The world’s soils became a major source of atmospheric carbon around 1860, when agriculture began rapid expansion onto previously uncultivated lands. Soil was the leading source of new atmospheric carbon until the burning of fossil fuels finally surpassed it in the late 1970s.
Most soil carbon is part of soil organic matter. The component parts of soil organic matter include living organisms, the slightly altered constituents of recently living organisms (lipids, proteins, carbohydrates, etc.) and humic substances. Humic substances are the most stable part of soil organic matter, lasting from many weeks to thousands of years.
Soil Carbon Balance
Carbon is added to soils each year, mostly through the decay of dead plant materials. Soil carbon is lost to the atmosphere when soil microorganisms mineralize the organic carbon, forming CO2 and in some cases methane. Over time, equilibrium between soil carbon additions and soil carbon losses is established under a particular set of environmental conditions and land management practices. Unfortunately, most traditional agricultural practices not only increase soil carbon loses, but also decrease the amounts of organic residues returned to the soil, compared to natural systems.
We reduce the amount of carbon added to the soil, because we remove grains, vegetables, hay and some of the other vegetable matter (straw and stover) when we harvest a crop. Also, modern chemical fertilizers replaced the organic amendments that used to be more widely used to supply plant nutrients on many farms.
We increase the microbial activity in the soil, and the associated carbon release through mineralization, by both increasing the soil temperature and increasing the amount of oxygen available to the soil microorganisms. Trees, shrubs, grasses and their natural plant litter shade the soil surface from the heating of the sun. When the shade is removed, soil temperatures increase. Tilling the soil increases the amount of oxygen available to microorganisms. Draining wet soils increases both the temperature and the amount of oxygen in the soil.
Increasing Soil Carbon in Agricultural Lands
The object of soil based carbon banking is to develop a new steady state between carbon additions and losses. Management practices that leave more crop residue in the field and reduce the need for tillage and soil drainage are key to developing this new carbon balance and increase the overall carbon content of the soil.
The outright conversion of crop land to hay, pasture and forests will obviously increase soil carbon. Less drastic land use changes on important croplands include no-till and other reduced till management in row crops, and incorporating more perennial crops in rotations. Organic farming, with its increased input of manures and other composted materials also increases soil carbon, but it has been less studied as a method for carbon sequestration.
Increasing soil carbon content will also enhance some of the soil’s physical and chemical properties important to plant growth and the environment. Fertility, water holding capacity, infiltration rate and the ability for soil to act as a filter for some environmental contaminants is all positively correlated to carbon content. Any effort at increasing atmospheric carbon sequestration in soils will result in a healthier, more productive landscape.
I would opine that carbon sinks are part of the solution but what is your comment on the charcoal burning for town dwellers who cannot afford LPG and or hydro electricity - a case of Uganda?