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

Abstract

Nitrogen-15 enriched ammonium sulphate was applied to micro-plots in a field in which two leguminous (white clover and peas) and two non-leguminous (ryegrass and winter wheat) crops were grown to produce 15N-labelled crop residues and roots during 1993/94. Nitrogen benefits and recovery of crop residue-N, root-N and residual fertilizer-N by three succeeding winter wheat crops were studied. Each crop residue was subjected to four different residue management treatments (ploughed, rotary hoed, mulched or burned) before the first sequential wheat crop (1994/95) was sown, followed by the second (1995/96) and third wheat crops (1996/97), in each of which residues of the previous wheat crop were removed and all plots were ploughed uniformly before sowing. Grain yields of the first sequential wheat crop followed the order: white clover > peas > ryegrass > wheat. The mulched treatment produced significantly lower grain yield than those of other treatments. In the first sequential wheat crop, leguminous and non-leguminous residues supplied between 29–57% and 6–10% of wheat N accumulated respectively and these decreased with successive sequential crops. Rotary hoed treatment reduced N benefits of white clover residue-N while no significant differences in N benefits occurred between residue management treatments in non-leguminous residues. On average, the first wheat crop recovered between 29–37% of leguminous and 11–13% of non-leguminous crop residues-N. Corresponding values for root plus residual fertilizer-N were between 5–19% and 2–3%, respectively. Management 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 leguminous residues compared to 2 to 4% from non-leguminous residues. The N recoveries tended to be higher under mulched treatments especially under leguminous than non-leguminous residues for the second sequential wheat crop but were variable for the third sequential wheat crop. Relatively higher proportions of leguminous residue-N were unaccounted in ploughed and rotary hoed treatments compared with those of mulched and burned treatments. In non-leguminous residue-N, higher unaccounted residue-N occurred under burned (33–44%) compared with other treatments (20–27%).