Abstract
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 d 15 N-NO 3 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 NO 3-N loads at the reach scale (100 m and 1000 m long), and (3) changes in d 15 N-NO 3 signature with increasing stream order (from first to third order). This study suggests that land cover and fertilization practices were the major determinants of d 15 N-NO 3 signature in first-order streams. NO 3-N loads and d 15 N-NO 3 signature increased with fertilization intensity. Small changes in d 15 N-NO 3 signature and minor inputs of groundwater were observed along both types of reaches, suggesting the NO 3-N load was slightly influenced by in-stream processes. The variability of NO 3-N concentrations and d 15 N signature decreased with increasing stream order, and the d 15 N 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 (d 15 N) 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.