Short-term cycling of15N-urea in a ryegrass—white clover pasture

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Abstract
A field 15N microplot experiment was conducted to determine the fate of urine N (simulated by 15N urea added at c. 300 kg N/ha) under dry summer conditions in a soil growing a ryegrass (Lolium perenne)-white clover (Trifolium repens) pasture. Ammonium sulphate and potassium nitrate treatments also were included. The amounts and 15N distribution of inorganic N, soil organic N, and plant top and root N were determined at 1, 3, 7, and 22 days after initiation of the experiment. Quantitative recovery of 15N was obtained on day 1, indicating minimal volatilisation losses of NH3 from the urea-treated plots. However, volatilised NH3-N may have been absorbed by the plant canopy, since 15N recovery was higner in the herbage from the urea-treated than from the ammonium sulphate-treated plots. Apparent 13N immobilisation was rapid with the urea and ammonium sulphate treatments but much less with the potassium nitrate treatment. Evidence indicated that with the formet two sources of N, immobilisation was due at least in part to microbial turnover. Plant uptake of 15N increased with time, and was 19.4%, 10.6%, and 5.7% of the added 15N by 22 days with the urea, ammonium sulphate, and potassium nitrate treatments, respectively. By 22 days, 15N recovery was about 89%, 61%, and 70% for the urea, ammonium sulphate, and potassium nitrate treatments, respectively. Some nitrate was leached from the potassium nitrate treated plots early in the experiment.