IIHR-Hydroscience and Engineering, University of Iowa, Iowa City, Iowa, United States of America.
Iowa Geological Survey, Iowa City, Iowa, United States of America.
PLoS One. 2018 Apr 12;13(4):e0195930. doi: 10.1371/journal.pone.0195930. eCollection 2018.
The main objective of this work was to quantify and update the U.S. Midwest agricultural state of Iowa's contribution of nitrate-nitrogen to the Mississippi River stream network against the backdrop of the ongoing problem of Gulf of Mexico hypoxia. To achieve this objective, we used stream nitrate and discharge data collected from 1999 until 2016 at 23 Iowa stream sites near watershed outlets, along with publicly-available data for sites downstream of Iowa on the Missouri and Mississippi Rivers. Our analysis shows that Iowa contributes between 11 and 52% of the long-term nitrate load to the Mississippi-Atchafalaya Basin, 20 to 63% to the Upper Mississippi River Basin, and 20 to 89% to the Missouri River Basin, with averages of 29, 45 and 55% respectively. Since 1999, nitrate loads in the Iowa-inclusive basins have increased and these increases do not appear to be driven by changes in discharge and cropping intensity unique to Iowa. The 5-year running annual average of Iowa nitrate loading has been above the 2003 level for ten consecutive years, implying that Gulf hypoxic areal goals, also based on a 5-year running annual average, will be very difficult to achieve if nitrate retention cannot be improved in Iowa. An opportunity exists for land managers, policy makers and conservationists to manifest a positive effect on water quality by targeting and implementing nitrate reducing-practices in areas like Iowa while avoiding areas that are less likely to affect Gulf of Mexico hypoxia.
这项工作的主要目的是，在墨西哥湾缺氧问题持续存在的背景下，量化并更新美国中西部爱荷华州农业区向密西西比河流域输送硝酸盐氮的情况。为了实现这一目标，我们使用了从 1999 年到 2016 年在 23 个爱荷华州靠近流域出口的溪流站点收集的硝酸盐和流量数据，以及密西西比河和密苏里河爱荷华州下游站点的公开可用数据。我们的分析表明，爱荷华州对密西西比-阿查法拉亚盆地的硝酸盐长期负荷的贡献率在 11%至 52%之间，对上密西西比河流域的贡献率为 20%至 63%，对密苏里河流域的贡献率为 20%至 89%，平均值分别为 29%、45%和 55%。自 1999 年以来，爱荷华州流域的硝酸盐负荷有所增加，而这些增加似乎并不是由爱荷华州独特的流量和种植强度变化所驱动的。在过去的十年中，爱荷华州硝酸盐负荷的 5 年滚动年平均值已经连续 10 年高于 2003 年的水平，这意味着如果不能改善爱荷华州的硝酸盐截留，那么基于 5 年滚动年平均值的墨西哥湾缺氧区域目标将很难实现。土地管理者、政策制定者和自然资源保护主义者有机会通过在爱荷华州等地区实施减少硝酸盐的实践，在避免对墨西哥湾缺氧影响较小的地区的同时，对水质产生积极影响。
