Institute of Surface-Earth System Science, School of Earth System Science, Tianjin University, Tianjin 300072, China; School of Geographical and Earth Sciences, University of Glasgow, Glasgow G12 8QQ, United Kingdom; Tianjin Key Laboratory of Earth Critical Zone Science and Sustainable Development in Bohai Rim, Tianjin University, Tianjin 300072, China.
Institute of Surface-Earth System Science, School of Earth System Science, Tianjin University, Tianjin 300072, China; Tianjin Key Laboratory of Earth Critical Zone Science and Sustainable Development in Bohai Rim, Tianjin University, Tianjin 300072, China.
Water Res. 2020 Nov 1;186:116388. doi: 10.1016/j.watres.2020.116388. Epub 2020 Sep 3.
Understanding where nitrate is mobilized from and under what conditions is required to reduce nitrate loss and protect water quality. Low frequency sampling may inadequately capture hydrological and biogeochemical processes that will influence nitrate behavior. We used high-frequency isotope sampling and in-situ nitrate sensing to explore nitrate export and transformation in a karst critical zone. Nitrate was mobilised during light rainfall, and transferred from soil layers to the karst matrix, where some nitrate was retained and denitrified. Nitrate isotopic composition changed rapidly during the rising limb of events and slowly during the falling limb. The main nitrate source was synthetic fertiliser (up to 80% during event flow), transported by conduit flow following high rainfall events, and this contribution increased significantly as discharge increased. Soil organic nitrogen contribution remained constant indicating at baseflow this is the primary source. Isotope source appointment of nitrate export revealed that synthetic fertilizer accounted for more than half of the total nitrate export, which is double that of the secondary source (soil organic nitrogen), providing valuable information to inform catchment management to reduce nitrate losses and fluvial loading. Careful land management and fertilizer use are necessary to avoid nitrate pollution in the karst agroecosystem, for example by timing fertilizer applications to allow for plant uptake of nitrate before rainfall can flush it from the soils into the karst and ultimately into catchment drainage.
了解硝酸盐的来源和在何种条件下会被移动对于减少硝酸盐流失和保护水质至关重要。低频采样可能无法充分捕捉影响硝酸盐行为的水文和生物地球化学过程。我们使用高频同位素采样和原位硝酸盐感测技术来探索岩溶关键带中的硝酸盐输出和转化。硝酸盐在小雨期间被活化，并从土壤层转移到岩溶基质中，其中一些硝酸盐被保留并反硝化。硝酸盐同位素组成在事件上升支期间迅速变化，在下降支期间缓慢变化。主要硝酸盐源是合成肥料（在事件流量中高达 80%），在高降雨事件后通过管道流动输送，随着流量的增加，这种贡献显著增加。土壤有机氮的贡献保持不变，表明在基流期间这是主要来源。硝酸盐输出的同位素源指派表明，合成肥料占总硝酸盐输出的一半以上，是次要来源（土壤有机氮）的两倍，为流域管理提供了有价值的信息，以减少硝酸盐流失和河流负荷。需要谨慎的土地管理和肥料使用，以避免岩溶农业生态系统中的硝酸盐污染，例如通过安排肥料施用时间，以便在降雨将硝酸盐从土壤冲入岩溶并最终冲入流域排水系统之前，让植物吸收硝酸盐。
