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

Abstract

AbstractThe 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 CO2efflux or soil respiration (Rs) is strongly influenced by soil temperature and moisture. Thus, climate extremes such as heat waves and extreme drought should have substantial impacts onRs. We investigated the effects of such climate extremes on growing seasonRsin a mesic grassland by experimentally imposing 2 years of extreme drought combined with midsummer heat waves. After this 2 year period, we continued to measureRsduring a recovery year. Two consecutive drought years reducedRsby about 25% each growing season; however, when normal rainfall returned during the recovery year, formerly droughted plots had higher rates ofRsthan control plots (up to +17%). The heat wave treatments had no effect onRs, alone or when combined with drought, and during the growing season, soil moisture was the primary driver ofRswith little evidence forRstemperature sensitivity. When compared to aboveground net primary production, growing seasonRswas much less sensitive to drought but was more responsive postdrought. These results are consistent with the hypothesis that ecosystems become sources of CO2during drought because carbon inputs (production) are decreased relatively more than outputs (respiration). Moreover, stimulation ofRspostdrought may lengthen the time required for net carbon exchange to return to predrought levels.