Practices are available that increase soil organic carbon, but fixing more carbon in soil can have unintended consequences
Crop production’s carbon footprint is becoming more important, especially when selling feedstock for biofuel into markets operating under a carbon cap and trade framework.
But how can growers reduce the amount of greenhouse gas that their farms emit?
“You’re not going to be able to control how much energy it takes to make fertilizer or diesel fuel or how efficient your equipment is, but you can control what you do to manage your cropping system, so there is a role for that,” said University of Saskatchewan associate professor Rick Farrell.
The agriculture sector contributes eight percent of the total amount of greenhouse gas produced in Canada, including 22 percent of methane and more than 80 percent of nitrous oxide.
The goals for greenhouse gas mitigation in the agriculture sector are twofold: enhance carbon sequestration and reduce N2O emissions.
Using practices that increase soil organic carbon in the soil, such as zero-till, does reduce the amount of net carbon dioxide that a farm produces.
However, there may also be unintended consequences of fixing more carbon in cropland, Farrell told the Canola Industry Meeting last month in Saskatoon.
“What you do to increase carbon also includes other nutrients, including nitrogen, which means that you can be doing something to produce carbon and put it into the soil and the same time you may be producing N2O emissions,” Farrell said.
Nitrous oxide is a potent greenhouse gas, which will trap 289 times more heat energy than the equivalent mass of carbon dioxide.
This means 3.5 kilograms of N2O will offset one tonne of carbon sequestered in the soil.
Nitrous oxide is naturally created in soil. Even unbroken soil produces the gas because of the nitrogen cycling processes, primarily nitrification and denitrification.
“But the thing is, we are adding in plant residue or fertilizer, so we are adding in a source that pumps this whole thing along and increase the amount of nitrogen and increases the amount of N2O,” Farrell said.
“So somewhere along here we have to be able to control or slow that down.”
Nitrous oxide emission factors in agriculture are usually looked at in terms of the percentage of applied nitrogen that is actually lost as N2O, which varies globally.
The emission factors on the Prairies are .2 to .8 percent of the applied nitrogen lost as N2O emission, while the Intergovernmental Panel on Climate Change guidelines for N2O emissions, which serves as a standard, are one percent.
“So out here in the semi-arid Prairies we are below that, which is a good thing, but because we have so much land, that means that cumulatively that a large amount of N2O is being produced,” Farrell said.
He said the best way to reduce N2O emissions is to get better use of the nitrogen fertilizer that is applied, and the best way to do that is the 4R nutrient management system.
Under this strategy, producers choose the right source, rate, timing and placement of fertilizer so that it is available when a crop needs it, which maximizes its use.
Advanced efficiency and smart fertilizers can also be used to increase fertilizer efficiency within the 4R nutrient management system.
Farrell said a 4R nutrient management study at the Canada-Saskatchewan Irrigation Diversification Centre in Outlook, Sask., found that split applications tended to result in lower N2O emissions than applying all of the urea at the same time.
“Part of that is by the time you get to the split application you have an established crop,” he said.
“The fertilizer goes in and once it become soluble and available, the crop just snaps it up and uses it and so it’s not available to be converted into N20.”
The study applied granular urea between zero and 90 kg per acre using both split applications and single applications during seeding.
Side-banded applications were also compared to broadcast applications with the urea incorporated into the soil.
The study found that the amount of nitrogen applied affected N2O emissions, especially when it was increased from 45 kg per acre.
Fertilizer placement also has a significant effect on N2O emissions.
Plots that received the side-banded application had greater nitrogen losses in the form of N20 than did plots where urea was broadcast and incorporated into the soil.
“(With) the side banded fertilizer, there is an entirely different pattern, and part of the reason is we’ve taken all this fertilizer and we’ve put it into a tiny little band, which basically changed the chemistry of what occurs,” he said.
Farrell said a study that started in 1998 at Agriculture Canada’s research centre near Scott, Sask., found that crop rotations matter when it comes to N2O emissions.
Wheat on canola had the highest total emission as well as the highest yield-scaled emissions.
“We’ve seen this in some other studies. For some reason, when growing wheat or another crop on canola stubble, you have higher emissions in that second year,” Farrell said. “Adding canola in the rotation has an environmental cost in that it adds emissions in the year following when it was grown.”
The study monitored five crop rotations:
- continuous wheat crop sequence with and without fertilizer
- continuous pea
Urea was banded at 30 kg per acre for canola, 25 kg per acre for wheat and three kg per acre for pea crops.
Adding a pulse crop into the rotation lowers N2O emissions because the crop’s nitrogen fixation allows less nitrogen to be applied during the rotation.
“Including a pulse in the crop sequence benefits the overall rotation on both a per area and a yield scaled emissions basis,” Farrell said. “When you look at rotations, the general feeling is that if you combine canola and a pulse in rotation, they will sort of balance each other out.”
To understand why canola residue seems to produce more greenhouse gas emissions, Farrell helped set up a study that looked at how different crop residues affect N2O emissions.
Another study, this one at four Agriculture Canada centres in the brown, dark brown, black and grey soil zones, collected soil from long-term continuous wheat plots to gauge emissions caused by different residues. The soil was adjusted so that 50 and 70 percent of the pore spaces were filled with water.
Nitrification occurred In the soil with 50 percent of the soil’s pores filled with water. Denitrification occurred in the soil with 70 percent water-filled pores.
Pea, wheat, canola, and flax residue was tested with the soil, and the study used N15-tagged nitrogen compounds to help track where the emissions originated.
“Any N15 that wound up as N2O had to have originated from the residue, so we could track how much N2O came directly from the residue and how much came from the soil,” Farrell said.
Canola, flax and peas added some nitrogen and produced slightly higher emissions in aerobic conditions with 50 percent water in pore spaces. Canola and flax always had the highest emission values.
Canola, flax and peas added a little bit of nitrogen under anaerobic conditions, but they also added a lot of energy, which resulted in denitrification.
Canola and flax residue were again the top emitters, and the canola residue emitted the most N2O seven out of eight times.
“When you move into anaerobic conditions, remember you are also adding a lot of carbon, and the carbon provides a lot of energy and electrons to drive denitrification and again drive up emissions,” Farrell said.
Emission factors did not change considerably in the black soils with 70 percent water in the soil pores. However, emissions were high in the gray soil with 20 percent of the nitrogen emitted as N2O.
The same patterns emerged In the dark brown and the brown soils: higher emissions in anaerobic conditions but with higher percentages of the emissions coming from plant residue.
Plant residue with high nitrogen content can act as another source of nitrogen when it is added to the soil, and an increase of N2O emissions follows, Farrell said.
Wheat has a high carbon-nitrogen ratio and tends to immobilize nitrogen, so it actually removes nitrogen from the available pool and reduces the amount of N2O that is produced.
Farrell said there is an opportunity to increase nitrogen efficiency because any N2O emissions that develop must have passed through an available form of nitrogen.
“It has to have been a nitrate at some point, so these residues also produce a lot of available nitrogen in the soil, and the question is, is that nitrogen available to the plants, when is it available? That is still to be determined.”