ABSTRACT

The Rhône River originates in the high Alps and drains an intensely cultivated and industrialised catchment before it discharges to the Gulf of Lion. We investigated the interaction of catchment geomorphology with nitrate sources (atmosphere, agriculture, and nitrification of soil organic matter) and removal processes in large and diverse watersheds on the basis of dual nitrate isotope signatures in river water.

In March 2015, we took surface water samples along the Rhône River, including its main tributaries, and measured nutrient concentrations and the stable isotopic composition of nitrate (δ¹⁵N, δ¹⁸O and Δ¹⁷O), and water (δ¹⁸O-H₂O).

Results show that high altitude regions are dominated by nitrate from nitrification in pristine soils and atmospheric deposition, while nitrate in the downstream Rhône River originates mainly from nitrification of agricultural/urban sources. Parallel increases in δ¹⁵N and δ¹⁸O reflect the influence of primary production. Previous studies suggested robust correlations between land use and ${\delta }^{15}\mathrm{N}\text{-}\mathrm{N}{\mathrm{O}}_3^{\text{–}}$. Based on our observation that nitrate δ¹⁵N values at higher altitudes are lower than expected, we assume that lower nitrate δ¹⁵N values likely reflect limited nitrate consumption and lower soil nitrogen turnover rates. We propose that correlation between land use and nitrate δ¹⁵N is sensitive to slope and geomorphology.