The soil carbon calculation is a factor of the change in soil carbon per year versus the soil reference carbon stock, and depends on the type of soil and climatic region for the location of the farm where the assessment is being completed. The methodology comes from IPCC 2019. The delta change in the soil carbon is a factor of three changes:
- Land Use: E.g. cultivated, paddy, perennial, set-aside, again this is climatic zone dependant and whether the moisture regime is moist or dry.
- Tillage: This is classified under three regimes – Full, reduces and no-till, for each the conditions as to whether it emits or sequesters carbon is determined by the climatic zone and the moisture regime.
- Carbon Inputs: Relative carbon stock change, is determined by the cover crop and organic amendment regime applied to the soil, against this is climate zone dependent and moisture regime.
Specifically looking at the tillage factors the range of whether tillage practice emits or sequesters carbon is broad. This is known as the Stock Factor in IPCC and the CFT takes its data from IPCC 2019 table 5.5 (see the full list in our technical documentation) but to highlight the range:
- Most sequestration potential comes from no-till in moist regimes in Tropical (including Tropical-Montane) and warm-Temperate climate zones, where sequestration could by up to 10% of the soil reference carbon stock.
- The most emissions potential comes from Reduced Till in Cool Temperate and Boreal climate zones under a dry moisture regime where up to 2% of the soil reference carbon stock could be lost.
IPCC also provide a % Error confidence (for 2 standard deviations) for each factor, and these range between 0 and 7%. So with the two examples above the no-till tropical moist potential of 10% has a potential error factor of 7%, so the sequestration could be anywhere between 3% - 17%. Similarly for reduced till in cool temperate in a dry moisture regime, the emissions of 2% has an error factor of 5%, so the emissions may actually be 7% or could be sequestering 3%.
This is the reason why there is no definitive answer as to whether tillage regime emits or sequesters carbon. It is dependent on your climate zone, your soil type and your moisture regime, and within that there is local variation incorporated in the error uncertainties in the original evidence from IPCC. It is context specific for the farm in question, and is also the reason why the cool farm tool does not make recommendations as to the regime that any particular farm should adopt. The results of the CFT should be discussed at farm level between the farmer, their agronomist and ecologist to determine the most appropriate regime for their farm.