Soils contain more than twice as much carbon as the atmosphere according to estimates (Food and Agriculture Association of the United Nations, FAO). Increasing the amount of carbon naturally stored in soils could provide the short-term bridge to reduce the impacts of increasing carbon emissions until low-carbon and sustainable technologies can be implemented. A group called Soil Carbon, based in Australia, makes the case for soil carbon storage in a presentation available in English, German, Spanish, Italian, Mexican and Portuguese. The Soil Carbon report includes impressive photographs, such as those above, demonstrating the difference between well-managed and poorly managed soils.
The Soil Carbon report makes a good read in a powerpoint format rich in pictures, and is an easy introduction to a complex topic for the interested layman. The more scientifically oriented, and truly committed, will want to review the FAO report, Carbon Sequestration in Dryland Soils which goes much more in depth in the science and facts behind soil carbon.
The FAO report sheds some doubt on the optimism in the figures presented by Soil Carbon. For example, Soil Carbon calculates the potential for CO2 sequestration in soil by starting from the assumption that soil organic matter can be increased 1% of the total weight of the soils to a depth of 1 meter. By this calculation, Soil Carbon claims a potential increase of 47 tons of carbon per hectare. As reasonable as a simple “1 % increase” may sound, it appears not to be scientifically valid.
According to the FAO (FAO report, page 28): the carbon content of dryland soils is estimated to be 4 tons/hectare. Carbon content ranges between 7 tons and 24 tons in normal (non-depleted) soils, depending on the climate zone and vegetation. Studies show that non-degraded savannahs can have up to 18 tons C/hectare (top 20 cm). Based on this, one can conclude that an increased carbon sequestration of 18 – 4 = 14 tons/hectare is the most optimistic potential achievement, well under the 47 tons/hectare that Soil Carbon suggests is achievable. Nonetheless, the FAO report point out that increasing the carbon content by only 1.5 tons/hectare on 2 billion hectares of degraded lands could balance out predicted increases in CO2 concentrations in the atmosphere due to annual emissions increases. (FAO report, page 6) This would buy time while fossil-fuel free technologies are developed.
Soil Carbon also targets exclusively the use of ruminant grazing as a soil restoration method. This is only one of many methods, which must be used in combinations depending on the local conditions. As much as the beef lovers amongst us may cheer the finding that cattle are an essential part of a healthy farming eco-system, the FAO points out that there is a large amount of disagreement about the value of ruminants in soil carbon cycling. That manure is the most efficient manner to incorporate carbon into soils is undebated. But some studies point out that feed must be grown on adjoining land, thereby depleting it, so the carbon added to one piece of land is in effect merely displaced from other land, rather than a net positive addition. The question of methane production, a 23-times more potent greenhouse gas than carbon dioxide, must also be considered. Somehow humorous in the multi-faceted evaluations required to make good decisions is the statement in the FAO report that when conducting carbon audits: “it is essential to remember that the purpose of agriculture is to feed people.”
The most interesting facet of the FAO report for the non-scientist may be the discussions of using funding available from carbon offsetting to implement soil restoration projects and help farmers apply methods which benefit soil carbon levels. The additional income from carbon offsetting would help alleviate poverty, and the more productive farming possible after restoration of soils could break farmers out of the cycle of land depletion for mere survival. Although the development of accurate models to measure carbon offsets and the implementation of measures to reduce the risk of reversal of the gains present obstacles, the prospect of carbon sequestration in soils is a win-win for developed and developing nations.