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

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Abstract
SUMMARY: The release and fate of nitrogen from15N‐labelled perennial ryegrass (Lolium perenneL.) and white mustard (Sinapis albaL.) catch crops were studied in field microplots. The initial decline in15N‐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 organic15N 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 ryegrass15N, 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 vulgareL.) 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. The15N that was not accounted for may be present in the 10–45 cm depth, or it may have been lost by denitrification.