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