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

A computer model is presented that describes the flow of nitrogen betw een crop and soil on the field scale. The model has a compartmental st ructure and runs on a weekly time-step. Nitrogen enters via atmospheri c 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. In organic nitrogen is held in two pools as NH4+ or NO3-. Nitrogen flows in and out of these inorganic pools as a result of mineralization, imm obilization, 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 an d weekly evapotranspiration. The model is designed to be used in a 'ca rry forward' mode - one year's run providing the input for the next, a nd so on. The model also allows the addition of N-15 as labelled ferti lizer, and follows its progress through crop and soil. Data from a Rot hamsted held experiment in which the fate of a single pulse of labelle d N was followed over several years were used to set the model paramet ers. The model, thus tuned, was then tested against other data from th is 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 s ites 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 betwe en modelled and measured recovery of labelled N by the crop was 0.6, c ompared with 0.3 kg/ha in the measurements themselves.