Between a rock and a hard place
Stones must be removed from grassland soils for accurate carbon measurements, but this process can be arduous. Teagasc research is examining improved protocols for soil carbon calculations.
New Teagasc research, published in the international soil science journal Geoderma Regional, highlights the consequences of not considering stone content when calculating soil organic carbon (SOC) stocks in Irish grassland soils. Sieving a sample and removing the stones from the calculations is important as these stones do not contain carbon.
Shortcuts taken to avoid arduous and costly sieving of field dry samples can lead to an overestimation of SOC stocks. Monitoring, reporting and verification of SOC stock changes is in a challenging position, as keeping costs low – by not sieving – will have consequences for the accuracy of SOC stock calculations.
Stock in trade
Changes in SOC stocks can indicate either carbon sequestration – the capture and storage of atmospheric carbon dioxide – or losses of carbon into the atmosphere, says Owen Fenton, a Principal Research Officer at Teagasc Environment Research Centre, Johnstown Castle.
Accurate calculation of soil organic carbon stocks needs information on three elements, explains Owen: depth of a soil layer, a representative bulk density of 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.”
Samples are then brought to a laboratory and dried, weighed and sieved into fine and coarse components. This part of the process is arduous and there can be a temptation to avoid this step altogether and proceed with a bulk density that represents the whole soil sample, Owen says. “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.”
Scanning the horizon
In the soil horizons examined, volumetric stone content – rock fragments greater than 2mm in size – 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, we need accurate methods for determining bulk density.”
Accurately measuring SOC and carbon content changes in soil over time requires more than the measurement of SOC concentration. The Teagasc study evaluated five commonly used calculation methods and compared the resulting SOC stocks using data archived within three prominent 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 overestimation of SOC by 18-388%.
Certain methods for measuring bulk density use arduous sieving processes to break field-dried samples into fine material (defined as less than 2mm) and coarse material (greater than 2mm). Methods that do not use sieving can overestimate SOC stocks by as much as 388%, whereas those methods that factored in these components were closer to reality.
“This research underlines the importance of robust, reproducible and accurate methods to measure, report and verify SOC stocks on Irish farms,” concludes 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.”
Sieving soil samples is a vital process for more accurately estimating SOC stocks
Acknowledgements
Other authors on the Geoderma Regional paper are Giulia Bondi, Conor Bracken and Lilian O’Sullivan.
Funding
Teagasc core-funded work.
Contributors
Owen Fenton, Principal Research Officer, Teagasc Environment Research Centre, Johnstown Castle, Wexford. owen.fenton@teagasc.ie
Patrick Tuohy , Senior Research Officer ,Teagasc Animal & Grassland Research and Innovation Centre, Moorepark, Fermoy, Cork.
Karen Daly, Principal Research Officer, Teagasc Environment Research Centre, Johnstown Castle, Wexford.
Luis Lopez-Sangil, Soil Quality Technologist, Teagasc Environment Research Centre, Johnstown Castle, Wexford.
Further reading
The pitfalls of measuring carbon stocks from grassland soils (podcast)
This article was first published in TResearch. Read more from TResearch