The immediate and prolonged effects of climate extremes on soil respiration in a mesic grassland

The predicted increase in the frequency and intensity of climate extremes is expected to impact terrestrial carbon fluxes to the atmosphere, potentially changing ecosystems from carbon sinks to sources, with positive feedbacks to climate change. As the second largest terrestrial carbon flux, soil CO2 efflux or soil respiration (R-s) is strongly influenced by soil temperature and moisture. Thus, climate extremes such as heat waves and extreme drought should have substantial impacts on R-s. We investigated the effects of such climate extremes on growing season R-s in a mesic grassland by experimentally imposing 2 years of extreme drought combined with midsummer heat waves. After this 2 year period, we continued to measure R-s during a recovery year. Two consecutive drought years reduced R-s by about 25% each growing season; however, when normal rainfall returned during the recovery year, formerly droughted plots had higher rates of R-s than control plots (up to + 17%). The heat wave treatments had no effect on R-s, alone or when combined with drought, and during the growing season, soil moisture was the primary driver of R-s with little evidence for R-s temperature sensitivity. When compared to aboveground net primary production, growing season R-s was much less sensitive to drought but was more responsive postdrought. These results are consistent with the hypothesis that ecosystems become sources of CO2 during drought because carbon inputs (production) are decreased relatively more than outputs (respiration). Moreover, stimulation of R-s postdrought may lengthen the time required for net carbon exchange to return to predrought levels.