English: Proposed model for soil organic carbon (SOC) cycling showing root carbon (C) inputs as the primary source of both SOC and dissolved organic C (DOC) in most ecosystems. Root-derived C is shown undergoing multiple stages of microbial processing, protection, and release into the DOC pool as it is transported vertically down the soil profile. Unlike shoot C inputs—which are often mineralized in the litter layer or undergo partial oxidation via microbial extracellular enzymes—most root C inputs will undergo microbial assimilation, biosynthesis, and turnover prior to SOC incorporation. Microbial processing increases the solubility and potential for protection of organic C compounds, which are protected primarily through abiotic mechanisms involving physical protection within soil micro- and macro-aggregates (upper right inset) and mineral sorption of DOC compounds. Microbial activities, and thus SOC decomposition, can be stimulated by multiple mechanisms, and the protection of SOC can be counteracted by physical or biochemical mechanisms, such as by certain root exudates fostering the release of organic C compounds from protective mineral associations (lower left inset). Soil fauna (e.g., detritivores) are represented by earthworms, which contribute to bioturbation. Respired C is shown for the whole soil (in relative amounts), as well as for individual processes. Differences in microbial communities between the litter layer, rhizosphere (i.e., the portion of soil in the immediate vicinity of roots), bulk soil, and with depth are indicated by color. Within the circles illustrating DOC cycling, thicker lines indicate more rapid rates. The size of the ends of wedges represents a relative increase or decrease. Controls on the processes shown include temperature, moisture, the flora and fauna present, and other ecosystem and soil properties (e.g., parent materials, texture, mineralogy, and pH).