R. S. Tubbs and J. S. Schepers. USDA-ARS, 3043 Fletcher Avenue, Apartment. 233, Lincoln, NE 68504
Fluxes of greenhouse gases (GHGs) (CO2 and N2O) from soils are influenced by soil water content, with evolution of N2O increasing as the soil becomes more saturated and anaerobic after rainfall or irrigation events. Biological processes associated with GHG evolution from soils are better documented than physical processes involved with initial displacement of gases already residing in the soil pore space as water infiltration begins. The goal of this work was to evaluate the physical displacement of GHGs as affected by soil water content. To accomplish this goal, bare soil plots were established under irrigated and non-irrigated conditions, and evolution of GHGs was monitored during and after irrigation. To monitor GHG evolution, vented closed chambers (76 cm x 76 cm x 30 cm) with sprayer nozzles for irrigating the soil were deployed over the plots on 12 June 2006. The irrigation event was initiated by applying 1.45 L min-1 for 12.7 min. Gas samples were collected at 5, 10, 15, 30, 60, and 90 min after initiation of the irrigation event in both irrigated and non-irrigated treatments. To analyze data, linear regression was performed relating GHG fluxes vs. time for two time periods; the irrigation period (5-15 min) and the infiltration period (15-90 min). Irrigation increased CO2 flux from the soil by 429% during the irrigation period and by only 82% during the infiltration period. Although N2O flux was not statistically different between irrigation treatments for either time period, the trend showed the irrigated treatment had a 142% increase over non-irrigated during the irrigation period, and a 99% increase during the infiltration period. This difference is expected to become statistically significant after data from other dates and sites are analyzed. These trends show that displacement of gases from the soil pore space occurs quickly upon wetting and water infiltration.