissions over the same period were 1.29, 1.07, 0.96, and 1.04 kg N₂O-N ha^–1 for CT-C, CT-Crc, NT-C, and NT-Crc, respectively. The modelled emissions underestimated the integrated measured emissions by 35–55%; however, the integrated measured emissions had an estimated uncertainty of ±35%. The model provided good predictions of the soil temperatures, moisture contents, and soil nitrate levels with no significant difference from the measured data. Correlations between modelled and measured values for these soil properties in the first 30 cm soil layer were positive and high with r ² 0.71–0.93.     

Crop production,nitrogen recovery and water use efficiency in rice–wheat rotation as affected by non-flooded mulching cultivation (NFMC)

Non-flooded mulching cultivation (NFMC) for lowland rice, as a novel water-saving technique, has been practiced in many areas of China since the 1990s. However, the information on NFMC effects on crop production, nitrogen and water use in rice–wheat rotations is still limited. A field experiment using ¹⁵N-labeled urea was conducted to evaluate the impacts of NFMC on crop yield, fertilizer N recovery and water use efficiency in rice–wheat rotations. Plastic film mulching (PM), and wheat straw and plastic film double mulching (SPM) resulted in the same rice grain yield (7.2 t ha^–1) while wheat straw mulching (SM) and no mulching (NM) led to 5 and 10% yield reduction, compared with rice under traditional flooding (TF). In the rice–wheat rotation, crop productivity in PM, SM or SPM was comparable to that in TF but greater than in NM. Weed growth and its competition with rice for nitrogen were considered the main reason that led to yield decline in NM. Compared with TF, NFMC treatments did not obviously affect fertilizer N recoveries in plant and soil in both rice and wheat seasons. The total fertilizer N recoveries in crop, weed and soil in all treatments were only 39–44% in R–W rotations, suggesting that large N losses occurred following one basal N application for each growing season. Water use efficiency, however, was 56–75% greater in NFMC treatments than in TF treatment in the R–W rotation. The results revealed that NFMC (except NM) can produce comparable rice and wheat yields and obtain similar fertilizer N recovery as TF with much less water consumption.     

Yield,water use and nitrogen uptake for different water and N levels in winter wheat

In an effort to establish an optimum combination of water and nitrogen for winter wheat a field investigation was carried out on a coarse loamy sand soil during 1984–85 and 1985–86 to assess effects of irrigation regime (IR) and N application on yield, water use and N uptake. The treatments compromised all combinations of three irrigation regimes (IR) based on ratios of irrigation water to cumulative pan evaporation viz.1.2 (I-1), 0.9 (I-2) and 0.6 (I-3) and four rates of N, viz. 0, 60, 120 and 180 kg ha^–1. Grain yield increased with increase in frequency of irrigation. In spite of wide differences in weather during the two years, scheduling of irrigation at IW/CPE = 1.2 gave the highest wheat yield on the coarse-textured soil. During 1984–85, the rainless year, grain yield under I-1 was 20 and 32 per cent higher than I-2 and I-3, respectively. With increasing N rate the yield and water use efficiency increased progressively upto 180 kg N under I-1 and upto 120 kg N ha^–1 under I-2 and I-3 regimes. During 1985–86, the wet year, grain yield response to IR was relatively low. Irrespective of IR, yield increased progressively upto 180 kg N ha^–1 during the wet year. Irrigation water regimes and N application also influenced leaf area index and root growth of wheat. The yield of unfertilized wheat was relatively less affected by seasonal rainfall and IR.Both N uptake and grain yield of wheat were found to increase linearly with increase in water use. Water use efficiency was highest under I-1 regime at all levels of N in the dry season of 1984–85 and under I-3 regime in the wet season of 1985–86. Increase in N uptake with increasing N rates was significantly higher under I-1 than I-2 and I-3 regimes. The N use efficiency being maximum at 60 kg N ha^–1, decreased at higher N levels irrespective of IR.     

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.     

Nitrogen losses from fertilizers applied to maize, wheat and rice in the North China Plain

Ammonia volatilization, denitrification loss and total nitrogen (N) loss (unaccounted-for N) have been investigated from N fertilizer applied to a calcareous sandy loam fluvo-aquic soil at Fengqiu in the North China Plain. Ammonia volatilization was measured by the micrometeorological mass balance method, denitrification by the acetylene inhibition – soil core incubation technique, and total N loss by ¹⁵N-balance technique. Ammonia loss was an important pathway of N loss from N fertilizer applied to rice (30–39% of the applied N) and maize (11–48%), but less so for wheat (1–20%). The amounts of unaccounted-for fertilizer N were in the order of rice > maize > wheat. Deep placement greatly reduced ammonia volatilization and total N loss. Temperature, wind speed, and solar radiation (particular for rice), and source of N fertilizer also affect extent and pattern of ammonia loss. Denitrification (its major gas products are N₂ and N₂O) usually was not a significant pathway of N loss from N fertilizer applied to maize and wheat. The amount of N₂O emission (N₂O is an intermediate product from both nitrification and denitrification) was comparable to denitrification loss for maize and wheat, and it was not significant in the economy of fertilizer N in agronomical terms, but it is of great concern for the environment.     

Efficiency of HCl- and H2SO4-acidulated rock phosphates for a rice based cropping system

Pot experiments were conducted with an acid laterite soil and a shallow black calcareous soil to study the effect of initial application of North Carolina and Udaipur rock phosphates, acidulated with HCl or H₂SO₄ to the extent of 25, 50, 75 or 100% of the requirement for complete conversion into superphosphate, on the grain yield and P uptake by crops in rice—wheat and wheat—rice cropping sequences. The products obtained on acidulation with HCl or H₂SO₄ at a given degree behaved similarly. Rock phosphates partially acidulated with HCl or H₂SO₄ to 50–75% could be used successfully for growing rice or wheat on both the soil types. In the rice—wheat sequence, the wheat crop following rice gave very low grain yields compared to the wheat crop in the wheat—rice rotation, while in the wheat—rice rotation the rice crop following wheat gave yields comparable to that of rice in the rice—wheat rotation. The reasons for this differential effect have been made plausible. The studies indicate that a 50–75% H₂SO₄ - or HCl-acidulated rock phosphate may be used as a single application to an upland crop in an upland crop—rice rotation especially on acid soils, where the water soluble fractions of the product are used by the wheat crop. During the process of growth of the upland crop under aerobic soil conditions, the citrate soluble and insoluble fractions undergo such transformations that make it possible for the following rice crop to utilize them under waterlogged conditions.     

Seasonal fluctuations of native available N and soil management implications

The concentration of native available N in tropical soils fluctuates considerably in response to seasonal changes in soil water potential. Such fluctuation reflects the net effect of inputs of N from mineralization, fertilizers and the atmosphere, and removal by plant uptake, immobilization, leaching and gaseous losses. The greatest concentrations normally occur during the transition between the dry and wet seasons. In East-Africa, up to 184 kg mineral N ha^–1 has been measured in the 0–40 cm soil layer and in Trinidad, 143 kg ha^–1 was found in the 0–10 cm layer. Release and accumulation of mineral N occur as a result of the influence of soil water potential on microbial activity. This is due to changes in microbial motility, solute diffusion, microbial survival and the release of protected organic matter. A quantitative understanding of these processes should increase the efficiency of use of this valuable N resource by crops. Current methods of forecasting mineralization under field conditions include measurement of the soil mineralization potential, the release of N from seasonal inputs of litter and model predictions. Litter quality is important. Its composition, in particular its nitrogen, lignin and soluble polyphenol content has a major impact on its N mineralization rate.Crop uptake, gaseous and leaching losses decrease the concentration of soil mineral N during the wet season. These losses are important under moist tropical conditions. For example, at Port Harcourt and Ibadan in Nigeria, leaching losses were large in spite of NO ₃ ^- adsorption which decreased the depth of NO ₃ ^- leaching relative to through-flow. To minimise these losses, it is essential to synchronise plant nutrient demand with supply by mineralisation. This is particularly important at the start of the tropical rainy season when high rates of mineralisation often in excess of the relatively low levels of crop demand, are observed. Fertilizer recommendation, the time table for cropping and the farming system used therefore need to take into account the seasonal availability of N. The CERES model simulates crop growth and development and the N-cycle. As development and validation continue, such models should provide a strong basis for better soil, crop and fertilizer management practices. A better understanding of the processes should provide a strong basis for futher development of such models.     

Nitrogen mineralization,nitrate leaching and crop growth following cultivation of a temporary leguminous pasture in autumn and winter

A field experiment was conducted to investigate the effect of timing and method of cultivation of a 3-year old ryegrass/white clover pasture on subsequent N mineralization, NO ₃ ^- -N leaching, and growth and N uptake of a wheat crop in the following season. The size of various N pools and decomposition of¹⁴C-labelled ryegrass material were also investigated. Cultivation method (mouldboard or chisel ploughing) generally had no significant effect on the accumulation of mineral N in the profile in the autumn or on the amount of NO ₃ ^- -N leached over winter.¹⁴C measurements suggested that initial decomposition rate of plant material was faster from May than March cultivation treatments. Despite this, overall net mineralization of organic N (of soil plus plant origin) increased with increasing fallow period between cultivation and leaching. The total amounts of mineral N accumulated in the soil profile before the start of leaching were 139, 119 and 22 kg N ha^–1 for the March, May and July cultivated soils respectively. Cumulative leaching losses over the trial calculated from soil solution samples were 78, 40 and 5 kg N ha^–1 for the March, May and July cultivated soils respectively. Differences in N mineralization over the season were generally not reflected by changes in amounts of potentially-mineralizable soil N (as measured by extraction or laboratory incubation) or levels of microbial biomass during the season. The amount of mineral N in the profile in spring increased with decreasing fallow period. This was reflected in an approximately 15% and 25% greater grain yield and N uptake respectively by the following wheat crop in plots cultivated in July rather than in March.     

Residual effects of growing mungbean and uridbean on the yield and nitrogen uptake of a succeeding wheat crop

A two year field experiment was carried out at the Indian Agricutural Research Institute, New Delhi - 110012, India to assess the effect of mungbean (Vigna radiata L.) and uridbean (Vigna mungo L.) residues on the yield and N uptake of a succeeding wheat crop as compared to sorghum fodder. Sorghum produced 3.5–7.5 times more dry matter and removed 2–3 times more nitrogen than mungbean or uridbean during same duration (80 ± 10 days) of their growth. Without N application the grain yield of wheat following mungbean and uridbean (without residue incorporation) was 0.45 and 0.48 t ha^–1 more than the yield of wheat following sorghum fodder. These yields were equivalent to that predicted when 36 and 38 kg urea-N ha^–1, respectively, was directly applied to wheat. The residual effects of these grain legumes were higher when succeeding wheat was fertilized with 60 kg urea-N ha^–1; at this level mungbean and uridbean spared 52 and 43 kg urea-N ha^–1, respectively, in succeeding wheat. The residual effect of mungbean and uridbean further increased when their residue was incorporated in soil; with this practice they spared 94 and 115 kg urea-N ha^–1, respectively, without N application to wheat and 74 and 82 kg urea-N ha^–1, respectively, with an application of 60 kg urea-N ha^–1 to wheat.Mungbean and uridbean, without residue incorporation, increased aboveground plant-N uptake of succeeding wheat by 11.5–34.9 and 10.8–34.0 kg N ha^–1, respectively; whereas with residue incorporation, they increased aboveground plant-N content of succeeding wheat by 26.1–45.8 and 32.7–47.7 kg N ha^–1, respectively.The results of the present study indicate that there is both an indirect sparing effect and a direct residual effect of mungbean and uridbean on the nitrogen needs of succeeding wheat, more so when their residues are incorporated in soil.     

Predicting the site specific soil N supply under winter wheat in Germany

The expected amount of plant nitrogen (N) at harvest which originates from soil N supply is of high relevance for N fertilization planning. Due to mineralization–immobilisation turnover processes, soil N supply is influenced by N fertilization which complicates its assessment. The soil N supply consists of two components: the soil mineral N measured at early spring and the ‘effective’ N mineralization (Min^eff) under winter wheat (Triticum aestivum L.). Min^eff was defined as the difference between crop N uptake (N^crop) at harvest and N supply. Our aim was the identification and quantification of climate and site-related factors in order to achieve an improved assessment of the site-specific (long term average) Min^eff. We used N rate experiments from 411 collective seasons, carried out at 98 sites across Germany in order to analyze the impact of climate and site-related factors on Min^eff. Quadratic curves were fitted in order to describe the grain N uptake as a function of N supply. A fixed marginal N efficiency was defined in order to analyze Min^eff at a reasonable N supply. Starting with estimates for Min^eff as function of preceding crop, we found that climate (average temperature during May, annual rainfall) and site-related factors have a significant influence on Min^eff. In order to ensure that the regression model is transferable to unknown sites, a “leave one site out” cross validation was carried out. Compared to considering preceding crop only (reference), the regression model reduced the RMSE by 9.5 (calibration) or 8.3 (cross validation) kg N/ha.     

The effects of a nitrification inhibitor on leaching losses and recovery of mineralized nitrogen by a wheat crop after ploughing-in temporary leguminous pastures

The effect of a nitrification inhibitor on the accumulation of ammonium (NH ₄ ⁺ -N) and nitrate (NO ₃ ^- -N) in the profile was investigated in two field experiments in Canterbury, New Zealand after the ploughing of a 4-year old ryegrass/white clover pasture in early (March) and late autumn (May). Nitrate leaching over the winter, and yield and N uptake of a following wheat crop were also assessed.The accumulation of N in the soil profile by the start of winter was greater in the March fallow (76–140 kg N ha^–1) than in the May fallow treatment (36–49 kg N ha^–1). The nitrification inhibitor dicyandiamide (DCD) did not affect the extent of net N mineralization, but it inhibited nitrification when applied to pasture before ploughing, especially at its depth of incorporation (100–200 mm). Nitrification inhibition in spring was greater when DCD was applied in May rather than in March due to its reduced degradation over the winter.Cumulative nitrate leaching losses were substantial from the March fallow treatment in both years (about 100 kg N ha^–1). A delay in the cultivation of pasture and the application of DCD both reduced nitrate leaching losses. When leaching occurred early in the winter (in 1991), losses were less when pasture was cultivated in May (2 kg N ha^–1) than when DCD was applied to pasture cultivated in March (68 kg N ha^–1). When leaching occurred late in the winter (in 1992), similar losses were measured from pasture cultivated in May (49 kg N ha^–1) and from DCD-treated pasture cultivated in March (57 kg N ha^–1).Grain harvest yield and N uptake of the following spring wheat crop were generally unaffected by the size of the N leaching loss over the winter. This was due to the high N fertility of the soil after four years of a grazed leguminous pasture.     

Simulation of soil nitrogen dynamics using the SOILN model

A model dealing with transport and transformations of nitrogen in soil is briefly described. The model has a one-dimensional layered structure and considers processes such as plant uptake, mineralization/immobilization, leaching and denitrification. A soil water and heat model provides daily values for abiotic conditions, which are used as driving variables in the nitrogen simulation. In this study, the model was run with data from a polder-soil area in the Netherlands, with winter wheat as the crop. The simulation results showed that if a measured time course of crop nitrogen uptake throughout the growing season is available, mineral-N dynamics in soil can be satisfactorily described with this model. The main problems identified in the simulations were related to the partitioning between above- and below-ground plant-N, and supplying the crop with sufficient N, as given by the measurements.     

Some results of nitrogen simulations with the model ANIMO

Simulation of the nitrogen behavior in the soil and the nitrogen uptake by winter wheat was performed using the model ANIMO. As input for the model ANIMO simulations of the hydrological conditions in the soil crop ecosystem were executed with the model SWATRE. Compared with measured data the simulation of nitrogen uptake by the crop was satisfactory. The simulation of mineral nitrogen in the soil agreed reasonably well with measured data for one of the experiments used for the analysis. The agreement was less for experiments with additional fertilizer applications in May and June.     

Impact of rice straw management practices on yield,nitrogen uptake and soil properties in a wheat-rice rotation in northern India

To study the long term effects of rice straw management practices in a wheat-rice rotation, experiments were started from the dry season (Nov–May) of 1984 to wet season (July–Nov) of 1989. Each year, six straw management practices, viz. control (C), straw incorporation (SI), straw mulch (SM), straw burning (SB), animal manure incorporation (AM), and straw and animal manure incorporation together (SI+AM) were imposed to wheat crop and their subsequent residual effect was studied on the following rice crop under three levels of N, viz. 0, 60 and 120 kg N ha^–1. The rate of straw and animal manure used was 5 t ha^–1 on dry weight basis.The wheat yield and N uptake did not vary significantly under control and SB throughout the experimental period. But, the production level of wheat and N uptake were consistently higher under AM and SM over these two treatments. The SI+AM which had significantly lower wheat yields and N uptake over the AM during the first crop, became equal to that of AM and SM during the second and third crops, and out yielded these two treatments from the fourth crop onward. Straw incorporation which produced wheat yield and N uptake even less than control and SB during the first two crops, resulted in wheat yield and N uptake equivalent to AM and SM from the fourth crop onward.None of the straw management practices had residual effects on the yields and N uptake during the first rice crop, except SM which reduced the rice yields and N uptake in the first two crops. However, AM and SI+AM in the second crop; AM, SI+AM and SI in the third crop; and AM, SI+AM, SI and SM from the fourth crop onward had significant and positive residual effects on rice yields and N uptake. Among these four treatments, SI+AM produced residual effects which were significantly higher than the remaining three treatments. Considering the production levels of wheat and rice, SI+AM treatment resulted in savings of 60 kg N ha^–1 each for wheat and rice.After five years of experimentation, the maximum soil build-up of organic carbon; available N, P and K; and DTPA-extractable Zn, Cu, Fe and Mn was observed under SI+AM, followed by AM and SM and it was minimal under SB and control treatments. The treatments of AM and SI+AM also resulted in a high percentage of water-stable aggregates of 70.25 mm in diameter (80.9%), larger mean weight diameter (0.82 mm), higher porosity (54.2%) and lower bulk density (1.19 Mg m^–3).