Human actions have altered nutrient and precipitation cycling and these changes are projected to intensify in the future. As soil respiration plays an important role in greenhouse gas (GHGs) fluxes between the terrestrial environment and atmosphere, it is necessary to determine how soil respiration will be affected by future conditions. We investigated how soil fluxes responded to changes in rainfall variability and nitrogen addition. Recent climate projections for the U.S. Midwest suggest there will be fewer but more intense precipitation events. Concurrently, nitrogen deposition rates will increase as a result of increases in fertilizer usage and fossil fuel combustion.
A factorial combination of rainfall manipulation and nitrogen addition (5 g N m-2yr-1) were applied to (4x5.5 m) plots in a restored tallgrass prairie. The rainfall manipulation excluded 50% of rainfall, but added this precipitation back in one rainfall event every 30 days, keeping the total rainfall amount uniform while creating intense precipitation events similar to 2-year historical rainfall events. Cavity ring-down spectroscopy soil chamber measurements captured CO2, N2O, and CH4 data for a single growing season. We hypothesized that the treatments would increase all GHG emissions, with the precipitation variability plus nitrogen treatment causing the strongest increase in GHG emissions.
Results/Conclusions
GHG emissions varied over time across all gases and treatments. Soils acted as a sink of CH4 throughout the growing season with July and August having the highest uptake and September the lowest. N2O emissions varied, with early summer and fall (June, October) operating as an overall sink, and August the largest source.
While treatments with increased precipitation variability and nitrogen addition produced the highest CO2 fluxes earlier (June-August), they were the lowest emitters later in the season (September-October). The interaction between precipitation and nitrogen increased N2O emissions, with the highest emissions occurring June-August. In contrast, increased precipitation variability caused the system to take up N2O. Precipitation variability plots consistently took up the least amount of CH4, with the interaction between nitrogen and precipitation creating the largest sink of CH4.
Our results indicate that the precipitation variability increased CO2 and CH4 and there was a strong opposite interaction of precipitation and nitrogen for CH4 and N2O. Soil GHG fluxes in this system were relatively insensitive to moderate changes in nutrient availability but were sensitive to precipitation variability. The strong seasonal response illustrates that it is important to capture variation throughout time to provide a full understanding of the environment.