Newsletter from Danish Research Centre for Organic Farming • December 2004 • No. 4

Simulating crop production and nitrate leaching losses from organic arable crop production

By Jørgen E. Olesen and Jørgen Berntsen, Danish Institute of Agricultural Sciences, Department of Agroecology

Crop models can be powerful tools to analyse the effect of changes in management of cropping systems on productivity and environmental impact, provided the models have been thoroughly tested.

The FASSET model was found to adequately reproduce effects of management (manure and catch crops) and of site (soil and climate) on crop yields and nitrate leaching from a 4-crop rotation with a grass-clover green manure crop in one out of four years. Scenario analyses show that the nitrate leaching increased relatively more with increasing N fixation in the grass-clover than did the crop yields. The yields could be increased and the nitrate leaching reduced by using catch crops and/or using the grass-clover for biogas instead of a green manure, assuming that the digested slurry was returned to the cereal crops.

Models in organic farming

The challenge in organic farming is to optimise short-term productivity while maintaining long-term soil fertility, increasing recycling of nutrients and resources, reducing nutrient losses to the surrounding environment and increasing natural control of pests and diseases. This is often a complex task, since there are multiple interactions in the agroecosystem and the various system responses depend on local soil and climate conditions. However, the system may be studied using mechanistic and dynamic simulation models, provided the models are general, reliable and validated against experimental data for whole organic crop rotations. Furthermore, valid and extensive models may be used to analyse consequences for the organic farm production and for the environmental impact.

The FASSET model

FASSET is a whole-farm model, capable of simulating major processes at the farm level, including processes and interactions of soil, crop, livestock and machinery and their effects on productivity, nutrient cycling, nitrogen losses and farm profitability. FASSET differs from other farm simulation models by dynamically simulating all these processes.

In the DARCOF project, BIOMOD, focus has been on improving the simulation of soil organic matter turnover and of simulating the effects of catch crops and grass-clover pastures on nitrate leaching and productivity of arable crops. This has involved testing the model against data from several experiments in organic farming. Here we present the results from the use of the model on an arable crop rotation, which includes comparison with data from a crop rotation experiment carried out at three sites in Denmark.

Crop rotation experiment

A crop rotation experiment was initiated in 1996/97 at three sites in Denmark with the aim of investigating the possibilities of increasing the organic production of cereals. The sites represent different soil types and climate regions in Denmark. Jyndevad is located in Southern Jutland on a coarse sand, Foulum is located in Central Jutland on a loamy sand and Flakkebjerg is located in Western Zealand on a sandy loam. The normal precipitation for the period 1961-90 was 859, 626 and 559 mm at Jyndevad, Foulum and Flakkebjerg, respectively.

Here we only use data from one of the crop rotations in the experiment. This is a 4-crop rotation with pea/barley, spring barley, grass-clover and winter wheat. The mixture of pea and barley was grown for maturity. The grass-clover was established by undersowing in the spring barley, and the grass-clover was followed by winter wheat.

Two experimental factors were tested in the experiment: (i) catch crop (with and without ryegrass as undersown catch crop) and (ii) manure (with and without animal manure applied as slurry to spring barley and winter wheat at a rate of 50 kg NH4-N/ha). The grass-clover was used solely as a green manure crop, and the cuttings were left on the ground. The crops were irrigated at Jyndevad. Measurements of grain yields and nitrate leaching were available for comparison with the model simulations.

Good simulation of productivity

The FASSET model was used to simulate crop yields and nitrate leaching for a 12-year period (three courses of the crop rotation) using climate and soil data for the three experimental sites and typical crop management for the sites. This 12-year period does not exactly match the measurement period in the experiment. The longer simulation period will tend to give higher soil fertility than the measurements, in particular for Flakkebjerg, where continuous cereal cropping had been practised prior to onset of the experiment.

The observed variation in grain yields between sites and treatments were well simulated by the model (Figure 1). The spring barley yields were overestimated with the model at Flakkebjerg, which can be attributed to the low initial soil fertility at this site. The simulations assumed higher soil fertility and this is the likely explanation for the higher simulated spring barley yields at Flakkebjerg. The model overestimated grain yields of winter wheat at all sites. There are two likely explanations for this, either a too high simulated N-fixation in the grass-clover or problems in simulating the proper timing of mineralisation of the grass-clover residues.

Nitrate leaching

The observed response of nitrate leaching to use of catch crops was well simulated by the model (Figure 2). However, the model overestimated nitrate leaching at Flakkebjerg and underestimated it at Jyndevad. The overestimation at Flakkebjerg may be due to the low initial soil fertility as mentioned above for the yield effects. The deviations at Jyndevad may have several causes. Thus stubble cultivation was used in the autumn to control perennial weeds, and this may have increased leaching. This tillage effect on soil nitrogen turnover is not included in the model. Also it is possible that the model does not properly capture the nitrogen turnover at low temperatures after ploughing a grass-clover in the autumn.

Response to variation in nitrogen fixation

The nitrogen supply in a crop rotation is strongly influenced by the N fixation in the grass-clover field. However, this N-fixation may vary greatly depending on the clover proportion in the grass-clover. A sensitivity analysis was therefore performed with varying rates of N-fixation (Figure 3). The grain yield increased with increasing N fixation. However, nitrate leaching increased relatively more than grain yields at high rates of N fixation. Thus, the N fixation in the grass-clover green manure crop is a source of nitrate leaching in this cropping system.

Better use of the grass-clover green manure

The use of the grass-clover as a green manure crop in the crop rotation means that the soil fertility is being built in just one out four years in the crop rotation. This gives a large nitrogen mineralisation after the incorporation of the grass-clover and this adds to nitrate leaching, if the mineralised N is not taken up by the following grain crops or catch crops.

An alternative is to harvest the grass-clover and to use this material in a biogas digester to produce biogas for energy production. The digested slurry may then be applied to the cereal crops as a fertiliser. In the biogas scenarios shown in Table 1 it was assumed that all of the grass-clover was digested and that the digested slurry was applied to the winter wheat (two thirds) and to the spring barley (one third). The results show that this increases grain yields and reduces nitrate leaching. The effects are largest for the sandy soil at Jyndevad, and there is almost no yield benefit from the biogas digestion at Flakkebjerg, although there is still a simulated reduction in nitrate leaching. The effects of using catch crops are also largest at Jyndevad and smallest at Flakkebjerg. There are thus large differences in the responses of crop management between sites (soils and climate) that should be accounted for in the design of cropping systems.