The carbon used by these bacteria is derived solely from atmospheric CO2. Mineralization is the process of converting organic N to plant available inorganic forms. It is a gradual breaking down of large molecules to smaller molecules by a succession of soil microorganisms. After these microbes complete their relatively brief life cycle, they are decomposed by other microbes. Energy for this process is obtained from carbon in the material being used, so introduction of fresh plant materials stimulates breakdown activity.
Immobilization is the process of incorporating inorganic into organic form by microbes or plants.
Because it is largely dependent on microbes, the availability of carbon and other nutrients determine the rate of immobilization. This effect eventually fades because, without external N, the microbial population dies off and decomposes, releasing N which is available to plants. The risk of immobilization is avoided by mixing plant residues into the soil well before the next cropping cycle. Nitrogen can be permanently removed from soil by erosion, leaching, denitrification, or volatilization. Organic matter, which contains the majority of native N, is concentrated in the plow layer of soils, and is thus susceptible to loss by erosion.