Microbial degradation of¹⁵N-labeled rice residues in soil during two years' incubation under flooded and upland conditions

The decay of rice residue was investigated after incubation periods of from 1 to 24 months at 30°C under both flooded and upland soil conditions. Tops and roots of rice plants were cut into about 10-mm length, and separately incorporated in soil which had been passed through a 0.5-mm sieve. Plant debris were fractionated physically according to their sizes and divided into five groups (>4 mm, 4-2 mm, 2-1 mm, 1-0.5 mm, and 0.5-0.25 mm). Carbon loss from the soils amended with rice residues and decrease in the weight of total plant debris proceeded at a rapid speed in the early periods (around 4 months) and then at a slow speed in the subsequent periods under both flooded and uplana soil conditions. The distribution of the plant debris in the decomposition processes differed under flooded and upland conditions. Under flooded conditions, 2–4 mm-sized plant debris were retained for a long period with slow transformation into the smaller fractions. In contrast, under upland conditions, change of plant debris from large to small size fractions proceeded gradually. This continuous change could be attributed to the high decomposing activities of fungi under upland conditions.