Multi-tracer identification of nutrient origin in the Hii River watershed, Japan

This study aimed to evaluate the loading of nutrients of agricultural origin. We investigated monthly nutrient concentrations at 11 stations located in the Hii River, Japan. The nitrogen and oxygen stable isotope ratios in nitrate were applied to distinguish the origin of nitrogen, i.e., from fertilizers applied to paddy fields or from sewage. Although total nitrogen (TN), presumably from transboundary air pollution, was mainly loaded during the cooler season, nitrate originating from fertilizers applied to paddy fields became the main source of nitrogen in the river water during the warmer season. Phosphorus was mainly added in particulate form, and showed increased loading at the upstream stations in the warmer season, but not in the cooler season. Potassium and magnesium—components of fertilizers—showed an increasing trend in the downstream section of the paddy fields. Our results suggest that controlled application of fertilizers is necessary to decrease the nitrogen loads originating from farmlands, particularly from paddy fields. Since the nitrogen isotope of TN in fertilizer showed significantly lower values (mean value −4.6 ‰) than that in river water (mean value 1.8 ‰) or treated water (mean value 21.9 ‰), we could use these values to determine the contribution of TN from fertilizers to river water quality, and can use them to monitor fertilization levels in watersheds.