Globally, depleted soils could reabsorb 80 billion to 100 billion metric tons of carbon, reducing atmospheric carbon dioxide by 38 to 50 parts per million. That does not include the carbon that could be simultaneously sequestered into vegetation, but the numbers are significant on their own, equaling up to 40 percent of the increase in concentrations since pre-industrial times.
An impressive study (“Greenhouse gas mitigation in agriculture”) reviewing the global potential of multiple practices estimated a theoretical capacity of 1.5 to 1.6 billion tons per year. The authors did not think this would be achieved and estimated 409 to 436 million, 681 to 736 million, and 1.0 to 1.2 billion tons respectively at carbon prices of $ 20, $ 50, and $ 100/ ton US.
How long does sequestered carbon remain in the soil? Scientists call this the mean residence time (MRT). Some soils have inherently better or worse MRT based on their composition. For instance, an abundance of clay particles provides long-term binding sites for humus (which is half carbon). Humus persists 100 to 5,000 years in soil in the absence of disturbances such as tillage.
The IPCC describes soil and biomass carbon reserves as non-permanent and reversible. Trees can be cut down or burned, and soil can be tilled again. There is also a point at which soil and biomass carbon sinks may become saturated and no longer able to absorb carbon. Some scientists think that sequestration slows down to zero, although others report that even in mature old-growth forests carbon stocks in soil and dead organic matter still grow even when living biomass virtually stops increasing. Biochar in soils is likely to persist for 100 to 1,000 years.
Many farming practices sequester carbon, some much more than others. Agricultural practices can have a profound impact on the mean residence time of carbon in soil. Farming practices and crops that sequester and increase the MRT of carbon in the soil include perennial crops, mulching, non-flooded rice, reduced tillage, managed grazing, crop– livestock integration, and continuous cover through green manures or cover crops. In contrast, annual crops, tillage, and bare soils can quickly release soil carbon and return it to the atmosphere.