Estimating soil carbon stocks on the Signpost Farms

There is a lot of interest in the potential of both our hedgerows and soils to store carbon as a means of offsetting agricultural emissions, writes Tom O’Dwyer, Head of Signpost Programme at Teagasc, who looks at how soil carbon is being monitored on the Signpost Farms.

While possible, there are a lot of factors which influence the rate at which both store carbon, also referred to as carbon sequestration. Additionally, under certain management conditions, both soils and hedgerows can lose carbon.

This short article focuses on the research being undertaken on the Signpost Farms relating to soil carbon sequestration, and more specifically to establishing the baseline of carbon stocks in the soils of the Signpost Farms.

This work is being led by Teagasc Researcher, Giulia Bondi. I spoke with Giulia recently about her work and she started by reminding me that “not all soils have the same capacity to store carbon; heavier soils have a better capacity to bind the carbon with fine soil particles like silt and clay, or to protect the carbon into aggregates, which are the basis of the physical structure of soil.”

Each soil is different and consequently each soil has different carbon stocks and carbon sequestration potential. Earlier this year, Giulia and her team started the second phase of the Signpost Soil Carbon project. This involved sampling up to four sites per Signpost Farm, with a profile pit being dug at each location to a depth of 60cm.

To date, 34 of the Signpost Farms have been visited (mainly in the south east), with the final farms for 2023 to be visited in early December; sampling will recommence in spring 2024.

In order to strategically select the appropriate sites on each farm, Giulia and her team used the agronomic soil sample results from the winter 2021/22 soil sampling campaign, plus relevant farm management information relating to each sample site. This farm management data includes: soil type; cropping history; farming system; farm stocking rate; and fertiliser nitrogen application rate data.

This data is critical to the completion of the deep soil sampling task, as it allows the research team to purposefully identify the most appropriate sites on each farm. The ambition is to achieve a “mosaic” of results covering as many as possible soil type, land use and management scenarios.

Once the profile pit was opened on each farm, a sample was taken at four depths down to 60cm, including samples at 0-15 cm, 15-30cm, 30-45cm and 45-60cm depth.

All samples have been submitted for analysis and results are expected for nutrients status, carbon stocks and carbon sequestered in aggregates (by depth) and a number of indicators of soil quality also. Initial results are expected in early 2024.

Returning to Giulia again, who said: “This research will help to develop a more accurate soil carbon stock baseline across various soil types, land-use scenarios and management practices on the Signpost Farms.

“This, in turn, is essential in improving the accuracy and precision of Irish soil carbon sequestration estimates for use in National Inventories, and in time for inclusion of agricultural soils in carbon farming schemes and life-cycle assessments, which will assist the sector in terms of carbon credits and a reduced carbon footprint on agricultural produce.

“The information and insights generated will benefit the Signpost Farmers in effectively managing their soils, so as to optimise their capacity to store carbon.”

Acknowledgement: With thanks to Giulia Bondi, Teagasc for her input to this article.

A more detailed overview of Giulia’s work can be found in the TResearch Winter 2023 Edition, which is available here.

This article first appeared in the Signpost Programme newsletter for December. For more information on the Signpost Programme, click here.

Photo caption: Giulia Bondi taking soil samples for measuring of carbon sequestration. Image source: TResearch