ORIGINAL PAPER Multi-tracer identification of nutrient origin in the Hii River watershed, Japan Yu Tabayashi 1,2,4,5 • Kentaro Miki 2 • Toshiyuki Godo 3 • Masumi Yamamuro 2,4 • Hiroshi Kamiya 3,6 Received: 24 December 2014 / Revised: 16 June 2016 / Accepted: 5 July 2016 / Published online: 20 October 2016 Ó International Consortium of Landscape and Ecological Engineering and Springer Japan 2016 Abstract 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 trans- boundary 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 neces- sary to decrease the nitrogen loads originating from farm- lands, particularly from paddy fields. Since the nitrogen isotope of TN in fertilizer showed significantly lower val- ues (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. Keywords River water quality Á Nitrogen Á Phosphorus Á Fertilizer Á Stable isotope Introduction Since the industrial revolution, the widespread con- sumption of fossil fuels and use of fertilizers have led to the accumulation of large quantities of reactive nitrogen, which has a significant impact on the envi- ronment and the economy. For example, in Europe, excessive release of nitrogen into the environment has been estimated to cause annual economic damage amounting to 70–320 billion Euros (Sutton et al. 2011). More than half of this nitrogen in river water originates from agriculture, and approximately one-quarter origi- nates from sewage. Many recent studies in Japan have focused on nitro- gen runoff and nitrogen pollution (Hayakawa et al. 2009; Woli et al. 2008). Although numerous studies have investigated denitrification in paddy fields (Bronson et al. 1997; Klu ¨ber and Conrad 1998), whether they act as a load source at the watershed scale is not yet known; only few studies have monitored denitrification activity at dozens of stations. Nitrogen is loaded into paddy & Yu Tabayashi forh-he@hotmail.co.jp 1 College of Economics, Kanto Gakuin University, 1-50-1 Mutsuura-higashi, Kanazawa, Yokohama 236-8501, Japan 2 Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa 277-8563, Japan 3 Shimane Prefectural Institute of Public Health and Environmental Science, 582-1 Nishihamasadacho, Matsue 690-0122, Japan 4 Institute of Geology and Geoinformation, National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba 305-8567, Japan 5 Center for Research in Isotopes and Environmental Dynamics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8577, Japan 6 Research Center for Coastal Lagoon Environments, Shimane University, 1060 Nishikawatsu, Matsue, Shimane 690-8504, Japan 123 Landscape Ecol Eng (2017) 13:119–129 DOI 10.1007/s11355-016-0307-5