The microbial metabolism in a heterogeneous and warming soil environment: A bioenergetic point of view

The physical colocation of decomposers and substrates has been proposed as being a determining factor of microbial metabolism in soil, which is also greatly modulated by environmental temperature. Moreover, spatial heterogeneity of insoluble substrates is hypothesized to favor the fungal energy channel, as fungi have a well-developed capacity to translocate resources within their mycelia thus overcoming local resource limitation. Here, the effects of warming, substrate spatial heterogeneity, and fungal translocation on microbial metabolism as indicated by substrate-derived CO₂ emission, heat production, and calorespirometric ratio (CR, the ratio of heat production to CO₂ emission) were tested, using cylinders with four compartments that either prevent or allow diffusion between compartments.

With increasing spatial heterogeneity, the CO₂ emission rate generally declined under ambient temperature. The emission rate was slightly higher when diffusion was not allowed across the compartments, except the second half of incubation in the most heterogeneous treatment (100-0-0-0%). In warming environment, the CO₂ emission rates were stimulated, but with diminished effect of spatial heterogeneity. The heat release in the most heterogeneous treatment was lower than the most homogenous (25-25-25-25%) and intermediate heterogeneous (50-0-50-0%) treatments. Under warming condition, the peaks of heat release were heightened, and the peak of the most heterogeneous treatment was brought forward. The heat release was higher when diffusion was not allowed across the compartments under ambient temperature, but insignificant difference among treatments were detected under warming environment. CR decreased rapidly in the first half of incubation, and remained stable during the rest . The difference in CR was mainly detected in the first half of incubation, with CR declining with the decrease of spatial heterogeneity.

Overall, our findings provide detailed information about microbial metabolism in response to substrate spatial heterogeneity and warming climate, and suggest that the degree of substrate spatial heterogeneity is an important boundary condition shaping the energy use channel in this soil compartment.