The fate at several time intervals of ¹⁵ N‐labelled ammonium nitrate applied to an established grass sward

SUMMARY 15 N‐labelled ammonium nitrate solution was applied in late April to circular, enclosed micro‐plots prepared by pressing open‐ended polypropylene cylinders into an established sward of perennial ryegrass. Cylinders were removed from the ground at intervals between 2 and 370 days after the application and assessments made of the distribution of 15 N in plant and soil components. Of the added labelled N, 54.7% was recovered in the herbage which was cut four times during the growing season and again at the final sampling date. After two days, 37% of the labelled N was recovered in the soil microbial biomass. Large fluctuations occurred in the amount of 15 N recovered in the soil microbial biomass over the next 14 days suggesting that rapid cycling of 15 N occurred between this fraction and the mineral N fraction. After the first cut in late May, translocation of 15 N occurred more slowly from the roots into the stubble than from stubble into new herbage, so that the amount in the stubble declined more rapidly than did that in the roots. During the winter, there was no net transfer of N from the roots to above‐ground components of the sward. By the end of the growing season, half the 15 N remaining in the sward was immobilized in the humified fraction of the soil organic matter; some of this was mineralized in the following spring.