How is microbial metabolic power distributed throughout the soil pore network?

Soil microbial communities live within a complex three dimensional pore network, the properties of which constrains microbial life and activity. The physical structure of soil, and the associated pore network, limit microbial access to resources. It also determines micro-environmental conditions (e.g. redox conditions) that can affect microbial use of the available resources and the rates at which they use energy. Whilst the distributions of different types of activities (CO2 production, enzyme activities) in the pore network have received some attention, the rate at which microbial communities use the energy available to them, i.e. metabolic power, has received little. Energy is required for most aspects of microbial functioning and the rate at which this energy is used determines the extent to microbial functioning proceeds.

Linking the energy available to the rate at which it is processed at the pore scale may help us to better understand how microbial growth and C dynamics are constrained by the physical environment in soil. In order to do so, we collected data from papers in which isotopically-labelled organic substrate was added to pores with different neck diameters and calculated the microbial community catabolic rates, the Gibbs energies of the reactions in oxic and anoxic conditions. This allowed us to estimate the distribution of microbial metabolic power in the pore network and of carbon use efficiency using the approach in LaRowe and Amend (American Journal of Science, Vol. 315, March, 2015,P.167–203, DOI 10.2475/03.2015.01). We then compare the calculations with laboratory measurements of the distribution of carbon use efficiency at the pore scale.