Nitrogen and Oxygen Isotope Composition of Nitrate in Stream Water of Fuji River Basin

Abstract

To identify the nitrate source and spatial distribution of nitrogen isotope values originated from forest and agricultural lands, fifty-seven stream water samples from Fuji River Basin were analyzed for nitrate concentration and nitrate-nitrogen (δ 15 N-NO 3-) and oxygen (δ 18 O-NO 3-) isotope values. The δ 18 O-NO 3 showed that the nitrate originated through nitrification. These values were plotted against fraction of each land use category in the study catchments. The δ 15 N-NO 3 was positively correlated with the fractions of agricultural and residential areas in the catchments. The increasing of δ 15 N-NO 3 with fraction of agricultural area converged to 5.4‰ which was similar to a reported nitrate isotope value of groundwater at orchard area in Kofu-Basin. The increasing δ 15 N-NO 3 with more than 1% residential area suggested that potential source of nitrate contamination was domestic waste water. An increase of δ 15 N-NO 3 in the catchments with less than 98% forest area suggested the sources of nitrate contamination were anthropogenic. The estimated value of δ 15 N-NO 3 in forest catchment was 0.9±1.2‰.

Hydrological Effects on Relationships Between δ15N of River Nitrate and Land Use in a Rural River Basin, Western Japan

“…Nakamura et al . () demonstrated that river δ 15 N NO3 remained low (approximately less than +2.5‰) in a basin along the Fuji River, Japan, that had a forested area ratio of more than 98%, whereas river δ 15 N NO3 exceeded +5‰ in a basin that had a residential area ratio of more than 1%. Therefore, the relationship between river δ 15 N NO3 and the land‐use ratio in a basin provides the threshold of land‐use ratio that can critically affect changes in river NO 3 − concentration.…”

“…However, the study conducted by Nakamura et al . () was based on data obtained during nonprecipitation periods, i.e., low flows.…”

“…Regression analysis of the relationship between NO 3 À concentration in rivers and land use within a basin is a simple and comprehensive approach in exploring which land-use type predominantly affects changes in river NO 3 À concentration (Johnson et al, 1997;Jordan et al, 1997;Brett et al, 2005). However, the relationship between NO 3 À concentration and land use is often unclear and varies depending on hydrological conditions (Jordan et al, 1997;Tufford et al, 2003;Brett et al, 2005;Nakamura et al, 2011). Making use of the isotopic composition of N compounds could overcome these shortcomings (Barnes and Raymond, 2010).…”

“…δ 15 N of NO 3 À (δ 15 N NO3 ) is a useful tool for distinguishing between anthropogenic-origin and natural-origin NO 3 À in rivers, because the δ 15 N NO3 originating from anthropogenic practices such as agriculture and wastewater is usually higher than that originating from natural sources, such as forest soil (Harrington et al, 1998;Rock and Mayer, 2004;Ohte et al, 2010). Nakamura et al (2011) demonstrated that river δ 15 N NO3 remained low (approximately less than +2.5‰) in a basin along the Fuji River, Japan, that had a forested area ratio of more than 98%, whereas river δ 15 N NO3 exceeded +5‰ in a basin that had a residential area ratio of more than 1%. Therefore, the relationship between river δ 15 N NO3 and the land-use ratio in a basin provides the threshold of landuse ratio that can critically affect changes in river NO 3 À concentration.…”

“…This threshold could be a useful criterion for determining which land use within a basin should be most intensively managed for controlling river NO 3 À concentration. However, the study conducted by Nakamura et al (2011) was based on data obtained during nonprecipitation periods, i.e., low flows.…”