Nitrogen accumulation in soybean [Glycine max (L.) Merr.] is increased by manure compost application in drained paddy fields as a result of increased soil nitrogen mineralization

The application of manure compost is an effective way to increase soybean [ Glycine max (L.) Merr.] yield and nitrogen (N) fertility in drained paddy fields. We investigated changes in soil N mineralization during soybean cultivation using reaction kinetics analysis to determine the contribution of increased N mineralization after manure compost application (at a rate of 0 to 6 kg m ⁻²) on N accumulation and seed yield of soybean under drained paddy field conditions. The seed yield and N accumulation decreased markedly in the second and third year of the experiment, but soil N mineralization increased in both years. No decrease in soil N mineralization occurred even after two soybean crops. Soil N availability was not the main cause of decreased soybean yield in the second and third years. The differences in plant aboveground N content between plots with and without manure compost was similar to the increase in N mineralization caused by manure compost application in the second and third years. The application of 6 kg m ⁻² of manure compost increased the amount of ureide-N and nitrate-N in soybean in the third year. Our results suggest that manure compost application increases soil N mineralization and soybean N ₂ fixation, resulting in increased N accumulation and seed yield. However, the soybean yield remained less than 300 g m ⁻² in the second and third years (i.e., below the yield in the first year) at all levels of manure compost application due to the remarkable decrease of N accumulation in the second and the third crops.