Soil greenhouse gas (GHG) fluxes play an important role in regulating carbon and nitrogen cycling and carbon sequestration. Changing abiotic conditions from climate change will affect these emissions. Already, heavy precipitation events have become more frequent in the Midwestern U.S., a trend projected to continue. Between inundation events, drought conditions will occur. Drought, compounded with increased nitrogen deposition from fertilizer and fossil fuel usage, affects nutrient availability and the soil processes that depend on accessible nutrients.
We evaluated the response of soil CO2, N2O, and CH4 emissions to changing precipitation regimes and nitrogen fertilization (5 g N m-2yr-1) in a tallgrass prairie. Rainout shelters intercepted 50% of incoming precipitation, and we added back that amount every 30 days to create heavier precipitation events (2-year historical rainfall events) while keeping total precipitation constant. GHG emission rates were measured between and after rainfall additions. We hypothesized that the heavy rainfall events would produce increased CO2 and N2O emissions. We also expected that brief periods of anaerobic conditions produced by the rainfall events would elevate CH4 emissions, but that emissions would be suppressed in the drier periods between rainfall events and treatments would follow typical diel flux patterns for CO2.
CO2 emissions varied by time of day, with a strong diel pattern, but precipitation variability nor nitrogen fertilization did not significantly reduce emissions between rainfall events. Pre-existing soil moisture conditions determined whether CO2 emissions would be suppressed or elevated in response to precipitation events, and all emission responses were of short duration. Nitrogen addition, and to a lesser extent its interaction with precipitation variability, influenced how N2O emissions responded to time of day. Similar to CO2, CH4 emission and sink strength depended on a relationship between pre-existing soil moisture conditions and the timing of rainfall events. As hypothesized, the precipitation events created anaerobic conditions and became a source of CH4, but only for a short period.
Our results indicated precipitation variability in this tallgrass prairie affects soil GHG emissions. While emissions responded to the treatments over the short term, these responses to changes in precipitation variability and nitrogen fertilization were moderate, indicating that potential environmental changes might not signficantly impact tallgrass prairie GHG exchange between the earth and atmosphere.