Fig. 1. Carbon fluxes through the crop–soil system. 1 = net crop production, generating organic matter from atmospheric CO₂. 2 = aboveground crop residues returned to the soil surface. 3 = belowground C allocation, that is, fluxes of C from shoots to root systems. 4 = root mortality and other inputs of organic matter from root systems to the soil. 5 = root, mycorrhizal, and rhizosphere respiration. 6 = CO₂ produced by the heterotrophic oxidation of detritus in the soil. 7 = surface soil CO₂ efflux, or soil respiration.