15N‐NITRATE SIGNATURE IN LOW‐ORDER STREAMS: EFFECTS OF LAND COVER AND AGRICULTURAL PRACTICES

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Many studies have shown that intensive agricultural practices significantly increase the nitrogen concentration of stream surface waters, but it remains difficult to identify, quantify, and differentiate between terrestrial and in-stream sources or sinks of nitrogen, and rates of transformation. In this study we used the delta15N-NO3 signature in a watershed dominated by agriculture as an integrating marker to trace (1) the effects of the land cover and agricultural practices on stream-water N concentration in the upstream area of the hydrographic network, (2) influence of the in-stream processes on the NO3-N loads at the reach scale (100 m and 1000 m long), and (3) changes in delta15N-NO3 signature with increasing stream order (from first to third order). This study suggests that land cover and fertilization practices were the major determinants of delta15N-NO3 signature in first-order streams. NO3-N loads and delta15N-NO3 signature increased with fertilization intensity. Small changes in delta15N-NO3 signature and minor inputs of groundwater were observed along both types of reaches, suggesting the NO3-N load was slightly influenced by in-stream processes. The variability of NO3-N concentrations and delta15N signature decreased with increasing stream order, and the delta15N signature was positively correlated with watershed areas devoted to crops, supporting a dominant effect of agriculture compared to the effect of in-stream N processing. Consequently, land cover and fertilization practices are integrated in the natural isotopic signal at the third-order stream scale. The GIS analysis of the land cover coupled with natural-abundance isotope signature (delta15N) represents a potential tool to evaluate the effects of agricultural practices in rural catchments and the consequences of future changes in management policies at the regional scale.