Soil Carbon Sequestration
Soil carbon sequestration is an important mechanism that removes carbon dioxide from the air and storing it in the soil, improving a number of aspects of soil health. Carbon stored in soils is often called soil organic carbon and it is good for soils, improving their workability, water holding capacity, and productivity. Ecosystems that can sequester more CO2 than they release are termed carbon sinks, while those that emit more than they sequester are termed carbon sources. Forestry is good for sequestering carbon, and agricultural soils can also be carbon sinks (but they can also be sources). Ireland must reduce greenhouse gas (GHG) emissions by 51% by 2030 and agriculture has a target to reduce emissions by 25%.
Teagasc has conducted a lot of research investigating carbon sequestration. Grassland soils have been shown to contain large stores or stocks of carbon, approximately 440 t CO2/ha or an estimated 1,800 Mt CO2 across all Irish mineral soils. To put this in context, national GHG emissions are about 60 Mt per year, so our mineral soils store 30 years worth of emissions. Our peat soils store even more per ha, about 4,000 t CO2/ha. So there are huge stocks of carbon in our soils, but are we adding to (sequestering) or running down these stocks?
Sequestration is also very difficult to measure as the amount of carbon sequestered (or lost in some cases) each year is tiny relative to the overall stock. All this complexity is not a reason to give up on carbon sequestration. Rather, it is a challenge for scientists and we have other tools to measure it such as computer modelling or flux towers that measure gaseous exchange above the soils. Given its importance to agricultural GHG balances, we need to be able to quantify it.
The research shows that grasslands are generally a carbon sink, with values for carbon sequestration ranging from 1.5 to 4 tonne CO2 /ha/yr. However, international accounting rules state that we can only include the additional sequestration that occurs due to management changes, the reported figures are much lower (-0.75 to 1.4 tonne CO2/ha/yr). In contrast to mineral soil, grasslands on drained organic (peat) soils are a substantial source of CO2 of circa. 20 t CO2 /ha/yr. This is due to the fact that they contain very large C stocks (approx. 4,000 tonnes CO2 per ha) and upon draining, this carbon is rapidly decomposed and released as CO2. These soils account for 5-6 Mt CO2 emissions in addition to the ~20 Mt CO2 from agriculture. Restoring small areas of peat soils can deliver large CO2 savings.
Can we increase carbon sequestration on a farm? There are ways to increase carbon sequestration as shown in Table 1.
Table 1. Ways to increase carbon sequestration
|Measure to increase C sequestration
|Avoid soil compaction
|Increase the proportion of grazing
|✓ to ✓✓
|Allow existing hedgerows to be taller & wider
|✓ to ✓✓
|Improve soil fertility
|Establish clover/multispecies swards
|Planting extra hedgerows
|Planting additional woodlands/forests
|Restoring a drained wetland
Research is currently focusing on establishing Irish specific emission factors for soil carbon sequestration for inclusion in the national inventory. Through the National Agricultural Soil Carbon Observatory, the Agricultural Catchments programme, the Signpost farms and Vista Milk Ireland is developing the largest infrastructure in Europe to measure and report emissions. The research is also investigating a number of the measures in Table 1 to generate scientific data and advise for farmers that will see major improvements to the national greenhouse gas inventory. Emissions from the Land-use, Land-use change and forestry sector have to be reduced in-line with all sectors to assist Ireland in achieving the 51% greenhouse gas reduction target. While science improves the measurement of carbon and refinement of emission factors, the measures identified in Table 1 above will deliver on carbon savings now. Many of these measures improve incomes, improve agronomic yields and have benefits to biodiversity and water quality.