Model predictions and empirical data
The model results are broadly consistent with empirical work showing that changes in soil C stocks are driven by changes in microbial enzyme activity (Carreiroet al. 2000; Waldrop et al. 2004), and that these changes in activity arise from the multiple effects of temperature on enzyme kinetics, microbial pool size, and microbial allocation to exoenzymes (Steinweg et al. 2013; Malik et al. 2019). Empirical data suggest that natural values of enzyme allocation fraction are low (Schimel & Weintraub 2003; Burns et al. 2013) and fall in the range for which our model predicts large evolutionary responses of decomposition to warming. Measured mass-specific potential enzyme activity, used as a proxy for allocation to exoenzymes, was shown to generally increase with warming, a pattern also predicted by our evolutionary model (Steinweg et al.2013). In particular, Steinweg et al . found that the rise in enzyme allocation was greatest for moderate warming and less pronounced for strong warming, which matches our model predictions under the scenario of temperature-dependent MGE.
A large body of empirical observations and controlled experiments at various time and spatial scales highlights a general response of soil respiration and soil C stocks to warming, involving an ephemeral increase of respiration, no significant change in SOC, and a decrease in microbial biomass. Only the temperature-dependent mortality scenario with evolution could match these patterns. It has been argued, however, that most experiments remain insufficient to rule out model predictions that depart from these patterns (Sulman et al.2018). Furthermore, the mechanism (physiological, ecological and/or evolutionary) by which exoenzyme allocation and soil respiration vary in these experiments is unknown (see Supplementary Note 6 for further discussion of ecosystem modeling predictions of soil C responses to warming). Next we discuss the potential interplay of evolutionary adaptation with physiological acclimation (phenotypic plasticity within individuals) and ecological community assembly (shifts in species composition in whole communities) in shaping soil decomposition responses to warming.