Recovery of N-15-labelled fertiliser by a perennial ryegrass seed crop anda subsequent wheat crop

Large amounts of nitrogen (N) fertiliser (150-200 kg N/ha) are currently be ing applied to perennial ryegrass (Lolium perenne L.) seed crops in New Zea land. Due to increasing requirements for efficient use of N fertilisers and minimising nitrate contamination of the environment, a field experiment wa s 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 N/ha), August (30 kg N/ha), September (60 kg N/ha) and October (60 kg N/ha) . The urea-N-15 was applied in solution and watered in to minimise volatili sation loss. At the time of harvest (December), 9% of the applied N-15 was in the seed, 29% 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 several applic ations, each one at a relatively low rate, and at times when leaching was u nlikely to occur. Ryegrass plants used a greater proportion of the N applie d in September and October (61-65%) compared with that applied in April (44 %). Consequently more N was recovered from the soil in the autumn applicati on (57%) than from the September and October applications (28-44%). The ava ilability of the residual fertiliser N to a subsequent wheat (Triticum aest ivum L.) crop was studied in a glasshouse experiment. The residual fertilis er N was present in the soil and ryegrass roots and stubble. The wheat plan ts only recovered 7-9% of this residual N. Most of the N taken up by the wh eat came from the soil organic N pool. Overall, applying a total of 180 kg N/ha to the ryegrass appeared to have minimal direct impact on the environm ent. In the short term N not used by the ryegrass plants contributed to the soil organic N pool.