Spatial Variabillity of Soil Nitrogen Dynamics slong a Slope in a Cryptomeria japonica D.Don Plantation

Spatial variabillity of soil nitrogen(N) dynamics was examined along a slope in a 45-year-old Cryptomeria japonica D.Don plantation in the southeastern paet of Shiga Prefecture. Net N mineralization showed no clear gradient along the slope, while net nitrification and percent nitrification were high at lower positions,and very low at upper positions of the slope. Principal component(PC) analysis showed soil properties were divided into three groups which were PC1(water content and pH), and PC3(total C and N). Regressions of net soil N transformations against PCs scores suggested net N mincralization was mainly regulated by PC3, while net nitrification and percent nitrification were mainly regulated by PC1 and PC2. The difference in the main form of inorganic N was expected to affect soil N availability for plants through the difference in the mobility in negatively changed forest soil. We evaluated the soil N availability that reflebted the ion mobility factors by ion exchange resins and found that greater mobility of nitrate lead to greater soil N availability at lower positions of the slope. In addition, gross N transformations were directly measured using 15N isotope dilution, and the influence of microbial caebon (C) availability on the internal soil N cycles were examined by long-term laboratory incubation. Gross nitrifivation was detected in both soils taken at upper and lower positions on the slope, suggesting that nitrification was also an important process at upper positions where almost no net nitrification was detected. Changes in net and gross N transformations, the organic C and N pools, and microbial respiration suggested that start of rapid net nitrifibation might be affected not only by the availability of C for microbial biomass, but also by the relative availability of C and N