Energy source

Soil organisms may be classified as either autotrophic or heterotrophic, based on where they obtain the carbon, needed to build their cell constituents, and energy.
The heterotrophic soil organisms obtain their carbon from breakdown of organic materials, previously produced by other organisms. Nearly all heterotrophic soil organisms also obtain their energy from the oxidation of the carbon in organic compounds.
The autotrophs obtain their carbon from CO2 gas or carbonate minerals, rather than from carbon already fixed in organic materials. Some autotrophs use solar energy (photo-autotrophs), while others use energy released by the oxidation of inorganic substances such as nitrogen, sulfur, and iron (chemo-autotrophs).