Seasonal changes and controlling factors of gross N transformation in an evergreen plantation forest in central Japan

Abstract

We conducted a year-round measurement of gross N transformation rates using the 15N dilution method, and analyzed seasonal changes and the mechanisms regulating gross N transformation in the Kiryu Experimental Forest in central Japan. While soil microbial biomass C (SMB-C) decreased from the dormant to growing seasons at the organic (O) horizon, no significant trend was observed in SMB-N. This resulted in SMB-C/N being high in the dormant season and low in the growing season, and suggests that the microbial composition changed seasonally. No clear seasonal trend was found in gross NH4+ production rates at either the O or surface mineral soil horizons. In contrast, the NH4+ consumption rate varied seasonally, with high values in January and April during the dormant season and low values in July and October during the growing season. There was no clear trend in seasonal fluctuation of net NH4+ production rates. Gross NH4+ production and gross NH4+ consumption rates were 10 times greater than the gross nitrification rate. Almost all of the produced NH4+ was immobilized, indicating that N tightly cycles at this study site. Considered together with results of the gross N transformation rates, the dominance of high SMB-C/N microbes might stimulate immobilization in the dormant season. At this study site, the change in microbial composition likely influences gross N transformation through immobilization efficiency.