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小溪、湖泊、河流和海洋的氮(N)负荷增加。水体中过量的氮会导致藻类的快速生长，这些藻类最终会死亡和分解，分解后会从水体中去除氧气，导致死亡区域。除了藻类大量繁殖外，过量的氮还会污染饮用水，并可能减少淡水生物多样性。反硝化作用是一种微生物介导的过程，它将溶解的硝酸盐氮(NO3-)转化为N气体，并从生态系统中永久去除N。溪流的底部是反硝化的理想位置，在那里它已被证明是永久脱氮的重要机制。相比之下，人们对反硝化作用在河流中的作用知之甚少；这一脱氮过程一直被忽视，主要是因为在这种规模的系统中与实地工作相关的挑战。这项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速率。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-梯度上的反硝化率。此外，这项研究还将比较由于反硝化作用引起的氮去除与其他临时氮去除机制，如将溶解氮加入生物质中(例如植物和藻类)。小溪已被确定为改善水质的“热点”，因此成为许多修复项目的重点。另一方面，河流一直被视为管道，它们的主要功能是向下游输出水(和污染物)。我们预测，河流和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，应该管理最大限度地提高生物反应性，以减少下游N污染的环境影响。

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.