Asymmetric and symmetric warming increases turnover of litter and unprotected soil C in grassland mesocosms

Abstract

The response of soil organic carbon (SOC) to increasing temperature is a critical and uncertain component of terrestrial ecosystems feedbacks to climate. To investigate warming impacts on soil and whole-ecosystem carbon cycling, we conducted a whole-ecosystem warming experiment in grassland mesocosms in Oregon, USA, with diurnal temperature profiles simulating both asymmetric and symmetric increases in daily minimum (Tmin) and maximum (Tmax) temperature. In 2011 we reported that after 3 years of warming, the ecosystem switched from a neutral C balance to a C source, with warming causing an average loss of 88 g C m−2 year−1. Because warming had no impact on aboveground net primary productivity, decomposition of SOC was initially the suspected source of these emissions. Here we report follow-up data from years 4–9 on the effects of warming on soil C stocks, litter, and soil respiration. We employed several complimentary methods to isolate the readily-mineralizable soil C fraction, but power analysis showed that only density fractionation provided sufficient power to detect warming impacts from natural variability. Collectively, the warming treatments reduced the C content in the light density fraction by 9 % after 6 years, and also dramatically reduced litter accumulation, with about 50 % less litter C in warmed chambers by year 9. Consistent with these losses, warming also increased soil respiration by an average of 27–29 %. We conclude that warming-induced losses of unprotected soil carbon and litter, but not protected C, were detectable within a decade.