MODELING MINERAL NITROGEN ACCUMULATION IN GRAZED PASTURE - WILL MORE NITROGEN LEACH FROM FERTILIZED GRASS THAN UNFERTILIZED GRASS CLOVER

A model of herbage growth and nitrogen dynamics for grazed pasture was developed. Grazed herbage nitrogen is partitioned between animal grow th, urine and faeces. After allowing for ammonia volatilization, the r ate and area covered by urinary nitrogen is simulated. The fate of eac h day' s urinary nitrogen deposition is followed separately through ti me. The mineral nitrogen remaining in the soil at the end of the growi ng season is assumed to be leached over winter. Leaching from fertiliz ed grass-only and unfertilized grass/clover pastures are compared at v arying levels of dry matter (DM) production. Biological fixation is as sumed to provide sufficient nitrogen to allow the grass/clover pasture to achieve the same seasonal distribution of production as the fertil ized grass-only pasture. There was a non-linear increase in predicted leaching with increasing DM production owing to the aggregation of uri nary nitrogen into urine patches; Leaching was lower from grass/clover than the grass-only pasture and the difference increased with DM prod uction. At levels of production common in the UK, the difference in le aching between the two pasture types was small and the spatial distrib ution of soil mineral nitrogen was uneven, so differences in leaching may be small and difficult to detect.