Stable isotopes of nitrate reveal different nitrogen processing mechanisms in streams across a land use gradient during wet and dry periods

Understanding the relationship between land use and the dynamics of nitrate (NO₃ ^-) is the key to constrain sources of NO₃ ^- export in order to aid effective management of waterways. In this study, isotopic compositions of NO₃ ^- (δ15N-NO₃ ^- and δ¹⁸O-NO₃ ^-) were used to elucidate the effects of land use (agriculture in particular) and rainfall on the major sources and sinks of NO₃ ^- within the Western Port catchment, Victoria, Australia. This study is one of the very few studies carried out in temperate regions with highly stochastic rainfall patterns, enabling a more comprehensive understanding of the applications of NO₃ ^- isotopes in catchment ecosystems with different climatic conditions. Longitudinal samples were collected from five streams with different agriculture land use intensities on five occasions - three during dry periods and two during wet periods. At the catchment scale, we observed significant positive relationships between NO₃ ^- concentrations (p < 0.05), δ¹⁵N-NO₃ ^- (p < 0.01) and percentage agriculture (particularly during the wet period), reflecting the dominance of anthropogenic nitrogen inputs within the catchment. Different rainfall conditions appeared to be major controls on the predominance of the sources and transformation processes of NO₃ ^- in our study sites. Artificial fertiliser was the dominant source of NO₃ ^- during the wet periods. In addition to artificial fertiliser, nitrified organic matter in sediment was also an apparent source of NO₃ ^- to the surface water during the dry periods. Denitrification was prevalent during the wet periods, while uptake of NO₃ ^- by plants or algae was only observed during the dry periods in two streams. The outcome of this study suggests that effective reduction of NO₃ ^- load to the streams can only be achieved by prioritising management strategies based on different rainfall conditions.