MODELING THE FATE OF NITROGEN IN CROP AND SOIL IN THE YEARS FOLLOWING APPLICATION OF N-15-LABELED FERTILIZER TO WINTER-WHEAT

A computer model is presented that describes the flow of nitrogen between crop and soil on the field scale. The model has a compartmental structure and runs on a weekly time-step. Nitrogen enters via atmospheric deposition and by application of fertilizer or organic manures, and is lost through denitrification, leaching, volatilization and removal in the crop at harvest. Organic nitrogen is contained within three of the model compartments - crop residues (including plant material dying off through the growing season), soil microbial biomass and humus. Inorganic nitrogen is held in two pools as NH4+ or NO3-. Nitrogen flows in and out of these inorganic pools as a result of mineralization, immobilization, nitrification, leaching, denitrification and plant uptake. The model requires a description of the soil and the meteorological records for the site - mean weekly air temperature, weekly rainfall and weekly evapotranspiration. The model is designed to be used in a 'carry forward' mode - one year's run providing the input for the next, and so on. The model also allows the addition of N-15 as labelled fertilizer, and follows its progress through crop and soil. Data from a Rothamsted held experiment in which the fate of a single pulse of labelled N was followed over several years were used to set the model parameters. The model, thus tuned, was then tested against other data from this and two contrasting sites in south-east England. Over a period of 4 years, the root mean square (R.M.S.) difference between modelled and measured quantities of labelled N remaining in the soil of all three sites was c. 7.5 kg N/ha, on average. The root mean square error in the measurements was c. 2.5 kg/ha. Similarly, the R.M.S. difference between modelled and measured recovery of labelled N by the crop was 0.6, compared with 0.3 kg/ha in the measurements themselves.