How is the carbon use efficiency of microbial communities distributed within the soil pore network

Soils play a key role in mitigating global warming due to their capacity to sequester CO₂, a process likely impacted by the efficiency with which microbial communities transform organic matter into biomass. The activity of microbes is affected by the conditions they find themselves in (moisture, oxygen, etc.). Soils are characterized by a network of pores of different sizes, which create a variety of microenvironments that can influence resource availability, microbial activity and soil C dynamics. However, the influence of conditions at the pore-scale on microbial carbon use efficiency remains poorly quantified. In this study, we conducted short-term incubations using a mixture of 13 C-labelled compounds to assess the carbon use efficiency of microbial communities residing in pores with maximum neck diameters between 15 and 30 μm and between 75 and 200 μm. The study was carried out in an agricultural soil (Eutric Cambisol) under three different management practices and a grassland (Sandy Cambisol) in order to have a range of pore structures, C contents, pH and different microbial communities. Our findings show that microbial mineralization of added substrate was higher in the large pores but that the carbon utilization efficiency was lower. The differences across pore sizes were likely due to the different constraints at the microenvironment scale (moisture, predation, available space, substrate abundance, etc.). These results suggest that microbes adopt distinct carbon processing strategies and functional roles, depending on pore size.