I believe the answer is respiration
The alluvial soils are somewhat enriched in Mn, Zn, Co, and Cu, which is related to the biological accumulation of these elements entering the trophic chains..
The series of processes by which carbon compounds are interconverted in the environment, chiefly involving the incorporation of carbon dioxide into living tissue by photosynthesis and its return to the atmosphere through respiration, the decay of dead organisms, and the burning of fossil fuels.