Ongoing land-use intensification in subtropical catchments is expected to release more inorganic nitrogen to downstream coastal waters similar to historical changes in temperate ecosystems. Here, we examined spatial and temporal drivers of stream nitrogen loads across a subtropical land-use gradient using the isotopic compositions of nitrate (NO₃^--N) and radon (²²²Rn), a natural groundwater tracer. We investigated eleven subtropical creeks/rivers over contrasting hydrological conditions in Australia. NOx-N (nitrite (NO₂^--N) + nitrate (NO₃^--N)) accounted for 13.1%, 34.0 %, and 42.6% of total dissolved nitrogen (TDN-N) in forest, peri-urban and agricultural creeks, respectively. Following an 80 mm rain event, loads of dissolved inorganic nitrogen (DIN-N) from agriculture catchments reached 368 mg N m^-2 catchment area day^-1. Forest and peri-urban catchments had aquatic TDN-N loads 17.8% and 31.1% of loads from agricultural catchments. Radon observations suggest that nitrogen and phosphorus loads were driven primarily by surface runoff rather than groundwater discharge. The δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- values in the agriculture, forest and peri-urban catchments indicate fertilisers and soil nitrogen as the main sources of NO₃^--N. However, one of the catchments (Double Crossing Creek) received a mixture of recirculated greywater and chemical nitrogen fertilisers. Isotopic signatures imply significant NO₃^--N losses via denitrification during dry conditions. Groundwater discharge played a minor role because regional aquifers were not contaminated by nitrogen. Overall, intensive agricultural land use and episodic rainfall events were the major spatial and temporal drivers of nitrogen loads.

预计亚热带流域持续的土地利用集约化将向下游沿海水域释放更多的无机氮，这与温带生态系统的历史变化相似。在这里，我们使用硝酸盐（NO ₃ ^-- N）和ra（²²² Rn）（一种天然地下水示踪剂）的同位素组成，研究了亚热带土地利用梯度上氮流的时空驱动因素。我们在澳大利亚不同的水文条件下调查了11个亚热带小河/河流。的NOx-N（亚硝酸钠（NO ₂ ^- N）+硝酸盐（NO ₃ ^--N））分别占森林，郊区和农业小河中总溶解氮（TDN-N）的13.1％，34.0％和42.6％。在发生80毫米降雨之后，第^-1天来自农业流域的溶解无机氮（DIN - N）负荷达到368 mg N m ^-2流域。森林和城郊集水区的水上TDN-N负载量分别为农业集水量的17.8％和31.1％。Rad的观察表明，氮和磷的负荷主要是由地表径流驱动的，而不是由地下水的排放驱动的。的δ ¹⁵ N-NO ₃ ^-和δ ¹⁸ O型NO ₃ ^-农业，森林和城郊集水区的数值表明肥料和土壤氮是NO ₃ ^-- N的主要来源。但是，其中一个流域（Double Crossing Creek）接受了再循环的灰水和化学氮肥的混合。同位素特征暗示在干燥条件下通过反硝化作用会导致大量的NO ₃ ^-- N损失。地下水排放的作用很小，因为区域含水层没有被氮污染。总体而言，密集的农业土地利用和突发降雨事件是氮负荷的主要时空驱动因素。