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.