Improving the management of grasslands on mineral soils for increased carbon sequestration is one of the measures set out in Ireland’s Climate Action Plan to reach carbon neutrality by 2050.
Researchers at the National Agricultural Soil Carbon Observatory (NASCO) have been tasked with measuring how much carbon dioxide (CO2) is currently being emitted in Ireland.
In an interview with Agriland, research officer at the Teagasc Crops, Environment and Land Use Programme, Gary Lanigan spoke about the “great difficulty” that comes with measuring carbon.
Carbon sequestration
Carbon gets into the system via photosynthesis and the CO2 gets fixed by chlorophyll in a plant. While some of that carbon gets respired by the plant, Lanigan said that some of it goes into the soil.
If the rate of respired carbon exceeds that of stored carbon, then the ecosystem is a source of carbon. If more carbon is stored than respired, it becomes as a carbon sink.
Carbon stored in soils is often called soil organic carbon which is good for soils, improving their workability, water holding capacity, and productivity, Teagasc said.
Different agricultural practices promote or demote the amount of carbon stored in soil. The rate of carbon sequestration on agricultural land is also impacted by climate and soil type, he said.
Grassland on mineral soils
In terms of a soil’s capacity to store carbon, Lanigan explained that the carbon sink potential of different soil types can be thought of as either a wash basin, a bath, or a swimming pool.
Grassland soils have been shown to contain large stores or stocks of carbon, approximately 440t CO2/ha or an estimated 1,800Mt CO2 across all Irish mineral soils, Teagasc said.
National greenhouse gas (GHG) emissions are about 60Mt CO2/yr, so Ireland’s mineral soils store 30-years worth of emissions. Peat soils store even more at about 4,000t CO2/ha.
Teagasc researchers found that Irish grasslands are generally a carbon sink, with values for carbon sequestration ranging from 1.5-4t CO2/ha/yr.
However, because of international accounting rules, only the additional sequestration that occurs due to management changes made after 2005 can be included.
Therefore, reported figures are much lower, ranging between -0.75t to 1.4t CO2/ha/yr, according to Teagasc. Negative figures indicate that more carbon is emitted than stored.
Lanigan explained that when grassland is ploughed, for example, oxygen is introduced into the system and the protective carbon breaks up and gets decomposed and lost.
Soil types
Looking at the carbon that is locked into Irish soils, peatland makes up less than 10% of the landmass but 50% of the carbon stock – a little bit like our “rainforests”, Lanigan said.
The researcher explained that as soon as a peatland is drained, oxygen is introduced into the system which starts the decomposing process and the peatland starts emitting carbon.
“We got about 4 million hectares of grassland on mineral soils which, according to the book values, is sequestering about half a tonne of CO2/ha/yr, or two million tonnes in total,” he said.
Grassland on drained organic peat soils is a substantial source of CO2 at 20t/ha/yr which equals to about 9 million tonnes in total. Due to these different types of grasslands – on mineral and on peat soil – they are a carbon source, he said.
Using “sink analogy”, Lanigan explained that with healthy peatlands it is like the tap is on drip but the stop is stuck up, so none of the carbon is leaving the system.
For grassland, the tap is half on, so there is a lot of carbon going in but equally the stop is only half in so carbon is leaving the system. In terms of crop land, the tap is on full, but there is no stopper so everything that goes in goes straight back out again.
Measuring carbon
Explaining the difficulty of measuring carbon – which is a small input to a very large sink – in simple terms, the Teagasc researcher said:
“If I give you a swimming pool, and I have the tap that is introducing water on drip, and I am asking you to measure the weight of water being introduced to that system simply by measuring the height of water in the swimming pool – that’s a challenge.”
Ireland must cut its GHG emissions by 30% by 2030. However, because agricultural emissions are difficult to reduce, 5.6% of national emissions can be offset by carbon sequestration, Teagasc said.
Currently, Ireland’s national GHG inventory is based on average international emission factors from the Intergovernmental Panel on Climate Change (IPCC).
However, these emission factors are highly uncertain as they don’t take into account the soil type, land use, management practices, and climates on Irish farms, Teagasc said.
The NASCO will therefore produce Irish-specific carbon emission and sequestration values using 32 “Eddy Covariance” Flux Towers on agricultural grasslands, mineral soils and peatlands.
Eddy Covariance is a micrometeorlogical technique used to directly measure the rate of CO2 exchange between the atmosphere and terrestrial ecosystems, according to Teagasc.
Lanigan explained that, using the towers, a meter is essentially put on the tap.
“Instead of measuring the carbon in the soil, we are measuring the carbon influx into the system and the carbon emitted out of the system,” he said.