Diel responses of soil greenhouse gas fluxes to fertilization and precipitation variability
Climate change and anthropogenic nitrogen alter fluxes of greenhouse gases (GHGs) between terrestrial ecosystems and the atmosphere. Recent climate projections for the U.S. Midwest anticipate that in the future there will be fewer but more intense precipitation events. This change, in combination with nitrogen deposition from fertilizer usage in agriculture and fossil fuel combustion threatens to alter soil fluxes of CO2, N2O, and CH4. Our objectives were to determine diel soil GHG fluxes (CO2, N2O, and CH4) and how they respond to a 50% rainfall reduction coupled with 2 year historical rainfall events, and nitrogen deposition over two times the amount currently deposited annually on a mixed-grass prairie. We employed rainfall exclusion plots, adding the omitted rainfall to treatment plots every 30 days to keep the total rainfall amount uniform while creating intense precipitation events. Slow-release polymer coated urea was applied at the beginning of the growing season to provide for constant nitrogen addition. We utilized cavity ring-down spectroscopy (CRDS) soil chamber measurements to measure CO2, N2O, and CH4 fluxes during six 24-hour periods throughout June-August.
All treatments produced strong diel patterns of respiration with the maximum CO2 flux rates occurring in the afternoon when the air temperatures were the highest and the lowest rates in the morning and evening. Overall, the soil functioned as a sink for CH4 throughout the day and a stronger source for N2O in the evening. Fluxes ranged from 2.099 – 5.327 µmol/m2s for CO2, -1.021 *10-4 to 8.288*10-4 µmol/m2s for N2O and -1.355 to 0.0424 µmol/m2h for CH4. Our results indicate that the treatments did not significantly impact the overarching diel flux curves, suggesting that microbial process rates in this system are relatively insensitive to moderate changes in water and nutrient availability.