Nitrogen pollution in subtropical waters is rapidly increasing due to land-use change, but specific sources, transformations, and attenuation rates remain understudied compared to cooler temperate catchments. Here, we quantify high-resolution nitrate (NO₃⁻-N) loads, sources and natural attenuation in a subtropical creek in Australia over contrasting hydrological conditions. We observed large creek NO₃⁻-N loads (ranging from 44 to 2938 μmol m⁻² catchment d⁻¹) exceeding the bottom-up estimates of nitrogen input to the catchment at the most upstream sites. Stable isotope natural abundances (δ¹⁵N and δ¹⁸O in NO₃⁻-N) and Bayesian analysis revealed that greywater was the dominant source accounting for ~55% of NO₃⁻-N in the upper creek, but fertilisers (~29%) and rainfall (~16%) were also relevant NO₃⁻-N sources. NO₃⁻-N loads at the most downstream site were only 0.2–9.7% of loads at the most upstream site. The resulting NO₃⁻-N attenuation efficiency (mainly via denitrification) was 52–84% of original upstream load per km of creek, depending on hydrological conditions. This large capacity to attenuate NO₃⁻-N during dry and first-flush events exceeds the attenuation found in temperate creeks subject to several decades of pollution in the northern hemisphere. During periods of high water flow and saturated soils, high NO₃⁻-N loads were exported downstream, turning the creek from a natural bioreactor to a system resembling a flow-through pipe. In spite of effective natural nitrogen attenuation providing a valuable ecosystem service, concentrations and loads remained well above expected for natural systems and water quality guidelines. Overall, our results highlight the need for modifying fertiliser use, capturing nitrogen on farms and reducing greywater nitrogen to prevent significant losses to subtropical waterways.

由于土地用途的变化，亚热带水域的氮污染正在迅速增加，但是与较凉爽的温带流域相比，具体的来源，转化和衰减率仍在研究中。在这里，我们量化了在不同的水文条件下，澳大利亚亚热带小溪中高分辨率硝酸盐（NO ₃ ^-- N）的负荷，来源和自然衰减。我们观察到较大的小溪NO ₃ ^-- N负载（范围从44到2938μmolm ^-2 流域d ^-1）超过了从最上游位置向流域输入的氮的自下而上的估计。稳定同位素丰度（δ ¹⁵ N和δ ¹⁸ ○在NO ₃ ^--N）和贝叶斯分析表明，灰水是主要来源占NO的〜55％的₃ ^-在上部小溪-N，但肥料（〜29％）和大（〜16％）也有关NO ₃ ^- - N个来源。最下游站点的NO ₃ ^-- N负载仅为最上游站点的0.2-9.7％。最终的NO ₃ ^-- N衰减效率（主要是通过反硝化作用）为每公里小河原始上游负荷的52-84％，具体取决于水文条件。如此大的衰减NO _3的能力^-在北半球受到数十年污染的情况下，在干燥和首次冲洗期间的-N超过了在温带小溪中发现的衰减。在水流量高和土壤饱和的时期，NO ₃ ^-- N高负荷向下游输出，使小河从天然生物反应器转向类似流通管的系统。尽管有效地减少了自然氮，提供了有价值的生态系统服务，但其浓度和负荷仍然远远超出了自然系统和水质准则的预期。总体而言，我们的研究结果突出表明需要改变肥料的使用，在农场中捕获氮并减少灰水氮，以防止对亚热带水道造成重大损失。