Fig. 1. This conceptual diagram depicts our current understanding of the microbial contribution to C sequestration in agroecosystems. Microbial C pool sizes are indicated by the relative size of the boxes, and the relative rate of C transfer from one pool to another (including CO₂ evolution) is represented by arrow thickness. In Step I, substrate C is partitioned between bacterial and fungal biomass. The amount of C incorporated into biomass and metabolite production versus that lost as CO₂ is dependent on the microbial growth efficiency of the microbial community. The amount of bacterial versus fungal biomass is also determined by the relative degree of protection conferred by the soil matrix (characterized by pore and aggregate size distribution and by clay type and content). In Step II, the rate of transfer of microbial biomass C to microbially derived organic matter is influenced by the chemical recalcitrance of microbial products, the sensitivity of decomposition to (micro)climatic factors, and differential interactions between bacterial and fungal products and the soil matrix. We suggest that a fungal-dominated microbial community improves the physical environment for C stabilization and produces more protected and stable C.