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Carbon Dioxide

Carbon dioxide (CO2) is the most important GHG in the Earth’s atmosphere. Global atmospheric CO2 concentrations reached 409.8 ± 0.1 parts per million (ppm) in 2019 and have been increasing at an average rate of 2.3 ppm per year since 2009. Approximately two-thirds of the total increase in atmospheric CO2 is derived from fossil fuel combustion. The other one-third is the result of land-use change - conversion of natural vegetation to managed land.

Carbon dioxide in Agriculture

In Ireland, lime and urea application are the main sources of CO2 from soil. The CO2 emitted accounts for only 2.7% our total agricultural GHG emissions.  Liming is a common agricultural practices that improves yields by raising soil pH. The adoption of urea, especially protected urea, over other fertilizer types may increase due its effectiveness in mitigating nitrous oxide (N2O) emissions without compromising yields. Farms also emit CO2 from activities such as ploughing, sowing and chemical treatments.

Plants and soils are also sources of CO2 emissions. Photosynthesis is the main CO2 sink on land. Plants use light to convert atmospheric CO2 and water into carbohydrates. Respiration uses oxygen to convert these carbohydrates into energy, releasing CO2 in the process.

Net Ecosystem Exchange (NEE)

The balance between photosynthesis and respiration is termed net ecosystem exchange (NEE). The NEE value can be summed to determine whether a land use is a source or sink of CO2. If photosynthesis exceeds respiration, the land is considered a sink of CO2 - net uptake. If respiration exceeds photosynthesis, it is a net source of CO2 - net emission.

When examining the impact of management or land use on GHG emissions, the global warming potential (GWP) is a useful metric. Compared to N2O and methane (CH4), CO2 traps less heat re-emitted from the Earth’s surface but is more abundant in the atmosphere. The global warming potential (GWP) estimates the heat trapping potential of each gas relative to CO2 (N2O=265; CH4=28). The GWP and total GHG emissions are often expressed in terms of carbon dioxide equivalents.  

A comparison across land-use types in Ireland indicates tillage farms are, on average, less GHG intensive compared to dairy and beef farms. This is due to greater CH4 emissions from cattle in dairy and beef enterprises. Tillage farms on mineral soils however contain less soil organic matter than grazed pasture.

Further reading