GRAIN LEGUMES AND GREEN MANURES AS PRE-RICE CROPS IN NORTHEAST THAILAND .2. RESIDUE DECOMPOSITION

The loss of dry matter lash corrected), nitrogen (N) and carbon (C) fr om residues of several tropical legume species was monitored using lit ter bags in the field over a three-month period in Northeast Thailand. This work was linked to an experiment in a farmers' field where the r esidual benefits of the same legume species grown before flooded rice were measured. Litter bags were incorporated in the flooded rice plots at the same time as residue incorporation in the field experiment. Th e species studied were Sesbania rostrata, Aeschynomene afraspera and a multi-purpose cowpea variety (Vigna unguiculata cv KVC-7). In the cas e of S. rostrata the breakdown of fresh and oven-dried residues and of residues buried at depths of 2-3 cm and 15 cm was also compared. Alth ough the initial N and C concentrations were similar for all the resid ues they exhibited differing dry matter, N and C loss patterns. With S esbania rostrata, 80% of the N was lost from the residues after 20 day s, however, there was only a 40% decline in C and weight during the sa me period. The rate and amount of N loss from Aeschynomene afraspera r esidues was much less than with S. rostrata, declining by approximatel y 35% during the first 40 days. There were marked differences in rates of N loss from stem and leaves of A. afraspera indicating that monito ring the decomposition of stem and leaves combined can be misleading. In multi-purpose cowpea, loss patterns of dry matter, N and C were all similar and 50-65% was lost after 40 days burial. There was little di fference between breakdown of fresh and oven-dried S. rostrata residue s nor were there noticeable differences between residues incorporated superficially (2-3 cm) and buried at 15 cm. Although both %N and ligni n:N ratios correlated well with weight and N loss from the residues, t his was only the case when leaf and stem material were analyzed separa tely for A. afraspera. Despite the slower rate and smaller total amoun t of N released from the A. afraspera residues compared with the S, ro strata residues, a similar amount and proportion (around 20 kg N ha(-1 ) or 22-28%) of the N was recovered from both residues by a crop of ri ce planted at the time of residue incorporation. This suggests a consi derably higher use efficiency by rice of the N released from the A. af raspera residues (approximately 40%) compared with that for S. rostrat a (30%).