Soil type determines the sequestration potential of soils, playing a crucial role in determining the income farmers stand to claim from the rollout of carbon farming.

Clay soils tend to have the best capacity to take in and retain carbon, senior research officer at Teagasc, Giulia Bondi, said when speaking at a Teagasc conference on counting carbon on Thursday.

“A sandy soil, for example, has a higher CO2 uptake but also a higher CO2 loss so the carbon sequestration potential is quite low,” Bondi explained.

“On the other hand, a clay soil has a similar CO2 uptake but the CO2 loss is much lower as the carbon tends to bind into the clay particles, so the clay protects the carbon and holds the carbon in a deeper layer of soil .

“And then you have the peat soils with a very particular situation of a lower CO2 uptake but a low CO2 loss in natural conditions so the soil organic carbon can build up in natural conditions for a very high carbon sequestration potential.”

Carbon storage limits

However, Bondi added that even in mineral soils with a high potential for storing carbon, there is a limit to how much carbon can be stored.

“Each soil type has a different capacity for carbon sequestration but once this capacity is reached, any additional carbon can be lost. It is important to say that it’s not an infinite container.”

The majority of carbon taken in by soils is also released within the short term and it is only the stocks which remain in soils over a long period of time, which deliver climate benefit and are deemed to be stored.

The typical sequestration rate for improved grasslands on mineral soils are equivalent to 0.4 to 4t CO2/ha/year.

“It is very difficult to measure carbon, it has a very small quantity and it takes a long time to build up,” she said.

Changing land use and farm management can store carbon, but if “you do something wrong, you can go back to square one very easily”.

“Carbon sequestered is considered reversible.

“This is the main message and [it can happen] very easily”.