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

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Abstract
Rice residues labeled with 15N were incorporated in soil and incubated under flooded and upland conditions, and the distribution and transformation of the residual nitrogen in various soil fractions were traced over 2 years. Immobilization of a large amount of soil nitrogen by plant residues brought about a decrease in net mineralized soil-nitrogen, though some residue-origin nitrogen was mineralized. Under flooded conditions large amount of nitrogen loss occurred, probably through denitrification. On the contrary, the loss was very small under upland conditions. Incorporation of soil nitrogen into the rice residues occurred mostly, within the first month, and thereafter the percentages of residue-origin nitrogen in the total plant debris nitrogen did not change so much in the four large size fractions, though decay continued to the end of incubation (24 months). Decrease of residue-origin nitrogen in the plant debris fractions partly resulted in nitrogen tranfer to the silt with fine sand fraction and the clay. This phenomenon occurred vigorously within the first 4 months under both flooded and upland conditions.