MSA: Quantifying whole-stream denitrification and nitrogen fixation with integrated modeling of N2 and O2 fluxes

人类在景观上的活动增加了淡水环境中氮和磷的输入。这种营养物污染导致了溪流、湖泊和沿海海洋中的藻类大量繁殖和低氧环境。氮可以通过反硝化去除，这是一种由微生物将溶解的氮（硝酸盐）转化为N2气体的自然过程。了解溪流和河流中反硝化发生的地点、时间和程度，对于了解如何管理和保护这些生态系统非常重要。然而，测量反硝化速率需要在封闭的室中使用单独的底物，这为微生物创造了不切实际的条件。在这个项目中，我们将开发建模方法，通过测量昼夜循环中N2气体的浓度来量化河流断面的反硝化作用。我们将在不同浓度和氮磷比例的实验和自然溪流中测试这些模型。我们将创建采样方法，并使用开源框架分享我们的模型，以便研究人员可以采用我们的方法来研究其他溪流和河流。对于更广泛的影响活动，我们将为本科生提供培训机会，并为高中生开设暑期生态学编码和数据培训短期课程。利用N2浓度昼夜循环估算淡水反硝化速率的模型最近得到了改进，该模型整合了利用氧气（O2）浓度估算初级生产速率和呼吸速率的反向建模方法。然而，除反硝化作用外，还有许多不同的过程导致了N2浓度的变化，这些过程尚未被纳入这些模型。例如，N2气体可以通过扩散和气泡形成等物理过程以及固氮（生物介导的N2气体转化为铵）等生物过程从生态系统中去除。我们的目标是通过实验和实地调查测量相结合，改进现有模型，同时解决反硝化和固氮问题。首先，我们将在实验溪流中应用开放水域的N2通量模型，其中反硝化和N2固定的相对活性将受到不同水柱N:P的控制，并且我们将量化气泡形成和气体交换以参数化我们的模型。其次，我们将开放水域模型应用于国家生态观测网络（NEON）的9条河流，这些河流被选中具有N:P梯度，并且先前nsf资助的研究已经记录了反硝化和固氮活性的梯度。总之，这些活动将大大提高对美国不同生态区溪流中N2通量的估计，并提供改进的建模技术，使我们和其他人能够更好地了解溪流和河流中的氮循环和去除。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Human activities on the landscape have increased the input of nitrogen (N) and phosphorus (P) to freshwater environments. This nutrient pollution has led to algae blooms and low-oxygen conditions in streams, lakes and the coastal ocean. Nitrogen can by removed by denitrification, a natural process carried out by microbes that transforms dissolved nitrogen (nitrate) into N2 gas. It is important to understand where, when, and how much denitrification occurs in streams and rivers to understand how to manage and protect these ecosystems. However, measuring denitrification rates requires using separate substrates in enclosed chambers, which creates unrealistic conditions for microbes. In this project, we will develop modeling approaches to quantify denitrification in stream sections by measuring concentrations of N2 gas over day-night cycles. We will test these models in experimental and natural streams across varying concentrations and ratios of N and P. We will create sampling methods and share our model using an open-source framework so that our approach can be adopted by researchers to study other streams and rivers. For broader impact activities, we will provide training opportunities for undergraduate students and create a summer short-course on coding and data training in ecology for high school students.Models that estimate freshwater denitrification rates using day-night cycles of N2 concentrations have recently been improved by integrating inverse modeling approaches that have been widely studied for estimating rates of primary production and respiration using oxygen gas (O2) concentrations. However, many different processes contribute to changes in N2 concentrations besides denitrification that have not yet been integrated into these models. For example, N2 gas can be removed from ecosystems by physical processes like diffusion and bubble formation, as well as by biological processes like nitrogen fixation (the biologically-mediated conversation of N2 gas to ammonium). We aim to improve existing models to simultaneously resolve denitrification and nitrogen fixation through a combination of experimental and field survey measurements. First, we will apply open-water N2 flux models in experimental streams where the relative activity of denitrification and N2 fixation will be manipulated by varying water column N:P, and where we will quantify bubble formation and gas exchange to parameterize our model. Second, we will apply open-water models to nine streams that are part of the National Ecological Observatory Network (NEON), selected to have a gradient of N:P and where prior NSF-funded research has documented a gradient of denitrification and N2 fixation activity. Together, these activities will both provide much improved estimates of N2 fluxes from streams in different ecoregions in the United States, and provide improved modeling techniques that can be applied by ourselves and others to better understand nitrogen cycling and removal in streams and rivers.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.