Grain legumes and green manures as pre-rice crops in Northeast Thailand

Abstract

The loss of dry matter (ash corrected), nitrogen (N) and carbon (C) from residues of several tropical legume species was monitored using litter 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 residual 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. The species studied were Sesbania rostrata, Aeschynomene afraspera and a multi-purpose cowpea variety (Vigna unguiculata cv KVC-7). In the case 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. Although the initial N and C concentrations were similar for all the residues they exhibited differing dry matter, N and C loss patterns. With Sesbania rostrata, 80% of the N was lost from the residues after 20 days, however, there was only a 40% decline in C and weight during the same period. The rate and amount of N loss from Aeschynomene afraspera residues was much less than with S. rostrata, declining by approximately 35% during the first 40 days. There were marked differences in rates of N loss from stem and leaves of A. afraspera indicating that monitoring 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 difference between breakdown of fresh and oven-dried S. rostrata residues nor were there noticeable differences between residues incorporated superficially (2-3 cm) and buried at 15 cm. Although both %N and lignin :N ratios correlated well with weight and N loss from the residues, this was only the case when leaf and stem material were analyzed separately for A. afraspera. Despite the slower rate and smaller total amount of N released from the A. afraspera residues compared with the S. rostrata 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 rice planted at the time of residue incorporation. This suggests a considerably higher use efficiency by rice of the N released from the A. afraspera residues (approximately 40%) compared with that for S. rostrata (30%).