Gross Soil N Transformations in a Coniferous Forest in Japan

Abstract

Topographic influence on gross soil N transformation was investigated along a slope in a conifer plantation forest (45-yr-old Cryptomeria japonica) on Mt Ryuoh, Shiga, Japan, by studying net N mineralization and net nitrification rates of soil samples in the laboratory. The samples were collected from 9 sites on an altitudinal gradient of 765 to 841 m, established across (up) the slope along a 120-m transect at 15 m intervals from the lowest site. Half of the mineralized N was immobilized and half was left in the NH4+ pool in samples from the upper part of the slope, while 93% of mineralized N was used for immobilization or nitrification in samples from the lower part of the slope. From the similarity of microbial biomass and microbial C/N, microbial flora was judged to be similar among the sites. However, the gross mineralization rate was 4 times faster below 15 m than above 30 m from the lowest site, indicating that the substrate was decomposable below 15 m. On the upper part of the slope with Oa horizon, humified organic matter with high C/N would be resistant to decomposition by microorganisms, resulting in relatively slow N cycling