Soil organic matter in the sustainable agriculture: source or sink of carbon?

Stocks of Soil Organic Matter (SOM) are controlled by the balance between carbon inputs derived from plant production and outputs through decomposition processes mediated by heterotrophs. The importance and function of SOM is largely due to its dynamic nature, being continuously synthesised, mineralised and rearranged. Besides CO2 production, SOM decomposition provides metabolic energy for microorganisms and mineral nutrients available for plant uptake. Intensive agriculture stimulates the rate of decomposition, because the physical disturbance exposes oxygen diffusion and enhancing C mineralisation. Moreover, a large amount of crop products are removed, thus subtracting C to the ecosystems. Raising the soil C content means increasing the C input, decreasing the output or a combination of the two through improved land management. In particular, in agricultural soils, increasing C inputs is the major pathway for increasing soil C storage, without depleting nutrients released through SOM mineralization. This could be achieved by the use of animal manure, crop residues, sewage sludge and compost, improving rotations with higher C inputs to the soil and reducing soil disturbance with zero tillage practices.