15N reflects wetland nitrogen processing on a national scale as predicted by soil chemistry stoichiometry

Wetlands provide critical ecosystem services by intercepting and retaining excess non-point nitrogen (N) inputs moving through the environment. However, not all wetlands have the same capacity to store and remove N via denitrification, plant uptake, or storage in soil organic matter. The extent of these N transformations depends on factors such as regional differences in climate, N loading to the wetland watershed, and hydrogeomorphic and biogeochemical properties of the wetland itself. This study aims to identify wetland characteristics that target N removal by processing within wetlands as indicated by d15N values within the top 10 cm of wetland soils. In 2016, the US Environmental Protection Agency’s National Wetland Condition Assessment (NWCA) collected soil samples from ~1000 wetlands across the conterminous United States (US) in addition to a range of chemical, physical, and biological variables. Stable N isotope ratios (d15N) were measured on soil cores because we expected them to integrate signals from ecosystem N sources and N transformations over time. Using generalized additive models, we tested the influence of other wetland variables across climate, in situ wetland characteristics, and catchment land use, hydrology and inputs on soil d15N values. The primary driver of the d15N values was in-situ soil chemistry that captures the pH and stoichiometric ratio of C:N associated with denitrification, explaining ~ 50% of the overall variance. Other factors such as N inputs and sources entering the wetland were far less important than the inherent soil chemistry, revealing the surprising result that wetland soils preserve a historic N processing signal largely independent of recent N loading. We developed a predictive model of wetland in-situ N processing that can be used to identify wetlands with strong N processing and removal capacity.  Excess N in US waters leads to ecosystem risks such as eutrophication, biodiversity losses, and more. Therefore, identifying wetlands that are efficient at processing non-point N pollution is a critical step to inform nutrient reduction strategies at watershed scales.

Impact/Purpose

Wetlands provide critical ecosystem services by removing and storing excess nitrogen that is produced in and applied across agriculture and urban land use settings. Preventing this excess nitrogen from entering waterways is important because in excess, nitrogen acts as a pollutant and degrades water quality and therefore impacts our downstream ability to use and enjoy the water. Not all wetlands have the same capacity to reduce and store nitrogen, therefore this study applies national level survey data from the United States Environmental Protection Agency’s (US EPA) National Wetland Condition Assessment (NWCA) to investigate what characteristics across the wetland, watershed, and climate enhance nitrogen removal in wetlands. Using stable isotope methods, we identified relationships with soil properties such as soil carbon and nitrogen concentrations, pH, and density that indicate potential nitrogen removal in wetlands across regional differences and wetland types. From these results we developed a predictive model for wetland N processing that can be used to identify wetlands with strong N processing and removal capacity.  This model application is important because identifying wetlands that are efficient at processing non-point N pollution is a critical step to inform nutrient reduction strategies at watershed scales. 

Citation

Nowakowski, Catrina, A. Nahlik, R. Brooks, J. Compton, M. Weber, R. Hill, R. Sabo, M. Brehob, L. Trine, AND W. Rugh. 15N reflects wetland nitrogen processing on a national scale as predicted by soil chemistry stoichiometry. American Geophysical Union Annual Fall Meeting, Washington, DC, December 09 - 13, 2024.