CAREER: Yin and yang - is there a balance between nitrogen fixation and denitrification in riverine ecosystems?

人类活动带来的氮负荷增加会使水质恶化，并威胁到水生生物多样性。长期以来，人们一直认为，随着河流氮负荷的增加，河流中通过生物固氮(将大气中的氮气转化为生物可利用的氮)产生的氮将停止。与此同时，通过反硝化作用(将有效氮转化回氮气)造成的氮损失将增加，从而导致河流和溪流中氮的净减少。然而，最近对河口和湖泊的研究表明，在一系列氮负荷中，固氮和反硝化的高速率可以同时发生，挑战了这一普遍存在的假设。这项研究将从根本上了解河流对人为氮素丰富的反应，并将能够在区域和全球范围内更好地管理氮循环。这项工作还试图通过将生态系统生态学技术整合到GK-12和本科教育中来创造一个更了解生态的公民。密歇根理工大学工程学和生物学方向的本科生和教职员工将通过独特的合作，建造一个流动分析实验室，以促进全美各地的样品流。移动实验室还将用于为本科生生态学课程设计新的实验室练习，支持学生研究项目，并促进与密歇根州上半岛西部的GK-12学生的联系。这项研究将在营养负荷的观察和实验操作的背景下应用新的分析技术和新的河流模型，以测试关于大陆尺度河流和河流中氮负荷梯度之间的固氮和反硝化之间的平衡的假设。这项研究将解决三个基本问题：(1)氮固定和反硝化是否像在其他水生生态系统中一样，在自然范围内的氮浓度范围内共存？(2)“热点”和“热点”，即时空异质性，是否促进了这种共存？(3)增加的营养负荷是否通过反硝化改变了平衡，有利于损失？这项研究将利用国家生态观测网(NEON)作为一个大陆尺度的实验框架，通过三个相互作用的研究要素来检验假设：氮气固定和反硝化的梯度研究(第1-4年)，确定氮气固定和反硝化热点和热点的多尺度时空研究(第2-5年)，以及营养物质对溪流中氮气固定和反硝化作用影响的实验研究(第4-5年)。该项目将使用MIMS作为一种新的分析技术，与乙炔还原和乙炔阻断试验一起测量氮的固定和反硝化作用，并将这些测量与REACH尺度模型结合起来，通常用于估计河流新陈代谢，以估计反硝化和氮气固定。

Increased nitrogen loading from human activities degrades water quality and threatens aquatic biodiversity. It has long been assumed that as nitrogen loading to rivers increases, nitrogen production in the stream via biological nitrogen fixation (the conversion of atmospheric nitrogen gas to biologically available nitrogen) would cease. At the same time nitrogen loss via denitrification (the conversion of available nitrogen back to nitrogen gas) would increase, causing a net reduction in nitrogen in rivers and streams. Yet, recent work in estuaries and lakes suggests that high rates of both nitrogen fixation and denitrification can co-occur across a range of nitrogen loadings, challenging this widely-held assumption. This research will provide fundamental understanding about the response of rivers to anthropogenic nitrogen enrichment and will enable better management of nitrogen cycling at regional and global scales. This work also seeks to create a more ecologically-informed citizenry by integrating ecosystem ecology techniques into GK-12 and undergraduate education. The construction of a mobile analytical lab to facilitate sampling streams across the United States will be accomplished through a unique collaboration among engineering and biology-oriented undergraduates and faculty at Michigan Tech. The mobile lab will also be used to design new laboratory exercises for an undergraduate ecology course, support student research projects, and facilitate outreach to GK-12 students in the western Upper Peninsula of Michigan.This study will apply new analytical techniques and new stream models in the context of observational and experimental manipulations of nutrient loading to test hypotheses on the balance between nitrogen fixation and denitrification across gradients of nitrogen loads in streams and rivers at the continental scale. The study will address three fundamental questions: (1) Do N fixation and denitrification co-exist in streams across a natural range of nitrogen concentrations, as they do in other aquatic ecosystems?, (2) Do "hot spots" and "hot moments", i.e., spatial and temporal heterogeneity, facilitate this co-existence?, and (3) Do increasing nutrient loads shift the balance to favor losses via denitrification? The study will capitalize on the National Ecological Observatory Network (NEON) as a continental-scale experimental framework to test the hypotheses through three interacting research elements: a gradient study of N2 fixation and denitrification (years 1-4), a multi-scale spatial and temporal study to identify hot spots and hot moments of N2 fixation and denitrification (years 2-5), and an experimental study of nutrient enrichment effects on N2 fixation and denitrification in streams (years 4-5). The project will use MIMS as a new analytical technique along with acetylene reduction and acetylene block assays to measure N fixation and denitrification, and to couple these measurements with reach-scale models commonly used to estimate stream metabolism to estimate denitrification and N2 fixation.