Recent weeks have been marked by the confirmation that the carbon footprint of Irish tillage is exceptionally low.
This is an issue that has been addressed directly by Teagasc’s head of Environment, Crops and Land Use programme, John Spink, courtesy of presentations that he has given over the past fortnight or so.
Speaking at the National Tillage Conference 2024, he indicated that the outworkings of the most recent carbon footprint calculations should not come as a total shock.
Spink cited the fact that Irish grain yields are among the highest in the world. Meanwhile, Irish farmers make very efficient use of applied nitrogen (N) – one of the most important contributors to overall output.
So, how are the carbon footprint values of a cropping enterprise actually calculated?
Spink explained: “There are two carbon accounting methods and people will often get confused between them.
“They have different system boundaries. So, different criteria are included within them.”
“The first one is the Inter-Governmental Panel on Climate Change (IPCC) approach. This is used for the purposes of the national inventory.
“The second option is the Life Cycle Assessment (LCA) model. This is used to calculate the actual carbon footprint of a product.
“The values here are worked through on the basis of per unit of output. This can relate to grain, milk and meat,” he added.
According to Spink, activity data are required for the operation of both models.
Where cropping systems are concerned, these criteria include the actual tillage systems employed, the amount of fertiliser used and the actual forms in which this was applied.
“Specifically, where LCA is concerned, the final yield of the crop must also be included. This allows for the emissions produced per hectare to be divided across the overall output of the crops being assessed,” he added.
Emission factors
Emission factors must also be included in all carbon footprint calculations. The basic level is a tier one default value. However, it is also possible to calculate more precise carbon value footprints by using tier two or tier three values.
Underpinning all of this is research work carried out by the likes of Teagasc, which can accurately link projected greenhouse gas (GHG) emission levels, post the application of specific crop inputs.
One of the most important of these, is the emission factor linking the application of nitrogen (N) fertiliser to the subsequent production of nitrous oxide gas.
“In the case of calcium ammonium nitrate (CAN) applied under Irish conditions, we know that 0.35% of the N applied.
“In total contrast, the international default figure currently stands at 1.0% of the N applied. Irish emissions from CAN are about one third where they would be in other parts of the world,” Spink said.
However, fertiliser is only one source of N emissions within a tillage system. The decomposition of crop residues and the N in applied organic materials must also be factored in.
Spink confirmed that further research will be undertaken by Teagasc to develop bespoke Irish emission factors, linking both the application of animal manures and crop residue decomposition to subsequent GHG production levels.
He continued: “Most people suspect that the final figures calculated for Ireland, in this regard, will be substantially lower that the international values that are currently available.”
Straw incorporation
Significantly, the latest carbon footprint values for Ireland’s tillage sector also include the sequestration impact of active straw incorporation.
Spink said: “This is assuming a straw yield of 4t/ha. On light soils, this equates to just under one tonne of carbon being sequestered.
“At the other end of the spectrum, clay soils will sequester twice this amount of carbon. We have included these figures into the overall carbon footprint calculations.
“We have also included five years of data, generated on the back of rotation-based crop trials.
“The figures have been generated from plough-based calculations. These indicate that continuous winter wheat has a slightly higher carbon footprint value than the other cropping options.
“This is because the amounts of applied N are slightly higher. Continuous wheat crops are also slightly lower yielding,” he said.
According to Spink, the carbon embedded in applied fertiliser is accounting for almost half the carbon dioxide emissions produced within a cropping regime.
“Emissions created when the fertiliser is actually applied to the soil must also be accounted for. This accounts for 24% of the carbon dioxide that is produced.
“Approximately three quarters of the GHG emissions with winter wheat production are linked directly and indirectly, with the application of nitrogen fertiliser.
“Fuel use accounts for 14% of total GHG production Crop residues currently come in at 10%.
“However, this figure may well decrease once we get bespoke crop residue emission factors that relate to Irish conditions.
“Seed accounts for 3.0% of the carbon footprint generated by a tillage system with crop protection accounting for a further 1.0%.
“Net emissions take account of the gross emissions generated by a crop. If we take away the amount of carbon sequestered by incorporated straw, this generates a net emissions value.
“In the case of winter oats, this is getting us very close to a zero value, where overall emissions from the crop are concerned,” Spink explained.