More carbon in tillage soils than expected

Soil organic matter and soil carbon are directly linked. Increasing soil organic matter and carbon helps to improve soil health and function.

Another benefit is that carbon taken from the atmosphere by plants can be incorporated into the soil in the form of straw, cover crops and organic manures.

Tillage soils hold an average of 90t of carbon per hectare. This is lower than the grassland average of 110t C/ha, but according to Guilia Bondi of Teagasc, it was expected to be much lower.

These figures come from the sampling of farms involved in the Teagasc Signpost programme. The carbon levels ranged from 48t C/ha to 208t C/ha. This is very high compared to continental Europe, with Mediterranean soil holding between 40 and 80t C/ha.

The soil samples analysed were taken to 60cm. Deep sampling to this depth is critical as 20 to 30% of the carbon is stored below 30cm.

Guilia noted that many tillage soils start from a low amount of carbon, and it is a slow process to build up the carbon levels in the soil

An equilibrium, a point at which no more carbon can be stored, will eventually be reached. However, a soil can lose carbon quickly, and care must be taken to ensure carbon sequestration is not reversed.

Soil type has a major role to play in carbon sequestration potential.

Sandy soils have a high carbon uptake, but also loses carbon easily, leading to a low potential for sequestration. Carbon losses are much reduced under clay soils, and so have a high potential to store carbon.

Clay particles in the soil bind with carbon and protect it from being lost to the atmosphere.

These soils are also saturated with water for longer, which slows down organic matter decomposition and subsequent carbon emissions. Therefore, the amount of clay in a soil determines the potential size of the carbon sink.

Where is carbon stored?

There is also a difference in where carbon is stored in the soil profile. Sandy soils will store most of their carbon in the topsoil, with less in lower layers.

However, heavy soils will contain much more carbon at greater depths due to higher amounts of clay.

When it comes to cultivation methods, Guilia explained there is not a huge difference between plough, min-till, and no-till systems.

However, ploughing can help to increase soil carbon in the whole soil profile if organic matter such as chopped straw or cover crops are ploughed down.

“The management of carbon inputs [straw, cover crops, organic manures] may be more important than cultivation type in Ireland,” added Guilia. This is due to our farming systems and wet, temperate climate.

In order to maximise carbon sequestration, Guilia proposes that soil-specific management needs to be applied.