Laboratory study on nitrate removal and nitrous oxide emission in intact soil columns collected from nitrogenous loaded riparian wetland, Northeast China

Nitrate pollution of surface and groundwater systems is a major problem globally. For some time now wetlands have been considered potential systems for improving water quality. Nitrate dissolved in water moving through wetlands can be removed through different processes, such as the denitrification process, where heterotrophic facultative anaerobic bacteria use for respiration, leading to the production of nitrogen (N2) and nitrous oxide (N2O) gases. Nitrate removal and emission of N2O in wetlands can vary spatially, depending on factors such as vegetation, hydrology and soil structure. This study intended to provide a better understanding of the spatial variability and processes involved in removal and emission of N2O in riparian wetland soils. We designed a laboratory experiment simulating surface water flow through soil columns collected from different sites dominated by different plant species within a wetland. Water and gas samples for and N2O analyses were collected every 5 days for a period of 30 days. The results revealed significant removal of in all the soil columns, supporting the role of riparian wetland soils in removing nitrogen from surface runoff. Nitrate removal at 0 and 10cm depths in sites dominated by Phragmites australis and Carex schnimdtii was significantly higher than in the site dominated by Calamagrostis epigeio. Nitrous oxide emissions varied spatially and temporally with negative flux observed in sites dominated by P. australis and C. schnimdtii. These results reveal that in addition to the ability of wetlands to remove , some sites within wetlands are also capable of consuming N2O, hence mitigating not only agricultural nitrate pollution but also climate change.

全球地表与地下水系统的硝酸盐污染是全球性重大环境问题。长期以来，湿地被视作改善水质的潜在系统。流经湿地的水体中溶解的硝酸盐可通过多种途径去除，例如反硝化作用（denitrification）：异养兼性厌氧细菌（heterotrophic facultative anaerobic bacteria）以硝酸盐为呼吸底物，最终生成氮气（N₂）与一氧化二氮（N₂O）气体。湿地中的硝酸盐去除与N₂O排放存在空间异质性，其影响因素包括植被、水文条件与土壤结构。本研究旨在加深对河岸湿地土壤（riparian wetland soils）中硝酸盐去除与N₂O排放过程及其空间变异特征的理解。我们设计了室内模拟实验，将从湿地内不同优势植物群落位点采集的土柱（soil columns）进行地表水流模拟培养。实验周期为30天，每5天采集水样与气样，用于硝酸盐与N₂O分析。结果显示，所有土柱均实现了显著的硝酸盐去除，印证了河岸湿地土壤在去除地表径流氮负荷中的作用。在芦苇（Phragmites australis）与短鳞薹草（Carex schnimdtii）主导的位点，0cm与10cm深度的硝酸盐去除率显著高于拂子茅（Calamagrostis epigeio）主导的位点。一氧化二氮排放具有空间与时间异质性，在P. australis与C. schnimdtii主导的位点观测到负通量。上述结果表明，除湿地本身具备的硝酸盐去除能力外，湿地内部分位点还可消耗N₂O，因此不仅能够缓解农业面源硝酸盐污染，还可助力气候变化减缓。