UW Discovery Farms, University of Wisconsin-Madison, Division of Extension, Madison, WI, United States.
Department of Agricultural and Biosystems Engineering, Iowa State University, Ames, IA, United States.
Sci Total Environ. 2023 Jun 15;877:162837. doi: 10.1016/j.scitotenv.2023.162837. Epub 2023 Mar 15.
Denitrification bioreactors are an effective edge-of-field conservation practice for nitrate (NO) reduction from subsurface drainage. However, these systems may produce other pollutants and greenhouse gases during NO removal. Here a dual-chamber woodchip bioreactor system experiencing extreme low-flow conditions was monitored for its spatiotemporal NO and total organic carbon dynamics in the drainage water. Near complete removal of NO was observed in both bioreactor chambers in the first two years of monitoring (2019-2020) and in the third year of monitoring in chamber A, with significant (p < 0.01) reduction of the NO-N each year in both chambers with 8.6-11.4 mg NO-N L removed on average. Based on the NO removal observed, spatial monitoring of sulfate (SO), dissolved methane (CH), and dissolved nitrous oxide (NO) gases was added in the third year of monitoring (2021). In 2021, chambers A and B had median hydraulic residence times (HRTs) of 64 h and 39 h, respectively, due to varying elevations of the chambers, with drought conditions making the differences more pronounced. In 2021, significant production of dissolved CH was observed at rates of 0.54 g CH-C m d and 0.07 g CH-C m d in chambers A and B, respectively. In chamber A, significant removal (p < 0.01) of SO (0.23 g SO m d) and dissolved NO (0.21 mg NO-N m d) were observed, whereas chamber B produced NO (0.36 mg NO-N m d). Considering the carbon dioxide equivalents (COe) on an annual basis, chamber A had loads (~12,000 kg COe ha y) greater than comparable poorly drained agricultural soils; however, the landscape-scale impact was small (<1 % change in COe) when expressed over the drainage area treated by the bioreactor. Under low-flow conditions, pollution swapping in woodchip bioreactors can be reduced at HRTs <50 h and NO concentrations >2 mg N L.
反硝化生物反应器是一种有效的田间边缘保护措施，可用于减少地下水排水中的硝酸盐（NO）。然而，这些系统在去除 NO 的过程中可能会产生其他污染物和温室气体。在这里，监测了一个经历极端低流量条件的双室木屑生物反应器系统，以了解排水中 NO 和总有机碳的时空动态。在监测的头两年（2019-2020 年），两个生物反应器室都观察到了几乎完全去除 NO，在监测的第三年，A 室也观察到了显著的（p<0.01）NO-N 减少，每年两个室平均去除 8.6-11.4mg NO-N L。基于观察到的 NO 去除，在监测的第三年（2021 年）增加了对硫酸盐（SO）、溶解甲烷（CH）和溶解氧化亚氮（NO）气体的空间监测。2021 年，由于室的不同高度，A 室和 B 室的水力停留时间（HRT）分别为 64 h 和 39 h，干旱条件使差异更加明显。2021 年，在 A 室和 B 室中分别观察到溶解 CH 的显著产生速率为 0.54 g CH-C m d 和 0.07 g CH-C m d。在 A 室中，观察到 SO（0.23 g SO m d）和溶解 NO（0.21 mg NO-N m d）的显著去除（p<0.01），而 B 室则产生了 NO（0.36 mg NO-N m d）。考虑到每年的二氧化碳当量（COe），A 室的负荷（~12,000 kg COe ha y）大于可比的排水不良农业土壤；然而，当表达为生物反应器处理的排水区的面积时，景观尺度的影响很小（<1%的 COe 变化）。在低流量条件下，当 HRT<50 h 且 NO 浓度>2mg N L 时，可以减少木屑生物反应器中的污染交换。
