Fate and recovery of N-15 derived from grass/clover residues when incorporated into a soil and cropped with spring or winter wheat for two succeeding seasons

The fate of N when N-15-labelled perennial ryegrass/white clover residues were incorporated into field lysimeters (rate equivalent to 150 kg N ha(-1)) and two successive crops of winter or spring wheat were grown was investigated. Loss of N-15 over the first winter amounted to 23% and 22% respectively for winter and spring wheat and corresponding losses in the 2nd year were 10% and 14%. Both winter- and spring-sown crops 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 spring 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 the soil organic matter fraction and at harvest of the 2nd year 55% of added N-15 was present as soil organic N. In order to investigate the nature of this soil organic N-15, soil was fractionated into different particle size separates. Both N-14 and N-15 were concentrated in the medium and fine silt and coarse, and to a lesser extent, medium and fine clay fractions. However, in spring of the first season, N-15 was preferentially present in the floating organic matter and to a lesser extent sand-sized fractions (i.e. as particles of decomposing residue). Between then and harvest of the second season there was a redistribution of N-15 from these fractions and the medium- and fine-sized clay particles toward the coarse and medium silt-sized fractions. This suggested a movement of N-15 toward more aromatic humified material in silt-sized fractions and away from decomposing organic material in sand separates and away from labile organic material in the clay-sized fractions. A laboratory incubation experiment showed that the availability (percentage mineralization) of recently immobilized N-15 was greater than that of native soil N-14.