The release and fate of nitrogen from catch‐crop materials decomposing under field conditions

SUMMARY The release and fate of nitrogen from 15 N‐labelled perennial ryegrass ( Lolium perenne L.) and white mustard ( Sinapis alba L.) catch crops were studied in field microplots. The initial decline in 15 N‐labelled organic N, after incorporation of the material in early December, was more rapid from mustard containing 2.6% N than from ryegrass containing 1.4% N. After 9 months of decomposition, the residual organic 15 N from the two materials declined at the same rate; the average decay constant for the following 2 years of decomposition was 0.30 a −1 . After 33 months of decomposition, 23% and 34% of the mustard and ryegrass 15 N, respectively, was recovered in organic residues in the topsoil. Seven per cent of the ryegrass N was leached below 45 cm in micro‐lysimeters during the winter following incorporation. Three spring barley ( Hordeum vulgare L.) crops, which succeeded ryegrass incorporation, accumulated 19%, 4% and 2%, respectively, of the ryegrass N in the above‐ground plant parts. Perennial ryegrass swards recovered a total of 26% of the ryegrass and 22% of the mustard catch‐crop N within 2 years. After 2 years of decomposition in unplanted soil, 82% of the ryegrass N was accounted for. The 15 N that was not accounted for may be present in the 10–45 cm depth, or it may have been lost by denitrification.