Department of Geology and Environmental Science, Wheaton College, 501 College Ave, Wheaton, IL, 60187, USA.
Department of Geological Sciences, Ball State University, 2000 W University Ave, Muncie, IN, 47306, USA.
Environ Monit Assess. 2020 Feb 7;192(3):168. doi: 10.1007/s10661-020-8126-4.
Agricultural nonpoint source pollution from the upper areas of the Upper Mississippi, Missouri, and Ohio River basins accounts for the majority of the excess nitrogen that leads to the Gulf of Mexico's hypoxic zone. However, agriculture landscapes across the USA, Europe, and China are undergoing major changes related to the proliferation of confined animal feeding operations (CAFOs) that account for a greater proportion of point source contamination. Mitigating long-term nutrient inputs at a large scale (Mississippi River Basin) requires understanding these microscale changes at the small watershed level (less than 100 km). To assess the control of land-use and subsurface hydrological processes on nutrient fate and transport, temporal patterns in nutrient concentrations in a mixed land-use karstic watershed were analyzed. To assess the control of differential land-use sources on total inorganic nitrogen (TIN) loads within the watershed, 4 months of weekly water sampling was performed at a spring location and at two underground conduit locations. The observed temporal variations in nutrient concentrations are primarily associated with seasonal changes in land-use associated with corn growth. Data show that land-use sources explain much of the temporal variability of TIN at the spring when weighted against the hydrological factor. End-member-mixing analysis of dissolved organic carbon (DOC) shows a progressive increase in the contribution of DOC-enriched sources and a more labile form of carbon toward the harvest time. Overall, during high flow, nonpoint source infiltration from manure-fertilized croplands in the northern branch (NB) dominate DOC loads. Because conduit-dominated karstic aquifers are more susceptible to contamination from direct and fast (< 7 h) subsurface infiltration during late summer rainfall period (July to August), a slight advance in the timing of manure application could substantially reduce nutrient loads to local groundwater. A combined evaluation of subsurface hydrological processes and land-use factors controlling nutrients at the scale of small watersheds is crucial to developing site-specific nutrient management plans and managing the Gulf of Mexico's hypoxic zone.
密西西比河、密苏里河和俄亥俄河流域上游地区的农业面源污染是导致墨西哥湾缺氧区氮过量的主要原因。然而，美国、欧洲和中国的农业景观正在发生重大变化，与养殖场（CAFOs）的扩散有关，养殖场造成的点源污染比例更大。要大规模减少长期养分输入（密西西比河流域），就需要了解小流域尺度（小于 100 公里）的这些微观变化。为了评估土地利用和地下水文过程对养分归宿和运移的控制作用，分析了混合土地利用岩溶流域养分浓度的时间变化模式。为了评估流域内不同土地利用源对总无机氮（TIN）负荷的控制作用，在一个泉水点和两个地下管道点进行了为期 4 个月的每周一次的水样采集。养分浓度的观测时间变化主要与与玉米生长相关的季节性土地利用变化有关。数据表明，在泉水点，土地利用源在很大程度上解释了 TIN 的时间变化，而与水文因素相比权重更大。溶解有机碳（DOC）的端元混合分析表明，DOC 富集源的贡献以及更具活力的碳形式在收获期呈逐渐增加的趋势。总的来说，在高流量期间，北部支流水肥农田的非点源渗透作用主导着 DOC 负荷。由于夏季末降雨期（7 月至 8 月）地下管道主导的岩溶含水层更容易受到直接和快速（<7 小时）地下渗透的污染，因此稍微提前施肥时间可以大大减少养分负荷对当地地下水的影响。对控制小流域养分的地下水文过程和土地利用因素进行综合评估，对于制定特定地点的养分管理计划和管理墨西哥湾的缺氧区至关重要。
