Compiled by Nick Greenacre & Anne Ehrlich, members of the EFO Energy Committee
Abridged extracts from"Soil Carbon & Organic Farming "
(A detailed review of 39 studies) The Soil Association, 2009.
Carbon Emissions by Human Activities since 1850
|Fossil Fuel Burning||65.5%|
|Vegetation Losses by Clearance for Agriculture||19.3%|
|Soil carbon losses by conversion of permanent grassland to cropland||9.7%|
|Deforestation for timber & fuel wood collection||5.5%|
Soil is a major store of carbon - 2,500 billion tonnes
3 x that of Atmosphere (760 billion tonnes)
5 x that of Forests and Other Vegetation.
60% of soil carbon is organic carbon.
Before 1850 - 320 million hectares
1850 to 2000 cultivated area increased four fold
Conversion of forest and grasslands to cropland releases 10-50 tonnes of soil carbon/hectare to the atmosphere, 25-30% of the organic carbon in the soil
Globally 10.6 billion tonnes before 1850
+ 44 billion tonnes since 1850
It is estimated that ½ to 2/3 of this "farming" carbon, some 27-37 billion tonnes, can be removed from the atmosphere and returned to the soil by appropriate land management practices.
Soil Carbon Sequestration
Climate Change Mitigation That Supports Adaptation & Food Security
Humus is the main soil carbon store
Fresh organic matter (OM) is source of all humus
Most carbon from fresh OM is released as CO2
Percentage of OM converted to stable carbon depends on:
• Biochemical composition of OM (lignin content, starting C:N ratio <32:1, e.g.%age conversions : straw 5-7%, legumes 17%, farmyard manure (FYM) 23%, compost 50%)
• An active healthy soil organism population.
• Good soil aggregation
• Large root systems (root exudates double carbon, roots have 2 x the lignin of shoots, roots promote soil aggregation)
Farming Practices that Increase Soil Carbon
• Maintaining fertility & increasing top soil depth by use and incorporation of green & animal manures (as FYM & compost)
• Maintaining continuous soil cover
• Maintaining grassland
• Using crop rotations that include hay/pasture (Low to high stable organic carbon = grains< legumes<grass)
• Pasture rotation for optimal grazing. (i.e. avoiding under & overgrazing)
• Grass fed livestock and hay silage.,(i.e. avoiding corn feed and corn silage. Grass fed beef can be carbon neutral).
• Using long straw varieties (increased root size + straw for FYM)