Linking the carbon cycle to climate change: Effects of warming and altered precipitation on organic matter decomposition in an old-field ecosystem

Background/Question/Methods

Quantifying the effect of temperature, moisture and substrate availability on soil respiration and its components is critical to predict the fate of soil carbon under changing climate scenarios. Since soils have large reserves of carbon, any increase in soil respiration due to climate change could accelerate global warming.  To determine how warming and altered precipitation affect the rate and temperature sensitivity of soil respiration (R_s), rhizosphere respiration (R_r) and heterotrophic respiration (R_h), we subjected a New England old-field ecosystem to four levels of warming (up to 4^oC) and three levels of precipitation (ambient, drought (-50%) and wet (+50%) treatments). We also removed aboveground vegetation in small patches of experimental plots to evaluate the effect of substrate availability. We measured R_s and R_h monthly, and estimated the rhizosphere component by subtracting R_h from R_s. 

Results/Conclusions

In this mesic ecosystem, R_s and its components responded strongly to precipitation. Drought reduced R_s, R_r and R_h, both annually and seasonally. Annual cumulative soil respiration responded non-linearly to precipitation treatments. During the summer, when R_h was highest, we found evidence of threshold, hysteretic responses to soil moisture. Warming increased R_s and R_r in spring and winter when soil moisture was optimal, but decreased these rates in summer when moisture was limiting. Annual apparent Q₁₀ of both R_s and R_r decreased with warming and drought. The effect of climate treatments on the temperature sensitivity of R_s and its components depended on the season. The warmest treatment decreased apparent Q₁₀ of R_s relative to that of unwarmed plots in spring and fall. In summer, due to severe moisture limitation, neither warming nor precipitation affected apparent Q₁₀ of R_s. Drought decreased apparent Q₁₀ in fall compared to the other precipitation treatments. Together, these results suggest that the response of soil respiration to warming was modified by soil moisture variability. Our study also showed that the response of soil respiration was largely driven by rhizosphere respiration. Our results highlight the annual, seasonal and diel variability of R_s, R_r and R_h in response to soil moisture, temperature and substrate availability and emphasize the importance of adequately simulating these responses when modeling trajectories of soil carbon stocks under climate change scenarios.