Modelling 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 growth, urine and faeces. After allowing for ammonia volatilization, the rate and area covered by urinary nitrogen is simulated. The fate of each day' s urinary nitrogen deposition is followed separately through time. The mineral nitrogen remaining in the soil at the end of the growing season is assumed to be leached over winter. Leaching from fertilized grass-only and unfertilized grass/clover pastures are compared at varying levels of dry matter (DM) production. Biological fixation is assumed to provide sufficient nitrogen to allow the grass/clover pasture to achieve the same seasonal distribution of production as the fertilized grass-only pasture. There was a non-linear increase in predicted leaching with increasing DM production owing to the aggregation of urinary nitrogen into urine patches; Leaching was lower from grass/clover than the grass-only pasture and the difference increased with DM production. At levels of production common in the UK, the difference in leaching between the two pasture types was small and the spatial distribution of soil mineral nitrogen was uneven, so differences in leaching may be small and difficult to detect.