The Importance of Carbon in the Soil and How it Gets Stored

Soil organic carbon, which makes up about 60% of the soil organic matter on average, has beneficial effects on many physical, chemical and biological functions of soil quality. It helps support the productivity and diversity of all living organisms in the soil. It influences water-holding capacity, aeration, soil aggregation, and other physical aspects. It affects cation exchange capacity, the supply and availability of other nutrient elements, buffering capacity and other chemical parameters of soil. Soil organic matter, with soil carbon, holds vast amounts of organic compounds, nutrients, trace elements, and cations that are essential to plant growth and biological activity.

Soil organic carbon is also important for another reason: it serves as a repository for carbon dioxide (CO2), a greenhouse gas. The amount of carbon in the soil has been estimated to be at least two times greater than carbon in the atmosphere and in vegetation. The problems associated with climate change might be alleviated if more carbon remains captured in the soil as organic carbon.

Plants get carbon mainly by absorbing CO2 from the air and they use it with water to manufacture organic material. Eventually, carbon may comprise at least 50% of plant structure. When plants and animals die, decomposition breaks down their organic material and releases them into the soil.

Soil organic carbon is usually divided into different fractions, determined by the ease of decomposition. Crop residues and the like are easily broken down and the organic carbon is used in biological activity. Particulate organic carbon breaks down more slowly and plays a key role in soil structure; it also contributes to energy and nutrients needed in biological activity. Humus decomposes much more slowly and is especially crucial in providing nutrient elements. Finally, there is the very stable fraction of recalcitrant organic carbon, usually charcoal, which resists further decomposition and serves as a carbon sink.

The manner in which land is used and managed affects the soil’s ability to retain organic carbon.

Soil organic carbon is reduced by farm management practices that raise the decomposition rate of soil organic materials and/or reduce carbon inputs. Such practices include fallowing, overgrazing, burning or removal of stubble, and excessive cultivation (tillage). Essentially, these practices increase the rate of decomposition and expose the soil to erosion — processes that release carbon dioxide into the air instead of allowing carbon to be assimilated into other organic compounds in the soil.

Soil organic carbon is increased by farming practices that increase carbon inputs and/or decrease losses of organic material. The practices involved include direct applications of living organisms (earthworms) or dead organic materials, animal manure, composts, reduced tillage and stubble retention for green manure. Theoretically, any practice that raises crop yields will increase soil organic carbon storage because there is greater absorption of atmospheric carbon. Thus, practices like crop rotation, crop intensification through multiple cropping, and improved cultivars are useful, provided inorganic fertilisers are not used to boost yields.

Many farmers have found that adopting farm management practices that increase soil organic carbon helps them achieve higher profitability and better sustainability. The mitigating effects on climate change provide an added bonus.