Diel responses of soil greenhouse gas fluxes to fertilization and precipitation variability

Background/Question/Methods

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.

Results/Conclusions

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.