Influence of the agricultural land agglomeration to the nutrients of the river water in the Tokachi River basin.

The deterioration of river water quality as a result of agricultural activities in the Tokachi River basin is concerning. However, it is not easy to reduce fertilization or to decrease agricultural land area while maintaining crop yields as an effort of water quality conservation. We investigated the total nitrogen (T-N) and the total phosphorus (T-P) concentrations and the N:P ratio (atomic and mass) in the Tokachi River basin during 2007-2011 under base flow conditions. The proportion of agricultural land and the Spatial Continuity (SC) of agricultural land were analyzed to examine effective land use management for water quality conservation. T-P showed comparatively low concentrations (0.03-0.26 mg/L) because Andosols in the agricultural land, which have high phosphate absorption coefficients. While the T-N concentrations (0.4-6.6 mg/L) and the N:P mass ratio (6.3-39) tended to high values. T-N concentration and the N:P ratio had positive correlations with the proportion and the SC of agricultural land. In particular, the correlation coefficients of the relationships with the SC of agricultural land were higher than those of the proportion of agricultural land. In watersheds having more than 40 % of agricultural land, it was confirmed that there were some tributaries which had a high proportion of agricultural land but had low SC of agricultural land. Besides, the results suggested that the nitrogen runoff could be suppressed by setting the connectivity of agricultural land with certain scale without reducing the proportion of agricultural land in the Tokachi River basin significantly. [ABSTRACT FROM AUTHOR]