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.     

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.     

Simulation of the nitrogen balance in the soil and a winter wheat crop

A simulation model for winter wheat growth, crop nitrogen dynamics and soil nitrogen supply was tested against experimental data. When simulations of dry matter production agreed with measurements, nitrogen uptake was simulated accurately. The total amount of soil mineral nitrogen as well as the distribution of mineral nitrogen over the various soil layers were generally simulated well, except for experiments in which fertilizer was applied late in spring. In these experiments, applied nitrogen disappeared because it could not be accounted for by the model. Some explanations for this disappearance are briefly discussed.     

Simulation of nitrogen dynamics and biomass production in winter wheat using the Danish simulation model DAISY

A dynamic simulation model for the soil plant system is described. The model includes a number of main modules, viz., a hydrological model including a submodel for soil water dynamics, a soil temperature model, a soil nitrogen model including a submodel for soil organic matter dynamics, and a crop model including a submodel for nitrogen uptake. The soil part of the model has a one-dimensional vertical structure. The soil profile is divided into layers on the basis of physical and chemical soil characteristics. The simulation model was used to simulate soil nitrogen dynamics and biomass production in winter wheat grown at two locations at various levels of nitrogen fertilization. The simulated results were compared to experimental data including concentration of inorganic nitrogen in soil, crop yield, and nitrogen accumulated in the aboveground part of the crop. Based on this validation it is concluded that the overall performance of the model is satisfactory although some minor adjustments of the model may prove to be necessary.     

Response of wheat to nitrogen fertilization,a data set to validate simulation models for nitrogen dynamics in crop and soil

A data set originating from winter wheat experiments at three locations during two years is described. The purpose is to provide sufficient data for testing simulation models for soil nitrogen dynamics, crop growth and nitrogen uptake. Each experiment comprised three different nitrogen treatments, and observations were made at intervals of two or three weeks. The observations included measurements of soil mineral nitrogen content, soil water content, groundwater table, dry matter production and dry matter distribution, nitrogen uptake, nitrogen distribution and root length density.     

A model to predict changes in soil moisture,NO3-N content and N uptake by wheat

A simulation model to predict fertilizer N behaviour in a soil-plant (wheat) system has been developed and tested. The model predicts components of field water balance (evaporation, transpiration, drainage and run-off) and changes in soil nitrogen amounts due to N transformations (urea hydrolysis, mineralization, nitrification and volatilization), N movement and plant N uptake using information on N transformation coefficients for the soil, atmospheric evaporative demand (E_pan), leaf area development and root growth characteristics of the crop. The model predicts N uptake by wheat through mass flow using a new simplified crop cover function. The coefficients of correlation between the measured and predicted N uptake by wheat grown under three different moisture regimes in the two years (1987–88 and 1988–89) approached unity. The computed amount of residual NO₃-N in the soil profile at wheat harvest matched well with the measured amount with a root mean square error of 13.7 percent. The close matching of the measured and model predicted components of nitrogen and water balances under three widely different set of irrigation treatments suggests of model's capabilities to help in on-farm N management both under irrigated and rainfed conditions.     

Simulation of the effects of nitrogen supply on yield formation processes in winter wheat with the model TRITSIM

An outline of the dynamic winter wheat model TRITSIM is given. The model describes in one-day steps growth, yield formation and development of a crop from post-winter tillering until harvest under various conditions of water and nitrogen supply. TRITSIM is coupled with a simple soil nitrogen model and a soil water model to describe effects of nitrogen and water on yield formation processes. Comparisons between model and experimental results for ontogenesis, grain biomass, nitrogen uptake and soil mineral nitrogen are given for a series of Dutch experiments. Simulations were satisfactory, except for the time course of soil mineral nitrogen.     

Simulating nitrate leaching in bare fallow soils: a model comparison

In Western Europe, agriculture is one of the major contributors to the pollution of ground- and surface waters. Environmental concern has created the need for protection of these waters against eutrophication, caused by nutrient losses from e.g. agricultural sources. Leaching models may be used to predict nitrate-N losses to the environment and a plethora of such leaching models already exist. Four nitrate leaching models (Burns model, SLIM, SACFARM and ANIMO), with varying degree of complexity and parameter requirements, were used to simulate leaching in a bare fallow soil on a number of fields in the Wijlegem catchment in Flanders, Belgium. The models' performance was evaluated both statistically and graphically. Although all models predicted nitrate content in the soil profile within acceptable limits, the slightly adjusted Burns model appeared to simulate both the nitrate nitrogen and the water content best for the calibration field. Similar results were obtained for the evaluation field experiments: the Burns_– model simulated the moisture and nitrate content fairly well, while SLIM performed well in simulating the nitrate content. In conditions with limited data availability, simple (management) models, needing only a limited number of parameters to be measured or calibrated, may yield better simulation results than complex mechanistic models.     

Evaluation of different agronomic strategies to reduce nitrate leaching after winter oilseed rape (Brassica napus L.) using a simulation model

Winter oilseed rape (OSR) demands high levels of N fertilizer, often exceeding 200 kg N ha⁻¹. Large amounts of residual soil mineral nitrogen (SMN) after harvest are regularly observed, and therefore N leaching during the percolation period over winter is increased. In this study agronomic strategies (fertilization level, crop rotation, tillage intensity) to control nitrate leaching after OSR were investigated by combining field measurements (soil mineral nitrogen, soil water content, crop N uptake) of a 2-year trial and another 5-year field trial with simulation modeling. The crop-soil model uses a daily time step and was built from existing and partly refined submodels for soil water dynamics, mineralization processes, and N uptake. It was used to reproduce the complex processes of the N dynamics and to calculate N concentration in the leachate and total volume of percolation water. Some parameters values were thereby newly identified based on the agreement between measured data and model results. Although SMN in the 60–90 cm layer was overestimated, the model could reproduce the measured data with an acceptable degree of accuracy. Overfertilization of OSR increased N leaching and therefore the precise calculation of N fertilizer doses is a first step towards prevent N leaching. Compared to ploughing, minimum tillage decreased N leaching when winter wheat was grown as the subsequent crop. Volunteer OSR and Phacelia tanacetifolia were grown as catch crops after OSR harvest. N leaching could be decreased especially when Phacelia was grown, but nitrate concentrations in the drainage water were higher and exceeded the European Union (EU) threshold for drinking water when volunteer OSR was grown. The results of this study provide strong evidence that reduced tillage or growing of noncruciferous catch crops decrease N leaching and may be used as an agricultural measure to prevent N pollution.     

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.     

Simulated cereal nitrogen uptake and soil mineral nitrogen after clover-grass leys

Simulations were made to test the effects of age and composition of red clover (Trifolium pratense) based leys on yield of two subsequent spring cereal crops, as well as nitrogen (N) uptake and soil mineral N content. The experimental plots in two trials were cropped for 2–3 years with spring cereals, or 1-, 2- or 3-year-old red clover based leys, followed by spring wheat and subsequent spring oats. CoupModel, a process oriented ecosystem model, was calibrated with measured values of above ground N uptake and soil mineral N contents from plots of cereal monoculture. Cereal N uptake was simulated for a 2 year period in cereals after leys. The calculations of N inputs in incorporated plant material of leys were also tested. Simulated N uptake in the above ground biomass generally agreed with the field data with default values of the model. Some parameters were increased in order to improve plant N uptake and keep the soil mineral N contents at the measured levels. The simulated soil mineral N contents were close to the measured values for surface layers and were more accurate than for deeper layers in the profile. However, the high simulated mineral N increase after harvest in one trial was not seen in field measurements, which remains difficult to explain. Most probably the C:N estimate for crop residues was set too low in the model, but calculated N input was on a reasonable level. These results show that further testing and adjusting of N dynamics in organic farming system using CoupModel should be continued.     

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.     

Development of nitrogen fertilizer recommendations for arable crops in the Netherlands in relation to nitrate leaching

In the Netherlands, current nitrogen fertilizer recommendations for arable crops are based on the amount of soil mineral nitrogen in early spring. The larger the amount of soil mineral nitrogen, the lower the recommended application rate of fertilizer nitrogen. A more refined method is to draw up a balance sheet in which the nitrogen requirement of the crop is given on the one side and the contributions of fertilizer nitrogen, soil mineral nitrogen, and the amount of nitrogen mineralized during the growing period on the other. The most refined method of nitrogen fertilizer recommendation is the use of a simulation model that predicts the daily crop nitrogen requirement and nitrogen supply to the crop from various pools during the growing period. A simulation model thus adds the time element to nitrogen fertilizer recommendations. Moreover, in contrast with the other two methods, a simulation model allows identification of environmental side-effects of nitrogen fertilizer application.The current Dutch nitrogen fertilizer recommendations aim at predicting the economically optimum application rate of fertilizer nitrogen. From the environmental point of view it is interesting to know how much soil mineral nitrogen has accumulated in the soil at harvest, because this nitrogen is a potential loss to the environment through nitrate leaching during the subsequent winter period. If the economically optimum application rate of fertilizer nitrogen is applied to arable crops, it is unlikely that soil mineral nitrogen accumulates, except in the case of potatoes. Model calculations have shown that accumulation of soil mineral nitrogen after potatoes can be prevented when the recommended nitrogen application rate is reduced by 25%. In that case tuber yield is reduced by only 2%.     

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.     

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.     

WHNSIM — a soil nitrogen simulation model for Southern Germany

A one-dimensional deterministic soil nitrogen simulation model (WHNSIM) is presented. With the model the leaching of soil nitrate, its uptake by plant roots and the mineralization of soil organic nitrogen can be simulated. Basic elements of WHNSIM are differential equations that describe soil water, soil heat and soil solute transport. The equations are solved with a fully implicit finite difference method for a variety of boundary and initial conditions. With WHNSIM the soil nitrogen behavior of arable fields for one or more consecutive years can be described. The model has been calibrated for typical site conditions in Southern Germany. The main features of WHNSIM are discussed, some simulation results are also presented. For site conditions of Southern Germany the model appears to perform adequately.     

Effects of plant residues on crop performance, N mineralisation and microbial activity including field CO2 and N2O fluxes in unfertilised crop rotations

The impacts of crop rotation and input of organic matter in the form of green manure crops, straw residues and incorporation of catch crops on crop yield, nitrogen uptake, microbial biomass and activity were studied in unfertilised crop rotations differing in input of plant residues, i.e., high-input rotations with a grass-clover crop and catch crops included and low-input cereal rotations without catch crops. The parameters studied included substrate induced respiration (SIR), hydrolysis of fluorescein diacetate (FDA), arylsulfatase activity (ASA), N mineralisation, N₂O emission, and soil respiration. These parameters were measured in bare soil plots, to estimate the effects of previous years' crops and input of plant residues. In neighbouring plots crop performances were registered by measuring yields, above-ground biomass and nitrogen uptake during the growing season. Generally, all measured parameters were significantly higher in the high-input than in low-input rotations. Estimates of metabolic quotients indicated that the microbial communities in the low-input rotations were less efficient in utilising the C sources than those in the high-input rotations. Calculations of N₂O emission factors indicate that the current IPCC methodology for estimating N₂O emission from plant residues needs to be improved.     

Measurement and Modelling of NO Fluxes on Maize and Wheat Crops During their Growing Seasons: Effect of Crop Management

Fertilized agricultural soils are a significant source of NO, a gas involved in tropospheric ozone formation. The aims of the research reported here were to measure NO fluxes over the length of the growing season of wheat and maize crops, and to build a model of soil NO emissions from arable land. Field experiments were carried out on a 1-ha field divided into two parts. The first one was cropped with wheat and harvested in late July, 2002, whereas the second part was sown with maize and harvested in October. The wheat and maize received 130 kg N ha⁻¹ and 140 kg N ha⁻¹, respectively. For each crop, NO fluxes were measured during 10 months every 2 weeks using manual closed chambers, and continuously with a wind tunnel immediately after nitrogen fertilization. Fertilizer application significantly affected NO emissions: the largest NO emissions were recorded a few days after nitrogen application. This delay depended on the kinetics of nitrogen incorporation in the soil, as influenced by rainfall. The emissions measured on the maize field (2.6% of the fertilizer amount applied) were more important than those on the wheat field (1.0% of the fertilizer amount applied), owing to differences in timing of nitrogen application, with respect to climate and crop growth. Relationships between soil nitrification rate and NO emission obtained from laboratory incubations, and experimental data appeared useful and relevant to predict NO emissions at the field-scale.