Warning some soil carbon estimates could be significantly overstated

Stones can lead to the overestimation of soil organic carbon by 18-388%, Teagasc researchers warn.

A soil sample taken from a grassland field that is dried and sieved in the laboratory into fine and coarse fragments. The coarse proportion must be removed from the calculations of SOC stocks. This is the gold standard approach for measuring bulk density and estimating SOC stocks and represents best practice.

New Teagasc research just published highlights the consequences of not measuring soil organic carbon stocks in Irish grassland soils precisely.

Quantifying changes in carbon levels, either carbon sequestration or losses into the atmosphere, requires accurate determination of soil bulk density, which is only achieved by accounting for soil stone content (rock fragments higher than 2mm in size).

The research by Professor Owen Fenton and colleagues highlights the critical importance of soil bulk density estimations for soil organic carbon stocks, particularly in Irish grassland soils, which can have relatively high stone content. 

Stones (i.e., rock fragments higher than 2mm in size), which ranged in content from 0% to 36% in the soil horizons examined, are mainly inert and do not interact with soil organic matter, but have a much higher density than the active part of the soil. So if stones are not discounted from soil organic carbon stock calculations, it can lead to gross overestimations of carbon stocks.

Professor Owen Fenton, principal research officer at Teagasc Johnstown Castle, said: “To fully understand soil organic carbon dynamics, especially in the context of Irish grassland soils with variable stone contents, we need accurate methods for determining bulk density. 

"Stones are often incorrectly accounted for in bulk density calculations and can lead to overestimation of soil organic carbon by 18-388%. Accurately measuring soil organic carbon and carbon content changes in soil over time requires more than the measurement of soil organic carbon concentration.” 

Teagasc evaluated five commonly used calculation methods and compared the resulting soil organic carbon stocks using data archived within three prominent soil databases, including the Irish Soil Information System (I-SIS), the Heavy Soils Programme (HSP), and the Soil Quality Assessment Research Project (SQUARE). 

Methods for measuring bulk density that do not break field-dried samples into fine (less than 2mm) and coarse material (above 2mm) through arduous sieving, overestimate soil carbon stocks by as much as 388%, whereas those methods that factored in these components were closer to reality.

Dr Karl Richards, head of the Teagasc Climate Centre, said: “This Teagasc research underlines the importance of robust, reproducible and accurate methods to measure, report and verify soil organic carbon stocks on Irish farms”.

These findings offer valuable insights for policymakers, agricultural practitioners, and environmental scientists seeking to enhance the accuracy of soil organic carbon stock assessments in grassland soils. 

Dr Karen Daly, Acting Head of Environment, Soils and Land Use Research Department in Teagasc, said: ‘‘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 soil organic carbon stocks calculated for future carbon farming schemes.”