THE DISTRIBUTION OF WATER AND NITROGEN IN THE SOIL-CROP SYSTEM - A SIMULATION STUDY WITH VALIDATION FROM A WINTER-WHEAT FIELD TRIAL

The realistic simulation of uptake processes for water and nitrogen, and of partitioning processes for nitrogen, are necessary to the accurate reproduction of water and nitrogen deficit effects on crop growth and yield. Mathematical descriptions of these processes, based on the findings of detailed studies, were used as part of a larger simulation model to calculate the extraction and uptake of water and nitrogen, and the distribution of nitrogen, by a winter wheat crop from a multi-layered soil profile over a growing season. Descriptions of water uptake processes allowed the model to reproduce the hourly dynamics of water uptake and redistribution through the root system, and to estimate the yearly dynamics of water depletion from the soil profile consistent with field data. Descriptions of nitrogen uptake processes allowed the model to reproduce the hourly dynamics of mass flow, diffusion and active uptake, but estimates of the yearly dynamics of mineral nitrogen depletion did not closely follow field data. This inconsistency arose largely from the partial disappearance of fertilizer nitrogen in the field plots shortly after application. This disappearance was not reproduced in the model. The model was able to reproduce the seasonal accumulation and redistribution of dry matter and nitrogen within the crop for fertilizer applications from 0 to 16 g m-2. These applications gave dry matter and nitrogen yields of phytomass from 1340 to 1600 and from 13.4 to 23.4 g m-2 respectively, and of grain from 673 to 810 and from 11.5 to 20.4 g m-2 respectively. However, the model tended to overestimate both dry matter and nitrogen yields under high rates of fertilizer application. This simulation study demonstrates that results from more detailed studies of water and nitrogen uptake may be used to understand the annual dynamics of water and nitrogen distribution in the soil-crop system.