Stability and carbon uptake of the soil microbial community is determined by differences between rhizosphere and bulk soil

The interactions between plants and soil microorganisms are fundamental for ecosystem functioning. However, it remains unclear if seasonality of plant growth impacts plant-microbial interactions, such as by inducing shifts in the microbial community composition, their biomass, or changes in the microbial uptake of plant-derived carbon. Here, we investigated the stability of the microbial community and their net assimilation of plant-derived carbon over an entire growing season. Using a C3–C4 vegetation change experiment, and taking advantage of a natural 13C label, we measured the plant-derived carbon in lipid biomarkers of soil microorganisms in rhizosphere and bulk soil in two soils with contrasting textures. We found that temporal stability was higher in bacterial than in fungal biomass, whereas the spatial stability of the fungal biomass was higher than that of bacterial biomass. Moreover, symbiotic AM fungi tended to be more stable in the uptake of plant-derived carbon than bacteria and saprophytic fungi. While soil texture did influence microbial community composition as expected, it had no effect on the microbial plant carbon assimilation and the differences between rhizosphere and bulk soil. In addition, the putative differences in carbon utilization between microbial groups, with the exception of AM fungi, were generally smaller than expected, reflecting opportunistic utilization of energy sources. Our results suggest that microbial uptake of plant carbon is primarily limited by plant carbon allocation rather than by environmental factors such as soil texture and seasonality. This indicates that the ongoing carbon assimilation during the growing season is supported by a functional redundancy within the microbial community, which, in turn, helps sustain ecosystem functioning.