Very interesting question ...

The decomposition of SOC over the long term ( often referred  as SOC stability) may offer more significant indications of the potential alteration of carbon release from soil . Numerous studies have implicated temperature as a primary controller of SOC stability by altering the quality and quantity of litter input into soil and soil physico-chemical characteristics.Radiocarbon dating and 13C enrichment differentiation for soils indicated that SOC stability along latitudinal and altitudinal gradients was negatively related to MAT. Litter quantity and quality can influence SOC stability by altering the relative abundance of labile and recalcitrant compounds returned to soil. Soil pH and mineralogy can also  affect soil carbon humification and accumulation. In order to determine the factors controlling SOC stability, it is essential to identify the substrate availability, i.e., the sizes of the carbon pools, acting as thermodynamic controls, and their corresponding decomposition rates, acting as kinetic controls.

We investigated the stability of carbon in deep soil layers in one soil profile by combining physical and chemical characterization of organic carbon, soil incubations and radiocarbon dating. Here we show that the supply of fresh plant-derived carbon to the subsoil (0.6–0.8 m depth) stimulated the microbial mineralization of 2,567 ± 226-year-old carbon. Our results support the previously suggested idea7 that in the absence of fresh organic carbon, an essential source of energy for soil microbes, the stability of organic carbon in deep soil layers is maintained. We propose that a lack of supply of fresh carbon may prevent the decomposition of the organic carbon pool in deep soil layers in response to future changes in temperature. Any change in land use and agricultural practice that increases the distribution of fresh carbon along the soil profile1, 8, 9 could however stimulate the loss of ancient buried carbon. Source : Nature 450, 277-280 (8 November 2007) | doi:10.1038/nature06275

Thank you A.K. Srivastava  sir 

Dr. Srivastava has rightly mentioned that litter input and output is a major factor for SOC stability. Beside this, I think the nutrient status, temperature  and microbial activities in the soil affect appreciably the SOC stability in the soil.

Dr.Yadav,the continuous maintenance of certain quantity of humic substances,humic acid,fulvic acid and humin etc provides stability  to soil organic matter.That is possible if continuous replenishment/addition of organic matter is made to soil in part to provide for decomposition and mineralization by microbes and another part to maintain the humus content.The minimum stable carbon that remains in a soil depends on soil temperature and tillage practices.Waterlogging as in paddy system favour organic matter accumulation.Minimum tillage,cover crops, grasses and agroforestry also help to build up stable carbon in soil.The resistant humus and the recalcitrant inert carbon like biochar may be stable carbon pools in soil.