National Marine Science Centre, Southern Cross University, Coffs Harbour, New South Wales, Australia.
National Marine Science Centre, Southern Cross University, Coffs Harbour, New South Wales, Australia.
Sci Total Environ. 2021 Mar 1;758:143669. doi: 10.1016/j.scitotenv.2020.143669. Epub 2020 Nov 21.
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 (NO-N) and radon (Rn), a natural groundwater tracer. We investigated eleven subtropical creeks/rivers over contrasting hydrological conditions in Australia. NO-N (nitrite (NO-N) + nitrate (NO-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 catchment area day. 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 δN-NO and δO-NO values in the agriculture, forest and peri-urban catchments indicate fertilisers and soil nitrogen as the main sources of NO-N. However, one of the catchments (Double Crossing Creek) received a mixture of recirculated greywater and chemical nitrogen fertilisers. Isotopic signatures imply significant NO-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.
亚热带集水区的土地利用不断集约化，预计将像温带生态系统历史变化那样，向下游沿海水域释放更多的无机氮。在这里，我们使用硝酸盐（NO-N）和氡（Rn）的同位素组成（一种天然地下水示踪剂），研究了亚热带土地利用梯度上的河流氮负荷的时空驱动因素。我们在澳大利亚调查了 11 个具有不同水文条件的亚热带小溪/河流。NO-N（亚硝酸盐（NO-N）+硝酸盐（NO-N））分别占森林、城郊和农业小溪中总溶解氮（TDN-N）的 13.1%、34.0%和 42.6%。在 80 毫米的降雨事件之后，农业集水区的溶解无机氮（DIN-N）负荷达到 368 毫克 N m 集水区面积天。森林和城郊集水区的水生 TDN-N 负荷分别为农业集水区负荷的 17.8%和 31.1%。氡观测表明，氮和磷负荷主要由地表径流驱动，而不是地下水排泄。农业、森林和城郊集水区的δN-NO 和δO-NO 值表明，肥料和土壤氮是 NO-N 的主要来源。然而，其中一个集水区（双十字溪）同时接收了再循环的灰水和化学氮肥。同位素特征表明，在干旱条件下，通过反硝化作用导致大量的 NO-N 损失。由于区域含水层没有受到氮的污染，地下水排泄的作用较小。总的来说，集约化农业土地利用和偶发降雨事件是氮负荷的主要时空驱动因素。
