The largest emissions from soils are as CO2 as a result of respiration by soil micro-organisms from the decomposition of the active SOM pool.
This active SOM pool has carbon constantly being added to by plants from their residues, roots and exudates, as these plants take CO2 from the atmosphere to produce simple carbohydrates and all other organic compounds that enable them to grow. The additions and losses of carbon are relatively equal in a steady state system with a (very) gradual increase in SOM.
Agriculture and forestry influence the rates of carbon addition and losses to the SOM.
The most significant effect is from mechanical cultivation of soils. By cultivating soils the disturbance allows for greater oxidation of the slow SOM by soil micro-organisms with a consequent release of CO2.
Soils contain between 30 – 90t carbon/ha at 30cm depth.
Depending on the a number of factors, primarily preceding cropping, soil type, intensity of the cultivation and moisture content, cultivated soils can lose 3t of soil carbon/ha/year.
The highest losses of SOM occur in the first year of ploughing out a permanent pasture and if cultivation continues over the next 25 years and can result in 25 – 40% of the original soil carbon being lost depending on the soil type.
Soil surveys in the England and Wales from 1978 – 2003 estimated that soil organic carbon decreased over that time by 0.6% /year, which would equate to a loss of 4.4 million t C/yr. Where SOM levels were higher than average, the rate of loss of soil carbon was also greater, as much as 2% per year.
Arable farming techniques for building carbon
Where land is under continual cultivation, as is much of UK arable land, reducing the frequency, depth and intensity of cultivations will reduce this soil carbon loss.
Changing your crop establishment system to reduce the frequency and intensity of cultivations will provide an immediate reduction in farm GHG emissions.
Techniques for arable cropping with no cultivations are being practised on a few farms in the UK but there is very little data yet to assess what is happening to the SOM under these systems.
Changes in land use (grassland to cropping, cropping to forestry etc) has a significant global impact on GHG emissions but the UK has been a net 'sink' for carbon over the last few years.
Peat soils and soils with very high levels of SOM (>10% SOM) pose additional GHG challenges. Because of the very high levels of SOM, if these soils are cultivated (or drained) the resulting GHG emission can be 4x higher than the same action on an 'average' 5% SOM soil
Grassland farming techniques for building carbon
See the FCCT Case Studies section for examples such as at Woodland Valley Farm. Techniques include optimising stocking rates, appropriate sward species and root depth, and adopting permanent pasture.