Physical Processes and Nitrous Oxide Emissions Pre and Post-Freezing

Abstract:

Soil nitrous oxide (N₂O) fluxes may be moderated by physical and biological processes, particularly when soils freeze and then thaw. There is a need to understand how physical processes affect above- canopy fluxes of N₂O. There is also a need to understand the magnitude and duration of N₂O emission peaks for agricultural fields, particularly the nitrogen-fixing legumes, which can produce and consume N₂O in the plant root symbiosome. There are multiple potential sources of N₂O, including bacteria and fungi in soil and in root symbiosomes. Further, N₂O can be released when trapped in ice or dissolved in solution. These physical and biological processes can contribute to N₂O fluxes measured above the canopy. In 2012-2013, we evaluated canopy, surface and belowground N₂O data for a field seeded to lucerne (Medicago sativa). We used high-frequency data to determine above-canopy N₂O fluxes using an Aerodyne Quantum Cascade Laser integrated with an eddy covariance system, and compared these with low-frequency flux and concentration data collected at the surface and belowground. Belowground moisture, temperature and soil data were used to partition measured N₂O and CH₄ into gaseous and dissolved phases. Pre and post-freeze data indicated the proportion of post-freeze flux previously trapped in ice. Nitrous oxide fluxes following a thaw event were compared with the amount of N₂O trapped during freezing to determine the proportion of the flux resulting from previously trapped gases versus de novo N₂O production.