Precipitation variability alters the responsiveness of soil CO2 evolution

Background/Question/Methods Terrestrial ecosystems are exposed to variation in precipitation due to natural processes and climate change. Precipitation variability can influence belowground ecosystem processes (e.g., CO₂ evolution) via its interactive effects on plant and soil communities. However, multiple factors (i.e., soil texture, temperature, and microbial community composition) mediate the impact of precipitation variability on CO₂ evolution. One complicating factor is the presence of plants. Plants alter the impact of precipitation on soil moisture through interception and transpiration, and contribute to soil CO₂ via root respiration independent of microbial responses. As we strive to understand how precipitation influences feedbacks between soil CO₂ evolution and atmospheric CO₂ concentrations, it is essential that we understand how changes in precipitation influence soil CO₂ evolution and how plants mediate this interaction. Therefore, we measured the responsiveness of soil CO₂ evolution in plant vs. no plant field manipulations exposed to low vs. high variability precipitation regimes in a grassland at the Kellogg Biological Station’s Long-Term Ecological Research site. Soil CO₂ concentrations, moisture, and temperature were measured every minute over two months with in situ real-time sensors. We found the best-fit model for CO₂ evolution dynamics using time-series regression modeling and maximum likelihood analysis.

Results/Conclusions Our model included individual parameters for soil moisture for each precipitation regime in the plant and no plant treatments, but only one temperature parameter for each precipitation regime. The low variability precipitation regime induced the most dramatic responses of CO₂ to fluctuations in soil moisture, regardless of the presence of plants. This precipitation regime produced at least 2.5-times more CO₂ per unit change in soil moisture than the high variability regime. Under both precipitation regimes, the presence of plants decreased the sensitivity of CO₂ to changes in soil moisture. This dampening effect was most pronounced under high precipitation variability, as the presence of plants caused a 4-fold decrease in CO₂ evolved per unit change in soil moisture. Our results suggest that CO₂ evolution is more sensitive to small fluctuations in soil moisture under stable soil moisture regimes and that the presence of plants decreases this sensitivity.