群馬県神流川渓流域の森林の窒素飽和現象に関する研究

Abstract

Nitrogen saturation (N-saturation) is a phenomenon caused when nitrogen is saturated in forest soil, which is input into river water. Our purpose was to estimate the degree of N-saturation in the upstream area of the Kanna River in Gunma Prefecture, which has never been contaminated by industrial wastewater and domestic drainage. Here, we analyzed changes in the concentrations of nitrogen species (NO3−) and coexisting components in the river water and the forest soil. The water and soil samples were collected from nine sampling points between 2015 and 2016. Soil samples collected near the river showed the same levels of metallic element content as those in typical forest soil. This NO3− concentration belongs to "Stage 1" of the four N-saturation stages (Stages 0 – 3). The NO3− concentration in the water samples had very low correlations to the concentrations of co-existed ions. In contrast, the nitrogen content in the soil samples had a positive correlation to Ca, while the detail mechanism on the relationship could not be elucidated in this study. Rather, the NO3− concentration was considered to be strongly affected by air pollution in the metropolitan due to geographical and meteorological factors in Gunma Prefecture. Because this stage is an initial step towards serious adverse effect with N-saturation, the maintenance such as forest thinning will be required in order to inhibit the progress.