Abstract While enabling economically viable use of poorly drained soils, artificial subsurface drainage has also been found to be a significant pathway for nutrient transfers from agricultural land to surface waters. Thus, mitigating the impacts of agriculture on surface water quality needs to address nutrient transfers via subsurface drainage. Woodchip bioreactors are a promising mitigation option as demonstrated under arable agriculture in the mid-west of the USA. However, research is needed to ascertain their efficiency in removing nutrients from very flashy drainage flows common in New Zealand (NZ) pastoral agriculture and any possible pollution swapping (e.g. reduction of leaching losses vs. greenhouse gas emissions). Accordingly, a lined 78-m3 woodchip bioreactor was constructed on a dairy farm in the Hauraki Plains (Waikato, NZ) with a drainage area of 0.65 ha. Rainfall, flow, hydrochemistry and dissolved gases in the inflow and outflow were monitored for two drainage seasons (part of 2017, 2018). Based on the nitrate-N fluxes, the estimated nitrate removal efficiency of the bioreactor was 99 and 48% in 2017 and 2018, respectively. The higher removal efficiency in 2017 could be attributed to two reasons. Firstly, the substantially longer hydraulic residence time (HRT) of the water in the bioreactor (mean = 21.1 days vs 4.7 days in 2018) provided more opportunity for microorganisms to reduce the nitrate. A strong positive relationship between HRT and removal efficiency was also observed within the 2018 drainage season. Secondly, denitrification was supported in 2017 by greater electron donor availability. Evidence of this was the higher mass of DOC discharge from the bioreactor (318 mg C L−1 of bioreactor volume vs 165 mg C L−1 in 2018). Removal rates in the bioreactor varied from 0.67–1.60 g N m−3 day−1 and were positively correlated with inflow nitrate loads. Pollution swapping was observed during the start-up phase of the bioreactor in both years (DOC, and DRP only in 2017) and during periods with very long HRTs (hydrogen sulphide (H2S) and methane (CH4) production). Substantially elevated discharges of DOC and DRP, as compared to inlet conditions, occurred during the initial start-up phase of the bioreactor in 2017 (3 to 3.5 pore volumes of the bioreactor), but only slightly elevated DOC and decreased DRP discharges were observed when drainage flow resumed at the start of the 2018 drainage season. Unexpectedly, cumulative DRP removal during the 2018 drainage season amounted to 89% of the DRP inflow into the bioreactor. Long HRTs (>5 days) enabled high nitrate removal efficiency (≥59%) and promoted complete reduction of nitrate to harmless dinitrogen gas but also promoted strongly reduced conditions, resulting in the production of H2S and CH4. On the other hand, short HRTs (

中文翻译：

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木屑生物反应器的硝酸盐去除和二次效应处理牧区动态流下的地下排水

摘要 在经济上可行地利用排水不良的土壤的同时，人工地下排水也被发现是将养分从农田转移到地表水的重要途径。因此，减轻农业对地表水质量的影响需要通过地下排水解决养分转移问题。正如美国中西部的耕地农业所证明的那样，木屑生物反应器是一种很有前途的缓解选择。然而，需要研究来确定它们从新西兰 (NZ) 牧区农业中常见的非常华丽的排水流中去除养分和任何可能的污染交换（例如减少浸出损失与温室气体排放）的效率。因此，在 Hauraki 平原（怀卡托，NZ)，流域面积为 0.65 公顷。监测了两个排水季节（2017 年和 2018 年的一部分）的降雨、流量、水化学和流入和流出中的溶解气体。根据硝酸盐-N 通量，估计生物反应器的硝酸盐去除效率在 2017 年和 2018 年分别为 99% 和 48%。2017 年较高的去除效率可归因于两个原因。首先，生物反应器中水的水力停留时间 (HRT) 显着延长（平均值 = 21.1 天，而 2018 年为 4.7 天）为微生物减少硝酸盐提供了更多机会。在 2018 年排水季节也观察到 HRT 和去除效率之间存在很强的正相关关系。其次，2017 年更多的电子供体可用性支持了反硝化。这方面的证据是生物反应器排放的 DOC 质量更高（生物反应器体积为 318 mg CL-1，而 2018 年为 165 mg CL-1）。生物反应器中的去除率在 0.67–1.60 g N m-3 day-1 之间变化，并且与流入的硝酸盐负荷呈正相关。在这两年的生物反应器启动阶段（DOC 和 DRP 仅在 2017 年）和非常长的 HRT（硫化氢 (H2S) 和甲烷 (CH4) 生产）期间都观察到了污染交换。在 2017 年生物反应器的初始启动阶段（生物反应器的 3 至 3.5 孔体积），与入口条件相比，DOC 和 DRP 的排放量显着升高，但仅在以下情况下观察到 DOC 和 DRP 排放量略有升高排水流量在 2018 年排水季节开始时恢复。不料，2018 年排水季节累积的 DRP 去除量占进入生物反应器的 DRP 流入量的 89%。长 HRT（>5 天）可实现高硝酸盐去除效率（≥59%）并促进硝酸盐完全还原为无害的氮气，但也促进强烈还原条件，导致产生 H2S 和 CH4。另一方面，短 HRT（