Patterns of Soil Nitrogen Release in Paddy Fields of Warm Regions in Japan and Diagnosis of Fertilizer Application : II. Characteristics of soil nitrogen mineralization and nitrogen uptake patterns of rice plants in Fukuoka Prefecture

Abstract

Topsoils collected from various regions in Fukuoka Prefecture were incubated and the amount of mineralized nitrogen was measured. The relationship between the mineralized nitrogen amount (N) and the incubation time (t) fitted to the formula : N=N₀ (1-e^-kt)+B, and the nitrogen mineralization was characterized by the characteristic parameters : N₀, N-mineralization potential ; k, mineralization rate constant at 25°C ; and Ea, apparent activation energy. The value of k differed remarkably among soils, and the amount of soil nitrogen released (SNR) increased in the case of paddy soils with high k values. In the same type of soils, or similar k values, the amount of SNR was influenced by the value of N₀. The amounts of soil nitrogen absorbed by rice plants (SNA) differed considerably among fields. In the field with low fertility, rice plants uptook soil nitrogen less especially in the early stages in comparison to the field with high fertility. The average value of SNA/SNR was about 35% from transplanting to 25 days after transplanting. The value was 70-80% from 35 days after transplanting to harvesting, indicating that the soil introgen released was efficiently absorbed by rice plants at the later stages. Though the amounts of fertilizer nitrogen uptake differed among fields, the difference was considerably less than that of soil nitrogen uptake. From results of the above field experiment, we proposed the recommended amounts of nitrogen uptake through growth stages for each level of rice yield.