Accelerated soil carbon loss does not explain warming related increases in soil CO2 efflux

Abstract

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 modeling 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 belowground 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: (i) increases in TBCF and litterfall explain >90% of the increase in FS with MAT; (ii) bulk SOC and SOC fraction size and turnover rate do not vary with MAT; and (iii) increases in TBCF and litterfall do not influence SOC storage or turnover on century to millennial time scales. This gradient study shows that for tropical montane wet forest, long-term and whole-ecosystem warming accelerates belowground carbon processes with no apparent impact on SOC storage.