School of Biological, Earth and Environmental Sciences, UNSW Sydney, NSW 2052, Australia; Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW, 2234, Australia.
School of Biological, Earth and Environmental Sciences, UNSW Sydney, NSW 2052, Australia; Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW, 2234, Australia.
Sci Total Environ. 2022 Apr 15;817:152606. doi: 10.1016/j.scitotenv.2021.152606. Epub 2022 Jan 8.
The application of N fertilisers to enhance crop yield is common throughout the world. Many crops have historically been, or are still, fertilised with N in excess of the crop requirements. A portion of the excess N is transported into underlying aquifers in the form of NO, which is potentially discharged to surface waters. Denitrification can reduce the severity of NO export from groundwater. We sought to understand the occurrence and hydrogeochemical controls on denitrification in NO-rich aquifers beneath the Emerald Irrigation Area (EIA), Queensland, Australia, a region of extensive cotton and cereal production. Multiple stable isotope (in HO, NO, DIC, DOC and SO) and radioactive isotope (H and Cl) tracers were used to develop a conceptual N process model. Fertiliser-derived N is likely incorporated and retained in the soil organic N pool prior to its mineralisation, nitrification, and migration into aquifers. This process, alongside the near absence of other anthropogenic N sources, results in a homogenised groundwater NO isotopic signature that allows for denitrification trends to be distinguished. Regional-scale denitrification manifests as groundwater becomes increasingly anaerobic during flow from an upgradient basalt aquifer to a downgradient alluvial aquifer. Dilution and denitrification occurs in localised electron donor-rich suboxic hyporheic zones beneath leaking irrigation channels. Using approximated isotope enrichment factors, estimates of regional-scale NO removal ranges from 22 to 93% (average: 63%), and from 57 to 91% (average: 79%) beneath leaking irrigation channels. In the predominantly oxic upgradient basalt aquifer, raised groundwater tables create pathways for NO to be transported to adjacent surface waters. In the alluvial aquifer, the transfer of NO is limited both physically (through groundwater-surface water disconnection) and chemically (through denitrification). These observations underscore the need to understand regional- and local-scale hydrogeological processes when assessing the impacts of groundwater NO on adjacent and end of system ecosystems.
在世界各地，应用氮肥来提高作物产量是很常见的。许多历史上或现在仍过量施用氮肥的作物，其氮素的一部分以硝酸盐的形式运移到含水层中，这可能会导致硝酸盐向地表水的排放。反硝化作用可以降低地下水硝酸盐输出的严重程度。我们试图了解氮素丰富的含水层中反硝化作用的发生情况及其水文地球化学控制因素，这些含水层位于澳大利亚昆士兰州翡翠灌溉区（EIA）之下，该地区广泛种植棉花和谷物。我们使用多种稳定同位素（HO、NO、DIC、DOC 和 SO）和放射性同位素（H 和 Cl）示踪剂来建立一个概念性的氮素过程模型。化肥衍生的氮素可能在其矿化、硝化和迁移到含水层之前被整合并保留在土壤有机氮库中。这一过程，以及几乎没有其他人为氮源，导致地下水硝酸盐同位素特征均匀化，从而可以区分反硝化作用的趋势。在从高地玄武岩含水层流向低地冲积含水层的过程中，地下水变得越来越缺氧，从而表现出区域性的反硝化作用。在渗漏灌溉渠道下的局部电子供体丰富的亚缺氧潜流区发生稀释和反硝化作用。利用近似的同位素富集因子，估计区域尺度的硝酸盐去除范围为 22%至 93%（平均值：63%），渗漏灌溉渠道下为 57%至 91%（平均值：79%）。在主要为好氧的高地玄武岩含水层中，地下水位升高为硝酸盐向相邻地表水的运移创造了途径。在冲积含水层中，硝酸盐的迁移受到物理（通过地下水-地表水的不连接）和化学（通过反硝化作用）的限制。这些观察结果强调了在评估地下水硝酸盐对相邻和系统末端生态系统的影响时，需要了解区域和局部尺度的水文地质过程。
