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Milica presents at SSSA!

Instantaneously Identifying Biological Sources of Nitrous Oxide Flux from Agricultural Soils via Position Specific Nitrogen Stable Isotope Compositions

Milica Radanovic1,2, David R. Huggins3, Benjamin Harlow2, C. Kent Keller4, Tarah S. Sullivan5, R. David Evans1,2

1WSU School of Biological Sciences, Washington State University, Pullman, WA, USA
2WSU Stable Isotope Research Facility, Washington State University, Pullman, WA, USA
3Northwest Sustainable Agroecosystems Research Unit, USDA-ARS, Washington State University, Pullman, WA, USA
4School of the Environment, Washington State University, Pullman, WA, USA
5Department of Crop and Soil Sciences, Washington State University, Pullman, WA 99164, USA

Nitrous oxide (N2O), a greenhouse gas, is produced by multiple biological reactions but their relative contributions in agricultural soils is unknown, preventing loss minimization. We hypothesized that creating favorable environmental conditions for one of these processes, denitrification, will lead to enhanced N2O loss. This was achieved in a field study by adding water to create an anerobic soil environment of 80% water filled pore space and monitoring N2O flux using an automated chamber system connected to a cavity laser absorption spectroscopy system with capabilities to measure site-specific nitrogen stable isotope compositions of N2O. We predicted that differences would indicate the biological sources of N2O. Results showed that biological processes contributing to N2O flux was driven by water application, natural precipitation events, and changes in soil temperature. Site-specific ẟ15N analysis of terminal and central N in the N2O molecule indicate that denitrification is the main process responsible for increased N2O flux as soil temperature decreases or following precipitation events. In contrast, nitrification is the dominant producer of N2O with increased temperature and decreased soil water. These findings are novel as they monitor real time, relative contributions of biological processes responsible for soil N2O flux. Additionally, biological communities responsible for nitrification and denitrification are highly influenced by soil temperature and water, respectively. There is still a fundamental lack in knowledge about which soil microbial populations are responsible and their relative contributions to N2O flux in field settings. The ability to trace N2O to its biological origins and identify environmental impacts on biological populations will aid the scientific community’s understanding of drivers behind critical soil N cycle processes

Meaghan presents at NADP meetings!

Meaghan gave an excellent talk at the National Atmospheric Deposition meeting!

Assessing nitrogen critical loads at North Cascades National Park Service Complex 

Meaghan Petix1, Michael D. Bell2, Tonnie Cummings3, Alida Melse4 and R. Dave Evans5

Anthropogenic nitrogen (N) deposition (Ndep) contributes globally to disruptions in nutrient cycling, ecosystem functioning, and shifts in community composition. National Park Service (NPS) lands, including the North Cascades National Park Service Complex (NOCA), contain ecosystems that are potentially sensitive to Ndep. Accurate measurements of Ndep are needed to determine N critical loads, levels of Ndep that can be sustained without adverse biological effects. However, in the western U.S., complex topography and weather patterns can lead to difficulty in accurately estimating Ndep with predictive models. The N concentration of epiphytic lichens can be utilized to monitor Ndep because the relationship between lichen N concentration and Ndep can be modeled for a given region.

The goal of this study is to assess patterns of Ndep in NOCA and determine which ecosystems are affected by Ndep. We established 30 plots across NOCA to determine lichen community composition and N content and stable isotope composition in the summer of 2018 and 2019. Lichen N content (% dw) varied between 0.25 and 0.49 and estimated throughfall total inorganic Ndep spanned a range from 0.10 to 0.63 kg N ha-1 year-1, indicating low levels of Ndep throughout the park. We found levels of Ndep increased moving east in the northern portion of the park. There was not a strong relationship for lichen N content along an elevational gradient. Results will be incorporated with lichen community composition and atmospheric chemistry models in a GIS framework to develop a powerful approach to evaluate N critical load exceedance in the North Cascades. Integration with U.S. Forest Service (USFS) lichen air quality datasets will allow for further assessment of patterns of Ndep across the region.

1Washington State University, meaghan.petix@wsu.edu 2NPS Air Resources Division, michael_d_bell@nps.gov 3NPS Pacific West Region, tonnie_cummings@nps.gov 4Washington State University, alida.melse@wsu.edu 5Washington State University, rdevans@wsu.edu