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. Organic carbon and therefore, SOM levels largely depend on vegetation cover and type of plant (root depth and spatial distribution), along with any land use chnages.
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, known as reduced/ zero tillage are being practiced in the UK. A report earlier this year (2019) from Defra stated that c.50% of the total SOC accumulation (after 100 years) occurs within the first 20 years after having introduced reduced tillage soil management. Maintaining SOC at the new equilibrium level then becomes the main priority, which may be dependent on continuing or finding new management practices. but there is very little data yet to assess what is happening to the SOM under these systems. For more information on how tillage methods can affect soil carbon content read our latest article via here.
This evidence demonstrates how changes in land use (grassland to cropping, cropping to forestry etc) has a significant global impact on GHG emissions both in terms of its source and sink. Recntly the UK has been a net 'sink' for carbon, however, since 2008 there has been no significant changes in emission reductions from the agricultural sector as a whole, largely due to high demand and intensification of food production.
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. As well as this, the positive effects of pasture-fed livestock on the environment and climate can be seen here, which includes returning manures back to the soils and information on Pasture for Life.