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

Michigan Technological University, Houghton MI

Investigators

Abstract

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.