Warming-related increases in soil CO2 effux are explained by increased below-ground carbon flux

The universally observed exponential increase in soil-surface CO2 efflux ('soil respiration'; F-S) with increasing temperature has led to speculation that global warming will accelerate soil-organic-carbon (SOC) decomposition(1), reduce SOC storage, and drive a positive feedback to future warming(2). However, interpreting temperature-FS relationships, and so modelling terrestrial carbon balance in a warmer world, is complicated by the many sources of respired carbon that contribute to F-S (ref. 3) and a poor understanding of how temperature influences SOC decomposition rates(4). Here we quantified F-S, litterfall, bulk SOC and SOC fraction size and turnover, and total below-ground carbon flux (TBCF) across a highly constrained 5.2 degrees C mean annual temperature (MAT) gradient in tropical montane wet forest(5). From these, we determined that: increases in TBCF and litterfall explain >90% of the increase in F-S with MAT; bulk SOC and SOC fraction size and turnover rate do not vary with MAT; and increases in TBCF and litterfall do not influence SOC storage or turnover on century to millennial timescales. This gradient study shows that for tropical montane wet forest, long-term and whole-ecosystem warming accelerates below-ground carbon processes with no apparent impact on SOC storage.