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Understanding the relationship between land use and the dynamics of nitrate (NO 3 − ) is the key to constrain sources of NO 3 − export in order to aid effective management of waterways. In this study, isotopic compositions of NO 3 − (δ 15 N-NO 3 − and δ 18 O-NO 3 − ) were used to elucidate the effects of land use (agriculture in particular) and rainfall on the major sources and sinks of NO 3 − within the Westernport 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 NO 3 − 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 NO 3 − concentrations, δ 15 N-NO 3 − and percentage agriculture 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 NO 3 − in our study sites. Artificial fertiliser was the dominant source of NO 3 − during the wet periods while nitrified organic matter in sediment and nitrified manure were more apparent sources of NO 3 − to the surface water during the dry periods. Denitrification was prevalent during the wet periods while uptake of NO 3 − by plants or algae was only observed during the dry periods in two streams. The outcome of this study suggests that effective reduction of NO 3 − load to the streams can only be achieved by prioritising management strategies based on different rainfall conditions.

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