Clearing the air

Carbon sequestration — the capture of carbon into soil — can be an effective tool to combat climate change.

Improved analysis of soil carbon levels is a key tactic in combatting greenhouse gas emissions

 Carbon sequestration — the capture of carbon into soil — can be an effective tool to combat climate change. Attaining more accurate measurements of soil carbon levels requires widespread and in-depth attention to land-use practices. Agriculture contributes significantly to greenhouse gas (GHG) emissions. Sustainable soil management to increase storage of soil organic carbon (SOC) can provide opportunities to offset these emissions. Accurately quantifying SOC levels in deeper soil layers across different soil types, land uses and management practices is essential to achieve accurate baseline measurements and monitoring.

Carbon sequestration refers to the process of capturing carbon from the atmosphere and storing it in soil over time. It involves increasing the amount of carbon in the soil compared to its previous baseline levels. Giulia Bondi, Senior Research Officer at Teagasc’s Environment Research Centre, explains: “Plants remove carbon from the atmosphere through photosynthesis, storing it in tissue such as leaves and stems. When this tissue eventually dies, the carbon is then deposited onto the soil surface. Over time this residue is decomposed by soil microorganisms which release some carbon back into the atmosphere as part of heterotrophic respiration. The leftover carbon in this cycle, not in flux, is the carbon that has been taken out from the atmosphere and fixed — or sequestered — in the soil.”

Stock options

These amounts of carbon sequestered in particular soils are referred to as SOC stocks, continues Giulia. “SOC stocks are typically expressed in units of mass per unit area, such as kilograms of carbon per hectare. Existing standard methods for assessing SOC stocks are based on field and lab measurements that are costly and time consuming.” These methods include SOC concentration, measured from soil samples digested and combusted in the lab, and soil bulk density, measured in-field at depths down to 60cm and extrapolated over a given area.

As Giulia points out, carbon sequestration has the potential to be a crucial tool to engage with climate challenges, such as global warming. “Global warming is the long-term increase in the Earth’s temperature due to the accumulation of GHGs in the atmosphere, such as CO2. Although the capacity for CO2 to trap heat is less than that of nitrous oxide and methane, the concentration of CO2 in the atmosphere tends to be higher.”

In 2022, CO2 emissions accounted for 60.4% of the total national GHG emissions (excluding those from the Land-Use, Land-Use Change and Forestry sector), while CH4 (methane) and N2O (nitrous oxide) accounted for 29% and 9.4% respectively. Thus, sequestration can help to reduce global warming by offsetting the warming affect associated with high concentrations of CO2 in the atmosphere. However, just as not all emissions are equal, not all soils have the same capacity to store carbon. For example, explains Giulia, “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. This carbon remains protected in the soil for longer timeframes — this is what we refer to as sequestered carbon.” Each soil is different and has different carbon sequestration potential. For this reason, management practices need to be tailored to the carbon sequestration capacity of different soils.

Improving practices

Increasing SOC is a long-term process, and results may not be immediately apparent. Sustainable soil management practices benefit both agriculture and the environment by sequestering carbon and improving overall soil health. This process can be achieved through various practices based on reducing soil disturbance and improving land-use management.

• Soil fertility management: Balanced nutrient management can promote healthy soils and plant growth, leading to improved SOC sequestration dynamics.
• Cover cropping: Planting cover crops like legumes and grasses between main crop seasons is known to increase SOC, especially in the lower soil layers, by adding an organic matter input.
• Crop rotation: Rotating crops with different root structures and residue qualities helps diversify the organic matter inputs to the soil, increasing overall SOC content.
• Reduce traffic: Limiting heavy machinery traffic on soil surfaces reduces the breakdown of soil aggregates that protect SOC and prevent losses to the atmosphere.
• Build up SOC: Applying organic materials rich in carbon, such as compost, manure and crop residues (i.e. straw) to the soil provides an immediate boost to SOC levels.
• Maintain good soil structure: Minimise soil compaction, as compacted soils have reduced organic matter decomposition rates.

Improving SOC practices across Ireland is a key element of Teagasc’s Signpost Programme, Giulia explains. “The Signpost Programme aims to quantify SOC stocks in depth, examine SOC landscape distribution patterns, and identify factors influencing SOC stability in agricultural systems in Ireland.” A core principle of Signpost involves strategically selecting locations across various soil types, land use, and management scenarios. “The Signpost Programme avails itself of more than 100 farms where deep soil sampling coupled with high-resolution analytical techniques, such as spectra analysis and SOC sequestration measurements, are employed to elucidate the composition and stability of SOC within the deep soil layers.” This research helps attain more accurate baselines of SOC quantification and SOC sequestration rates across various soil types, land-use scenarios and management practices. This, in turn, is essential in improving the accuracy and precision of Irish soil carbon estimates from Tier 1 to Tier 2.

Irish-specific SOC sequestration factors across the main mineral soils will be produced that can then be inputted into national inventories, continues Giulia. “This will provide the basis for inclusion of agricultural soils into carbon trading schemes and life-cycle assessments, which will assist the sector in terms of carbon credits and a reduced carbon footprint on agricultural produce. “Signpost will contribute to developing and implementing tailored management practices for increasing SOC stocks and robust carbon accounting frameworks for sustainable agricultural systems. This will benefit farmers, land managers, and society in effectively managing their soils.”  

Giulia Bondi taking soil samples for measuring of carbon sequestration

Funding

Jointly funded by Teagasc Grant-in-Aid and industry funding from Signpost funding partners; additional support from VistaMilk Research Centre funded by Science Foundation Ireland and DAFM.

Contributors

Giulia Bondi
Senior Research Officer, Teagasc Johnstown Castle.
giulia.bondi@teagasc.ie

Rachael Murphy
Research Officer, Teagasc
Johnstown Castle.

Conor Bracken
Research Officer, Teagasc
Johnstown Castle.

Karen Daly
Head of Department
(Environment, Soils and Land-Use)
Teagasc Johnstown Castle.

[Pic credit] Teagasc