Experimental Warming and Drying Increase the Age of Soil Respired Carbon in Lowland Tropical Forests

Abstract Tropical forests account for over 50% of the global terrestrial carbon sink, but climate change threatens to alter the carbon balance of these ecosystems. We show that warming and drying of tropical forest soils may increase soil carbon vulnerability, by increasing the degradation of older carbon and decreasing decomposition of fresh inputs. In situ whole-profile heating by 4ºC and 50% throughfall exclusion both increased the average radiocarbon age of soil CO2 efflux by ~2–3 years, but the mechanisms underlying this shift differed. Warming accelerated decomposition and loss of older carbon as soil CO2 emissions reflecting increased decomposition and loss of older carbon. Drying suppressed microbial decomposition of fresh carbon inputs as soil CO2 emissions decreased. These findings imply that both warming and drying, by accelerating the loss of older soil carbon and reducing the incorporation of fresh carbon, will exacerbate soil carbon losses in tropical forests under climate change.