MODELING THE CHANGE IN SOIL ORGANIC C AND N AND THE MINERALIZATION OFN FROM SOIL IN RESPONSE TO APPLICATIONS OF SLURRY MANURE

A computer simulation model of the turnover of organic matter in soil was adapted to simulate the change in soil organic C and N contents of soil during several years following annual additions of farm slurry t o maize fields. The model proved successful in estimating the build-up of both C and N in soil and the leaching of N to ground-water in resp onse to applications of slurry ranging from 50 to 300 tons per hectare per year. The model was then used to estimate the build-up of organic matter in soil under crops of fodder maize that were grown using the excess of manure produced during the last 20 years in the Netherlands. The build-up of organic matter from these applications was estimated to lead to about 70 kg extra nitrogen mineralized ha(-1) yr(-1). As a result of legislation manure applications have decreased and are expec ted to decrease further in the immediate future. Calculations suggest that after 10 years of manure applied at rates no longer exceeding the amount needed to replace the phosphorus removed by crops, the extra m ineralization of N will still be between 45 and 60 kg ha(-1) yr(-1). I f manure applications cease altogether then the extra mineralization w ill be about 25-30 kg N ha(-1) yr(-1).