A few hours before COP26, we are only talking about CO emissions2 That it is imperative to reduce if we want our planet to remain habitable. But we too often neglect to stress that we will also have to reduce our nitrogen emissions, because in particular one of its compounds, nitrous oxide, is a greenhouse gas (GHG) 300 times more powerful than carbon dioxide. . Two researchers from McGill University have measured the nitrogen footprint of Canada and its provinces, and their calculations show that the inhabitants of our great country have a significantly higher average nitrogen footprint than that of Germans, Austrians, Dutch and Indians, in particular.
Let us first remember that these are only reactive nitrogen compounds, such as nitrous oxide (N2O), nitrogen oxides (NOx and No2), ammonia (NH3), nitrates (NO3–) and nitrites (NO2–), which have an impact on the environment. Nitrogen gas as N2, which constitutes 78% of the volume of the atmosphere, is itself completely inert and does not participate in the nitrogen footprint. On the other hand, nitrous oxide (N2O) is a powerful GHG. One kilogram of N2O has the same heating power as 298 kg of CO2. And since one kilogram of methane is equivalent to the heating power of 25 kg of CO2, the N2O is therefore also a more powerful GHG than methane.
A source of GHGs
Even though the N2O represents only 5% of total GHG emissions in Canada (in 2019), while methane accounts for 13% and CO2 for 80%, “reducing the sources of emission of this gas would have a big impact, because the N2O is a powerful source of GHGs even though, in absolute terms, it is less abundant than CO2 Remarked Graham MacDonald, professor in the Department of Geography at McGill University.
Besides the N2O, there is also ammonia and other nitrogen oxides (NOx) which play a role in air quality. In particular, nitrogen oxides (NO and NO2) greatly contribute to the smog that occasionally invades Montreal. Nitrates and nitrites that migrate through the soil and enter groundwater, then streams, rivers, streams and coastal waters also have impacts on water quality.
“These various reactive nitrogen compounds therefore have impacts on GHGs, as well as on air and water quality”, summarizes the researcher.
Postdoctoral fellow Sibeal McCourt and Graham MacDonald therefore counted nitrogen emissions in these different forms that took place over the three years around 2018, and they came to the conclusion that Canada was emitting 996 gigagrams (million kilograms) of reactive nitrogen in the environment each year, which equates to an average nitrogen footprint of 27 kg per Canadian per year. This footprint is a little smaller than that of our neighbor, the United States, where it reaches around 40 kg / capita / year, but it is comparable to that of the United Kingdom (27 kg / capita / year) and Japan ( 28 kg / inhabitant / year). On the other hand, it is clearly higher than that of Austria (20 kg / person / year), Germany (10 kg / person / year), as well as “of the developing countries of the southern hemisphere which often have a different diet, less rich in meat and less abundant in per capita serving than Canada, where consumption of animal products, as well as emissions from the combustion of fossil fuels for transportation and oil extraction and natural gas are significantly higher, ”says MacDonald.
It is in fact the food sector that contributes the most (up to 50%) to Canada’s nitrogen footprint, particularly the consumption of beef which comes first in Ontario, New Brunswick and Quebec ( 4.7 kg / person / year).
Nitrogen is a crucial ingredient in agriculture, MacDonald points out, before recalling that commercial fertilizers contain primarily nitrogen, phosphorus and potassium (NPK).
Another source of nitrogen in agriculture is manure from farm animals, which is also used as a fertilizer for crops. “The nitrogen present in synthetic fertilizers and manure that are applied to fields tends to leak, to escape from crops, to disappear from the soil. Particularly if too much manure or fertilizer is spread on the ground: there may be emissions of N2O and NH3, two forms of nitrogen that diffuse into the atmosphere, as well as a loss of nitrogen [sous forme de nitrates et de nitrites] towards groundwater and surface water (streams, rivers, etc.). Farmers should therefore apply nitrogen as precisely as possible to where the plant can use it and when it needs it most. When the plant absorbs the fertilizer that has just been applied, much less is then lost in the environment, ”explains the researcher.
Animal husbandry involves another important issue, namely that of manure management, which is very rich in nitrogen. “Nitrogen losses from manure can vary depending on the management method adopted by farmers. Farmers must pay attention to the way they collect manure, store it and spread it in order to reduce the emission of nitrogen gas, mainly in the form of NH.3, but also of N2O. In particular, manure must be stored in containers where these gases cannot escape into the atmosphere, ”says Mr. MacDonald.
Burning fossil fuels is responsible for 40% of Canada’s nitrogen footprint – making it the world’s second largest contributor – due to emissions of nitrogen oxides (NOx) and N2O that result.
Transportation, mainly that associated with heavy commercial vehicles used in particular in the mining industry and the extraction of various resources, is the sector that most influences this nitrogen footprint generated by fossil fuels. In some provinces, the oil and gas industry itself and the use of fossil fuels to generate electricity are also major sources of nitrogen emissions, says MacDonald.
The third largest source of nitrogen emissions in Canada is from wastewater, which contains nitrogen from food consumed by people and which is then excreted. However, in several provinces, a large part of this wastewater does not undergo an adequate treatment (called tertiary) allowing the nitrogen to be removed. “But the biggest problem is when you don’t get treatment, especially in rural areas where people have septic tanks. There are then losses of nitrogen in forms dissolved in the water which percolate in the soil before joining streams, rivers, and so on, ”explains the scientist.
In their study which is published in Environmental Research Letters, the two researchers indicate that the provinces which emit the most reactive nitrogen into the environment are of course the largest and most populous provinces, namely Ontario, Quebec, Alberta and British Columbia. . However, they note that Saskatchewan has the highest per capita nitrogen footprint (50.3 kg / capita / year), while Ontario (22 kg / person / year) has the highest nitrogen footprint. is the smallest, then in Prince Edward Island and Quebec (23.8 kg / person / year).
The largest per capita footprint in Saskatchewan can be explained in large part by the fact that electricity is produced there mainly from natural gas and coal, but also because this province devotes a very large part of its economy to electricity. agriculture relative to the small size of its population, notes MacDonald. This province produces over 50% (by weight) of the major grains grown in Canada, canola, lentils and wheat. In contrast, in Quebec, where the individual nitrogen footprint is much smaller, more than 95% of electricity is produced from hydroelectric dams.
Reduce our nitrogen footprint
“Collectively, we need to continue to use more renewable energy sources to generate our electricity and electrify our transportation. That could have a big impact, ”says MacDonald.
“In agriculture, we need to take care to properly collect and store manure. In addition, farmers should take care to apply the right amounts of fertilizer, where and when it is most needed, and avoid applying it to fields at times when it would be easily washed away by rain or water. melt water, ”he warns.
“On an individual level, we can focus on our food and the waste associated with it. This does not mean that we should all become entirely vegetarians and no longer eat meat, but rather we should balance our diet by paying attention to the amount of meat, especially red meat, such as beef, that we eat. , and therefore eat meat in moderation and integrate other types of protein into our diet, ”he recommends.
“Also, uneaten food that is thrown away contributes two ways to our nitrogen footprint: some nitrogen oxide and N2O can be released from this food waste when it decomposes, and any remaining nitrogen in it will eventually be released into the environment. But the biggest problem stems from all the nitrogen emissions that take place during the production of these foods on the farm and the energy that has been required to then bring them to us. If these foods are not eaten and are thrown away, it means that all these emissions have happened for nothing. This represents a great waste of resources! Graham MacDonald points out.