Stones have to be accounted for when measuring soil carbon

Skipping the labour-intensive and costly process of sieving field-dry samples can lead to significant overestimations of SOC stocks. This poses challenges for monitoring, reporting, and verifying SOC stock changes, as reducing costs by not sieving compromises the accuracy of the calculations.

Stoney soils can overestimate carbon levels (Image: web)

Key Findings

Changes in SOC stocks can indicate either carbon sequestration – the capture and storage of atmospheric carbon dioxide – or carbon losses to the atmosphere, says Owen Fenton, principal research officer at the Teagasc Environment Research Centre, Johnstown Castle.

Accurate SOC stock calculation requires information on three key factors: the depth of the soil layer; a representative bulk density for that layer; and a representative value for the organic carbon content of that layer.

“In a depth-based approach, the closest approximation of the SOC stock in the field can only be achieved using bulk density corrected for rock fragments, or stones, greater than 2mm in size. To accurately determine soil bulk density, cores (of known volume) are extracted from specific depth intervals within a soil profile in the field,” explains Owen.

The samples are then brought to a laboratory where they are dried, weighed, and sieved into fine and coarse components. Although this process is time-consuming and laborious, avoiding it and using a bulk density value that represents the whole soil sample can lead to inaccurate results.

“The present research highlights the critical importance of soil bulk density estimations for SOC stocks, particularly in Irish grassland soils, which can have a relatively high stone content,” adds Owen.

Stone Content and SOC Stock Calculations

In the soil horizons examined, volumetric stone content – rock fragments larger than 2mm – ranged from 0-36%. “These fragments are mainly inert and do not interact with soil organic matter, but have a much higher density than the active part of the soil. If they are not discounted from SOC stock calculations, this can lead to gross overestimations of SOC stocks,” Owen explains.

Peat soils, or organic horizons, have the lowest stone content, whereas mineral soils tend to have higher volumes of stones. To fully understand SOC dynamics, especially in the context of Irish grassland soils with variable stone contents, accurate methods for determining bulk density are essential.

Accurately measuring SOC and changes in soil carbon content over time requires more than just measuring SOC concentration. The Teagasc study evaluated five commonly used calculation methods and compared the resulting SOC stocks using data from three major soil databases, including the Irish Soil Information System, the Heavy Soils Programme, and the Soil Quality Assessment Research Project.

For the horizons examined, inappropriate accounting of stone content in bulk density calculations could lead to SOC overestimations of between 18% and 388%. Methods that do not involve sieving can overestimate SOC stocks by as much as 388%, while those that account for these components offer a more accurate assessment.

“This research underlines the importance of robust, reproducible and accurate methods to measure, report and verify SOC stocks on Irish farms,” says Owen. “These findings offer valuable insights for policymakers, agricultural practitioners, and environmental scientists seeking to enhance the accuracy of SOC stock assessments in grassland soils.

“This work provides a protocol for implementing best practices in measuring bulk density for carbon stocks, and will minimise uncertainty and give confidence to the SOC stocks calculated for future carbon farming schemes.”