Hydrological effects on relationships between δ¹⁵N of river nitrate and land use in a rural river basin, Western Japan

This study aimed to examine how the relationship between δ¹⁵N of nitrate (δ¹⁵N_NO3) in rivers and land use within a river basin changes with varying hydrological conditions. This information would aid in identifying the dominant source contributing to increased nitrate concentrations in rural rivers. For this, δ¹⁵N_NO3 in river water was investigated monthly in the five subbasins of the Hii River basin (area: 911km²), western Japan, for 1year and 3months. There were significant correlations (p<0.05) between δ¹⁵N_NO3 and the land-use ratio (i.e. ratios of forested, agricultural and residential areas in a subbasin) for the majority of the observation days, indicating that δ¹⁵N_NO3 reflected land use within the basin. δ¹⁵N_NO3 ranged from +1.4‰ to +8.5‰ and was lower in a subbasin with a higher forested area ratio. We found that the absolute value of the regression slope of the relationship between δ¹⁵N_NO3 and the land-use ratio decreased with increasing river discharge. This finding demonstrates that differences in δ¹⁵N_NO3 among subbasins with different land-use compositions became smaller under higher flow conditions. Because δ¹⁵N_NO3 decreased with increasing river discharge, the small absolute value of the regression slope under high flow conditions indicates that forested areas could be the dominant source of river nitrate during high flows in all subbasins investigated regardless of land-use composition. The results suggest that forested areas make a large contribution to the increase in nitrate concentration in downstream rivers during high flows, because the nitrate concentration increased with increasing river discharge.