Abstract
To elucidate the pattern of nitrogen N mineralization in paddy soils, we used moist and air-dried soil samples with various fertility levels collected from 100 fields throughout Japan. We incubated moist soils for 4, 7, or and 10 weeks and air-dried soils for 1, 2, 4, or and 10 weeks at 30℃ under submerged conditions and measured N -mineralization during incubation. In addition, we investigated the effect of initial soil moisture content before submergence on the N -mineralization rate, as well as and the relationship of rice yield to the amount of nitrogen N supplied by soil N -mineralization and nitrogen N fertilizer. (1) The pattern of N -mineralization during incubation of moist soils varied by the geographic location and soil group of the collected soil samples. Soils collected from the Sea of Japan sea side of the Tohoku region and Niigata areas exhibited rapid N mineralization after 7 weeks of incubation. Different patterns of N -mineralization increase were observed in Soils amended with organic matter (OM) in the form of compost or rice straw showed different patterns of N mineralization increase. (2) The pattern of N -mineralization during incubation of air-dried soil was did not found to vary by geographic location or OM amendment. Substantial N -mineralization occurred by 2 to 4 weeks of incubation, resulting in clear differences in N -fertility among soil samples. In addition, after 4 weeks of incubation of air-dried soil, we observed a positive correlation between the changes in total nitrogen N and total carbon C resulting from OM amendment and showed a positive correlation with the change in N -mineralization. (3) In soil samples with different As the initial moisture content decreased, the amount of N -mineralization at 4 weeks of incubation was found to increased with decreasing moisture content. The moisture content of soils collected from fields cultivated underused for rice-soybean rotation was lower than that of soils collected from nearby fields used for in the same geographical area undergoing continuous rice production, and the increase in N -mineralization associated with the decrease in lower moisture content was smaller. A strong positive correlation was observed between The difference in the moisture content of OM-amended or wetland rice-soybean rotation soils and that of the control soil and showed a strong positive correlation with the difference in N -mineralization during the incubation of air-dried soil. (4) We observed a strong quadratic correlation between The yield of staple wetland rice cultivars showed a strong quadratic correlation and with the combined nitrogen N supply from soil N -mineralization (estimated at rice maturity) and nitrogen N fertilizer.
