FATE AND RECOVERY OF N-15 DERIVED FROM GRASS CLOVER RESIDUES WHEN INCORPORATED INTO A SOIL AND CROPPED WITH SPRING OR WINTER-WHEAT FOR 2 SUCCEEDING SEASONS/

The fate of N when N-15-labelled perennial ryegrass/white clover resid ues were incorporated into field lysimeters (rate equivalent to 150 kg N ha(-1)) and two successive crops of winter or spring wheat were gro wn was investigated. Loss of N-15 over the first winter amounted to 23 % and 22% respectively for winter and spring wheat and corresponding l osses in the 2nd year were 10% and 14%. Both winter- and spring-sown c rops recovered about 10% of residue N-15 in the first season and about 1.5% in the second. The percentage of total crop N uptake originating from residue N-15 was only 14% and 12% respectively for winter and sp ring wheat in the first season and 2.5% and 1.9% respectively for the second season. The bulk of the N-15 recovered was incorporated into th e soil organic matter fraction and at harvest of the 2nd year 55% of a dded N-15 was present as soil organic N. In order to investigate the n ature of this soil organic N-15, soil was fractionated into different particle size separates. Both N-14 and N-15 were concentrated in the m edium and fine silt and coarse, and to a lesser extent, medium and fin e clay fractions. However, in spring of the first season, N-15 was pre ferentially present in the floating organic matter and to a lesser ext ent sand-sized fractions (i.e. as particles of decomposing residue). B etween then and harvest of the second season there was a redistributio n of N-15 from these fractions and the medium- and fine-sized clay par ticles toward the coarse and medium silt-sized fractions. This suggest ed a movement of N-15 toward more aromatic humified material in silt-s ized fractions and away from decomposing organic material in sand sepa rates and away from labile organic material in the clay-sized fraction s. A laboratory incubation experiment showed that the availability (pe rcentage mineralization) of recently immobilized N-15 was greater than that of native soil N-14.