Nitrogen Dynamics of Nitrogensaturated and Unsaturated Deciduous Forest Ecosystems on Toyama Plain, Japan

Abstract

Kureha Hill (KR) and Imizu Hill (IM) are located adjacent to one another in the center of the Toyama Plain. Though the recent nitrogen (N) loadings on them are almost same, N output from KR exceeded about 17 times that from IM. Therefore, a previous study indicated that the forest ecosystem at the KR site was N-saturated. To elucidate the mechanism underlying N-saturation in KR using a kinetic N-saturation model, we examined the N sink components of the ecosystem, i.e., N retention in plants and soil and N leaching into hillside streams. The soil N pools did not differ significantly between the two sites, but the net N mineralization and nitrification rates of surface soils (depth: 0–10 cm) were significantly higher in the KR. Foliar N concentrations and litter N cycling rates from plants to soil were higher on the KR hillside, indicating that plant N retention in this site exceeded retention levels in IM. Stream nitrate concentrations were higher in water samples collected in KR, indicating that outward N leaching rates were higher in this site than in IM. High N retention levels in plants in the KR likely increase the rate of litter N cycling, which may in turn accelerate rates of surface soil N mineralization, nitrification, and N leaching. These mechanisms may explain the kinetic N-saturation levels at the KR.