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 (inline-formula $M1inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 25pt16ptsvg-formulamathimg57a4663cbf0d11bf294d99bb32c9ae29 bg-15-3953-2018-ie00001.svg25pt16ptbg-15-3953-2018-ie00001.png ) is the key to constrain sources of inline-formula $M2inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 25pt16ptsvg-formulamathimg4c315b3ea451cf26923ad12993612b33 bg-15-3953-2018-ie00002.svg25pt16ptbg-15-3953-2018-ie00002.png export in order to aid effective management of waterways. In this study, isotopic compositions of inline-formula $M3inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 25pt16ptsvg-formulamathimga186e28964d6ae507e65dbc91f8b1f71 bg-15-3953-2018-ie00003.svg25pt16ptbg-15-3953-2018-ie00003.png (inline-formulaδ¹⁵N–inline-formula $M5inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 25pt16ptsvg-formulamathimg91b2e19ca239409a7665981c17575147 bg-15-3953-2018-ie00004.svg25pt16ptbg-15-3953-2018-ie00004.png and inline-formulaδ¹⁸O–inline-formula $M7inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 25pt16ptsvg-formulamathimga02883d0956e7dc256b9fe9fffa70b09 bg-15-3953-2018-ie00005.svg25pt16ptbg-15-3953-2018-ie00005.png ) were used to elucidate the effects of land use (agriculture in particular) and rainfall on the major sources and sinks of inline-formula $M8inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 25pt16ptsvg-formulamathimg8a872e45f44a0fc3c08e466e371cfb3a bg-15-3953-2018-ie00006.svg25pt16ptbg-15-3953-2018-ie00006.png 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 inline-formula $M9inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 25pt16ptsvg-formulamathimgb3512ed4eb493ff037a5c39221523c47 bg-15-3953-2018-ie00007.svg25pt16ptbg-15-3953-2018-ie00007.png 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 inline-formula $M10inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 25pt16ptsvg-formulamathimgdd23f13eb24280cbe650be4567ce8571 bg-15-3953-2018-ie00008.svg25pt16ptbg-15-3953-2018-ie00008.png concentrations (inline-formulap < 0.05), inline-formulaδ¹⁵N–inline-formula $M13inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 25pt16ptsvg-formulamathimg9712381780fcc4de6c4d72f703a8771c bg-15-3953-2018-ie00009.svg25pt16ptbg-15-3953-2018-ie00009.png (inline-formulap < 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 inline-formula $M15inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 25pt16ptsvg-formulamathimgecc3e6dd5af0ffb1da8bfbfcb16b8e8b bg-15-3953-2018-ie00010.svg25pt16ptbg-15-3953-2018-ie00010.png in our study sites. Artificial fertiliser was the dominant source of inline-formula $M16inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 25pt16ptsvg-formulamathimg48a6d5724cc017ced9c974ab9a81c03a bg-15-3953-2018-ie00011.svg25pt16ptbg-15-3953-2018-ie00011.png during the wet periods. In addition to artificial fertiliser, nitrified organic matter in sediment was also an apparent source of inline-formula $M17inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 25pt16ptsvg-formulamathimg822fcc3376206f5298bc14405cca7022 bg-15-3953-2018-ie00012.svg25pt16ptbg-15-3953-2018-ie00012.png to the surface water during the dry periods. Denitrification was prevalent during the wet periods, while uptake of inline-formula $M18inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 25pt16ptsvg-formulamathimg7dd3c683c0655cd2a5c1ed2d08ea01e9 bg-15-3953-2018-ie00013.svg25pt16ptbg-15-3953-2018-ie00013.png by plants or algae was only observed during the dry periods in two streams. The outcome of this study suggests that effective reduction of inline-formula $M19inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 25pt16ptsvg-formulamathimg7248c728767abac31fc80ac33e5f4469 bg-15-3953-2018-ie00014.svg25pt16ptbg-15-3953-2018-ie00014.png load to the streams can only be achieved by prioritising management strategies based on different rainfall conditions.