Microorganisms are centrally important to the soil organic carbon (C) cycle as decomposers that also contribute to long term C sequestration through physical stabilization and de novo production of soil organic matter (SOM). Bacterial secretions adhere to soil particles and colloidal materials, and these organo-minerals agglomerate into macroaggregates that are physically stabilized by fungal mycelia and plant roots. Since microbial populations are intrinsically linked to SOM dynamics, the objectives of this review are to discuss: (1) microbial processes that generate SOM, including microbial growth, C use efficiency (CUE), and microbial byproducts; (2) the contribution of saprophytic microorganisms and their trophic strategies (i.e., r-strategist versus K-strategist) to SOM dynamics, and (3) soil management strategies that influence microbial-mediated SOM formation. Although microorganisms have strict homeostatic stoichiometry in their cellular carbon:nitrogen:phosphorus (C:N:P) ratio, microbial groups vary in their use of different C sources; for instance, r-strategists (i.e., gram negative bacteria) efficiently colonize substrates with a low C:N ratio and easily-degradable C, while K-strategists (i.e., gram positive bacteria and fungi) are adapted to degrade complex organic compounds such as cellulose and lignin. Soil organic matter is replenished by microbial byproducts, including cellular debris that is referred to as necromass. Applying organic residues with low C:N ratios to agroecosystems is likely to increase microbial-derived products in the persistent SOM pool. Periodic inputs of substrates rich in lignin and keratin is recommended to sustain microbial diversity in agricultural soils, as K-strategists will gradually transform these complex compounds into simpler substrates that can be metabolized by all soil microorganisms.
