Dissertation Research: Quantifying the role of denitrification as a mechanism for inorganic nitrogen removal in Midwestern rivers

农业和城市发展导致全球溪流、湖泊、河流和海洋的氮负荷增加。水体中过量的N导致藻类快速生长，这些藻类最终死亡并分解，分解过程将水体中的氧气带走，导致？死区。除了藻华，过量的氮还会污染饮用水，并降低淡水生物多样性。反硝化是一种微生物介导的过程，它将溶解的硝酸盐N （NO3-）转化为N气体，并永久地从生态系统中去除N。溪流的底部是反硝化的理想场所，在那里它已被证明是永久去除N的重要机制。相比之下，人们对河流中反硝化作用的了解较少；这种脱氮过程一直被忽视，主要是因为在这种规模的系统中进行现场工作存在挑战。本研究将量化中西部农业地区5条河流的水柱和河底的反硝化率。利用在河口开发的一项新技术，该项目将比较农业诱导的NO3-梯度的反硝化速率。此外，本研究将比较反硝化去除N与其他临时去除N的机制，如将溶解的N纳入生物质（如植物和藻类）。小溪已被确定为改善水质的“热点”，因此成为许多修复工程的重点。另一方面，河流一直被视为管道，其主要功能是向下游输送水（和污染物）。我们预测，河流与溪流一样具有类似的生物反应性，在改善水质方面发挥着重要作用，应加以管理，使生物反应性最大化，以减少下游N污染对环境的影响。

Agricultural and urban development has resulted in increased nitrogen (N) loading to streams, lakes, rivers, and oceans across the globe. Excess N in water bodies causes rapid algal growth and these algae eventually die and decompose, with the decomposition removing oxygen from the water column, resulting in ?dead zones.? In addition to algal blooms, excess N contaminates drinking water and can decrease freshwater biodiversity. Denitrification is a microbially-mediated process that converts dissolved nitrate-N (NO3-) to N gases, and permanently removes N from ecosystems. The bottoms of streams are ideal locations for denitrification, where it has been shown to be an important mechanism for permanent N removal. In contrast, less is known about the role of denitrification in rivers; this N removal process has been overlooked mainly because of the challenges associated with field work in systems of this size. This study will quantify denitrification rates in the water column and river bottom of 5 rivers in the agricultural Midwest. Using a new technique developed in estuaries, this project will compare denitrification rates across an agriculturally induced gradient of NO3- . Additionally, this study will compare the N removal due to denitrification with other temporary N removal mechanisms like the incorporation of dissolved N into biomass (e.g. plants and algae). Small streams have been identified as "hotspots" for water quality improvement and have thus been the focus of many restoration projects. Rivers, on the other hand, have been treated as pipes, with their major function being to export water (and pollutants) downstream. We predict that rivers are similarly bioreactive, like streams, and play a significant role in improving water quality and should be managed to maximize bioreactivity to reduce the environmental effects of N pollution downriver.