Disentangling residence time and temperature sensitivity 1 of microbial decomposition in a global soil carbon model

12 Recent studies have identified the first-order representation of microbial decomposition as a 13 major source of uncertainty in simulations and projections of the terrestrial carbon balance. 14 Here, we use a reduced complexity model representative of current state-of-the-art models of 15 soil organic carbon decomposition. We undertake a systematic sensitivity analysis to 16 disentangle the effect of the time-invariant baseline residence time (k) and the sensitvity of 17 microbial decomposition to temperature (Q10) on soil carbon dynamics at regional and global 18 scales. Our simulations produce a range in total soil carbon at equilibrium of ~592 to 2745 Pg 19 C which is similar to the ~561 to 2938 Pg C range in pre-industrial soil carbon in models 20 used in the fifth phase of the Coupled Model Intercomparison Project. This range depends 21 primarily on the value of k, although the impact of Q10 is not trivial at regional scales. As 22 climate changes through the historical period, and into the future, k is primarily responsible 23 for the magnitude of the response in soil carbon, whereas Q10 determines whether the soil 24 remains a sink, or becomes a source in the future mostly by its effect on mid-latitudes carbon 25 balance. If we restrict our simulations to those simulating total soil carbon stocks consistent 26 with observations of current stocks, the projected range in total soil carbon change is reduced 27 by 42% for the historical simulations and 45% for the future projections. However, while this 28 observation-based selection dismisses outliers, it does not increase confidence in the future 29