Comparing the ECOS and EVOL responses to warming
The evolutionary ecosystem response of SOC equilibrium to warming
combines the non-evolutionary response of the ecosystem and the
evolutionary adaptive response of exoenzyme production (Fig. 1c). In all
three scenarios of temperature dependence, the SOC non-evolutionary
equilibrium generally decreases as temperature or resource allocation to
exoenzymes increases (Fig. 1b, Supplementary Figs. 3 and 4,
Supplementary Note 3). The negative effect of temperature results from
the SOC equilibrium being mostly sensitive to the maximum decomposition
rate (\(v_{\max}^{D}\)) (Supplementary Note 3). As the maximum
decomposition rate increases with warming, the SOC equilibrium
decreases. The SOC equilibrium is always lower in systems where microbes
invest more resources in exoenzymes because, all other parameters being
fixed, a larger exoenzyme allocation fraction entails that more
exoenzymes are produced per unit time, which leads to more SOC
decomposed per unit time (Supplementary Note 1).
We therefore predict that, without evolution, the equilibrium soil
carbon stock shrinks as the climate warms, in all scenarios of
temperature dependence (Fig. 3). The strongest losses occur in initially
cold systems due to the non-linear response of the maximum decomposition
rate (\(v_{\max}^{D}\)), hence of the SOC equilibrium, to temperature
(Fig. 3, Supplementary Figure 4). We expect microbial evolutionary
adaptation to alter the non-evolutionary response according to the
scenario, from aggravating soil carbon loss in systems where microbes
adapt to warming with higher exoenzyme production, to buffering carbon
loss from soils where microbes adaptively respond to warming with a
lower exoenzyme allocation fraction (Fig. 1c).
As expected, the simulated ecosystem-evolutionary change in SOC
equilibrium in response to warming (Figs. 2, 3) mirrors the evolutionary
adaptive response of the exoenzyme allocation fraction to warming in all
scenarios of temperature dependence (Supplementary Figs. 5a-d). In the
baseline scenario, microbes always evolve a higher enzyme allocation
fraction in response to rising temperature; as a consequence,
evolutionary adaptation amplifies the non-evolutionary soil carbon loss
due to warming (Fig. 3a). The effect of evolution is strongest in
ecosystems characterized by conditions that are hostile to microbial
growth (low initial temperature, T 0; high
mortality, \(d_{M}\); low MGE, \(\gamma_{M}\); low maximum uptake rate,\(v_{0}^{U}\)) (Fig. 2, Fig. 3a, Supplementary Fig. 6), where the
adaptive response of the exoenzyme allocation fraction is most
pronounced (Supplementary Fig. 5a). Strong evolutionary effects are
robust to the other model parameters – enzyme parameters (efficiency,
production) and environmental parameters (litter input, leaching)
(Supplementary Figs. 6 and 7, Supplementary Note 5), which have no
influence on the relationship between φ * and temperature
(equation (8)). The model also predicts stronger evolutionary effects
when the differential access to resources between microbial strains is
small (low c 0) (equation (8), Fig. 2c, d). Small
competition asymmetry selects for microbes allocating little to
exoenzymes (low φ *); due to the non-linear response of SOC
equilibrium to φ , this causes the SOC equilibrium to be more
sensitive to variation in the exoenzyme allocation fraction
(Supplementary Note 1, Supplementary Fig. 3).
In the temperature-dependent mortality scenario, the strength of the
effect of temperature on mortality is an important determinant of the
evolutionary adaptive response to warming. When mortality is moderately
sensitive to temperature, the positive response of φ * to warming
is attenuated. As a result, the evolutionary aggravation of soil carbon
loss is less severe (Fig. 3b). When mortality is strongly sensitive to
temperature, warming creates more hostile conditions for microbial
growth, therefore φ * decreases with warming. Evolutionary
adaptation then buffers the loss of soil carbon (negative EVO effect,
Fig. 3c).
In the temperature-dependent MGE scenario, the direction (positive or
negative) of the response of φ * to warming is determined by the
initial temperature T 0. The evolutionary effect
parallels the response of φ *. At low T 0,φ * increases strongly with temperature, causing an aggravation of
soil carbon loss (positive evolutionary effect). In contrast, at highT 0, φ * decreases strongly with warming,
thus opposing the non-evolutionary response (negative evolutionary
effect) and promoting carbon sequestration instead (Fig. 3d). At
intermediate T 0, φ * is weakly sensitive to
temperature, and the effect of evolutionary adaptation to warming is
negligible.