Influence of a perennial ryegrass seed crop on soil nitrogen status

Increasing amounts of nitrogen (N) fertiliser are being used on ryegrass seed crops to ensure high yields are obtained. However, there are increasing concerns about environmental contamination due to excess nitrogen. Consequently, a field experiment was established using N-15-labelled fertiliser to follow the fate of applied N. Urea-N-15 was applied to a perennial ryegrass seed crop in April (30 kg/ha of N), August (30 kg/ha of N), September (60 kg/ha of N) and October (60 kg/ha of N). At the time of harvest, only 10% of the applied N-15 was removed in the seed, 29% remained in the straw, 19% in the roots and 39% in the soil organic matter. Losses of N-15 were minimal as the N was applied in relatively low amounts and at times when leaching was unlikely to occur. Compared with the amount of fertiliser N applied, small amounts of N were removed in harvested seed and baled straw, resulting in a net accumulation of N in the soil. The plant availability of the residual fertiliser N was very low with only 6.6% of the fertiliser remaining in the soil and stubble recovered by a subsequent wheat crop. Most of the N taken up by the wheat plants came from the soil organic N pool. Overall the high rate of N applied to the ryegrass appeared to be efficiently used by the crop with minimal impact on the environment. Nitrogen not used by the ryegrass plants contributed to the soil organic N pool.