Nonpoint-source nitrogen (N) loads in the U.S. Corn Belt are a major concern both for local impacts on receiving waters and for contributing to hypoxia in the Gulf of Mexico. Nonpoint-source nutrient loads can be ameliorated by a combination of in-field and offsite practices, and wetland restoration is a particularly promising approach for reducing N loads from agricultural drainage. However, there is considerable variability among wetlands, and adequate performance data are available for relatively few systems receiving unregulated nonpoint-source loads. We measured N mass balances of 26 restored wetlands receiving a wide range of unregulated, naturally varying hydraulic and nutrient loads to evaluate the N removal performance of these systems and the effects of major factors controlling their performance. Nitrogen loads were primarily in the form of nitrate, and all of the wetlands were effective in reducing both nitrate and total N loads. Nitrate N and total N removal rates averaged 1,500 and 1,440 kg N ha-1 yr-1 , respectively, with the slightly lower total N removal rates reflecting a small net export of reduced N (averaging 66 kg N ha-1 yr-1 ). Average nitrate and total N removal rates were substantially higher than typically reported for Corn Belt wetlands but comparable with highly loaded systems elsewhere. Nitrate removal efficiency ranged from 9 to 92% and was strongly related to hydraulic loading rate and temperature. Results demonstrate the substantial capacity of wetlands to reduce unregulated and highly variable nonpoint-source N loads over a broad range of weather and loading conditions and provide a reasonable basis for predicting average wetland performance based on hydraulic loading rate, temperature, and nitrate concentration.

中文翻译：

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接受非点源氮负荷的湿地水质性能：硝酸盐和总氮去除效率及控制因素

美国玉米带中的非点源氮 (N) 负荷是当地对受纳水域的影响和导致墨西哥湾缺氧的主要问题。非点源养分负荷可以通过现场和场外实践相结合来改善，湿地恢复是减少农业排水中氮负荷的一种特别有前途的方法。然而，湿地之间存在相当大的可变性，并且对于接收未调节的非点源负载的相对少数系统而言，可以获得足够的性能数据。我们测量了 26 个恢复的湿地的 N 质量平衡，这些湿地接受了广泛的未调节的、自然变化的水力和营养负荷，以评估这些系统的 N 去除性能以及控制其性能的主要因素的影响。氮负荷主要以硝酸盐的形式存在，所有湿地都能有效减少硝酸盐和总氮负荷。硝酸盐 N 和总 N 去除率平均分别为 1,500 和 1,440 kg N ha-1 yr-1 ，总 N 去除率略低反映了减少的 N 的少量净出口（平均 66 kg N ha-1 yr-1 ） . 平均硝酸盐和总氮去除率大大高于玉米带湿地的典型报告，但与其他地方的高负荷系统相当。硝酸盐去除效率从 9% 到 92% 不等，并且与水力加载速率和温度密切相关。