Abstract
We investigated the relation of the amount of available N to soil biomass and organic material amended in order to examine the source of available N and process of accumulation of available N by the amendment of organic material. The soil samples used were collected from 3 upland fields (volcanic ash soil) amended with organic material continuously. Available N was measured by the incubation method (the aerobic incubation for 4 weeks at 30℃) using air-dried soil. High positive correlation was found between the amount of available N and soil biomass (biomass N and ATP) R^2 : 0.66-0.92). Available N was slao highly correlated with the amount of hot water-soluble organic substance C in soil (R^2 : 0.76-0.85). The amounts of soil biomass N (17.3-76.5 mg kg^<-1> dry soil) were 46-112% of that of available N (30-81 mg kg^<-1> dry soil) ; both values were almost identical. The amount of available N was highly correlated with the amount of easily decomposable fraction (hot water-soluble substance, carbo-hydrate, and crude protein) C in organic material amended (R^2 : 0.64-0.95). Based on these results, we concluded that biomass fraction in soil was a main source of available N, and the increase in soil biomass by the amendment of organic material resulted in the accumulation of available N in soil.
