"Respiration carbon" refers to the rate at which biomass is respired expressed as the mass of carbon which it contains. Soil respiration is the sum of respiration in the soil by animals and decomposers of litter (heterotrophs or "consumers"), which have not produced the biomass they respire, and respiration by the roots of plants (autotrophs or "producers"), which have themselves produced the biomass they respire. In accordance with common usage in geophysical disciplines, "flux" implies per unit area, called "flux density" in physics.