THE RELEASE AND FATE OF NITROGEN FROM CATCH-CROP MATERIALS DECOMPOSING UNDER FIELD CONDITIONS

The release and fate of nitrogen from N-15-labelled perennial ryegrass (Lolium perenne L.) and white mustard (Sinapis alba L.) catch crops were studied in field microplots. The initial decline in N-15-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 N-15 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 N-15, 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 N-15 that was not accounted for may be present in the 10-45 cm depth, or it may have been lost by denitrification.