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

Reduced soil-carbon storage in response to warming is a potential reinforcing feedback that could enhance climate change. A study now shows that for tropical montane wet forest, long-term warming (represented by an altitudinal gradient) accelerates below-ground carbon processes but has no apparent impact on soil-organic-carbon storage. The universally observed exponential increase in soil-surface CO2 efflux ('soil respiration'; FS) with increasing temperature has led to speculation that global warming will accelerate soil-organic-carbon (SOC) decomposition1, reduce SOC storage, and drive a positive feedback to future warming2. 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 FS (ref. 3) and a poor understanding of how temperature influences SOC decomposition rates4. Here we quantified FS, litterfall, bulk SOC and SOC fraction size and turnover, and total below-ground carbon flux (TBCF) across a highly constrained 5.2 °C mean annual temperature (MAT) gradient in tropical montane wet forest5. From these, we determined that: increases in TBCF and litterfall explain >90% of the increase in FS 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.