Effects of N-15-labelled crop residues and management practices on subsequent winter wheat yields, nitrogen benefits and recovery under field conditions

Nitrogen-15 enriched ammonium sulphate was applied to micro-plots in a fiel d in which two leguminous (white clover and peas) and two non-leguminous (r yegrass and winter wheat) crops were grown to produce N-15-labelled crop re sidues and roots during 1993/94. Nitrogen benefits and recovery of crop res idue-N. root-N and residual fertilizer-N by three succeeding winter wheat c rops were studied. Each crop residue was subjected to four different residu e management treatments (ploughed, rotary heed. mulched or burned) before t he first sequential wheat crop (1994/95) was sown, followed by the second ( 1995/963 and third wheat crops (1996/97), in each of which residues of the previous wheat crop were removed and all plots were ploughed uniformly befo re sowing. Grain yields of the first sequential wheat crop followed the ord er: white clover > peas > ryegrass > wheat. The mulched treatment produced significantly lower grain yield than those of other treatments. In the firs t sequential wheat crop, leguminous and non-leguminuus residues supplied be tween 29-57% and 6-10% of wheat N accumulated respectively and these decrea sed with successive sequential crops. Rotary heed treatment reduced N benef its of white clover residue-N while no significant differences in N benefit s occurred between residue management treatments in non-leguminous residues . On average, the first wheat crop recovered between 29-37% of leguminous a nd 11-13% of nonleguminous crop residues-N. Corresponding values for root p lus residual fertilizer-N were between 5-19% and 2-3%, respectively. Manage ment treatments produced similar effects to those of N benefits. On average . between 5 to 8% of crop residue-N plus root and residual fertilizer-N was recovered by each of the second and third sequential wheat crops from legu minous residues compared to 2 to 4% from non-leguminous residues. The N rec overies tended to be higher under mulched treatments especially under legum inous than non-leguminous residues for the second sequential wheat crop but were variable for the third sequential wheat crop. Relatively higher propo rtions of leguminous residue-N were unaccounted in ploughed and rotary heed treatments compared with those of mulched and burned treatments. In non-le guminous residue-N, higher unaccounted residue N occurred under burned (33- 44%) compared with other treatments (20-27%).