四万十川源流部の森林における降雨時の硝酸態窒素流出特性

Abstract

The characteristics of NO3--N cumulative specific load during stormy flood events were investigated in a forested, mountainous headwater catchment of Shimanto River basin, located at the southwestern part of Shikoku Island, Japan. The vegetation in the catchment was a natural forest. Seventeen flood events were investigated. The intercept of the regression line between the cumulative specific discharge and the NO3--N cumulative specific load in term A (from the end of the rainy season in July through September) was significantly different from that in term B (other than term A) (p < 0.01). The slope of the line was smaller (approximately 0.8) than those in previous studies (greater than 1). NO3--N in surface soil (0 to 5 cm depth from the surface) was most abundant in August, and less in winter and spring than in summer. It was considered that NO3--N in surface soil increased owing to active mineralization and nitrification in summer. These results indicated that the NO3--N cumulative specific load under the same cumulative specific discharge conditions tends to increase in summer, and that the increase rate of the NO3--N cumulative specific load relative to cumulative specific discharge tends to decrease in the studied catchment.