Reducing nitrous oxide emissions benefits farmers

Farmers who use best management practices for agronomic reasons also gain environmental benefits, says a University of Saskatchewan researcher.

However, soil scientist Dr. Richard Farrell told the recent provincial irrigation conference there is room for improvement around nitrogen use.

Agricultural soils produce about half of total greenhouse gas emissions for the sector but 90 percent of the nitrous oxide.

“Carbon dioxide has a global warming potential of one,” he explained. “For methane, that global warming potential is 25, so one kilogram of methane is equivalent to 25 kg of carbon dioxide. For nitrous oxide, it’s 298.”

Put another way, it takes about 3.3 kg of nitrous oxide to offset one tonne of sequestered carbon, he said.

Emissions will never be zero because nitrous oxide forms naturally in soil.

But farmers do want to keep emissions as low as possible as they adapt to climate change and do their part.

Nitrous oxide emissions are worst during snow melt.

“This is the time we should be focusing on for our mitigation,” Farrell said. “Spring thaw and early season.”

He outlined research done by graduate students looking at the 4R management system in irrigated canola and at the mitigation potential of enhanced efficiency fertilizer products on irrigated and dryland crops.

The goal of the first project was to identify the nitrogen rate that produced the greatest yield increase with the lowest fertilizer-induced emissions.

It used urea, at pre-planting and split applications, at rates of zero to196 pounds per acre, and broadcast and side-banded.

Soil tests were carried out to obtain the recommended rate. The study was conducted over three years.

Overall, there were no yield effects beyond the recommended fertilizer rates.

“Essentially, what we’ve seen so far from this study is, in terms of yield response, rate, timing, placement really didn’t have a significant effect on anything,” he said.

There were huge nitrogen losses, though.

“What we observed to be the optimum N rate was well below the recommended rate,” he said. “We’ve seen this in a number of other studies as well. There’s an implication we need to work on our nitrogen testing.”

The fertilizer rate, timing and placement had significant effects on nitrous oxide emissions.

“Total emissions and the emission intensity increases with the increasing N rate,” Farrell said.

“Split applications decreased both the total and the emission intensity. For the placement, there was a significant effect every year but it was inconsistent and moderated by weather.”

In wet years, side-banding didn’t work well but in dry years it did.

The second study of the enhanced efficiency products was part of a larger study that included the universities of Manitoba and Alberta.

It also used the 4R principles, with both urea and anhydrous-based products applied in fall and spring at the soil-test recommended rate. They were placed in a mid-row band, every second row, on wheat.

Farrell cautioned that the intent was to examine nitrous oxide mitigation potential, not the agronomics of the products.

“In theory, they should have gone in at lower rates,” he said.

This study found similar results in that the fall applications resulted in huge emission events the following spring. Farrell said even the polymer-coated products, which he expected to perform better, didn’t do as well as expected.

That’s because when applied in fall, they get wet. The coating absorbs the water, which dissolves the nitrogen source, which then swells.

“Then they freeze, which causes it to swell again and the pellets crack open,” he explained. This becomes a carbon source for denitrification to occur.

Non-irrigated fields had lower emissions than the irrigated fields, which is probably due to field history, he said.

“Fields that have a long history of irrigation, they get a little bit higher N rates, they have more crop residue returned to them, they just, over time, develop microbial systems that are almost looking for nitrogen to form nitrous oxide,” Farrell said.

The enhanced efficiency products were a little more effective on the non-irrigated sites.

One farmer at the meeting asked if applying coated products in January at two-thirds the recommended rate would help avoid that freeze-thaw effect and too much nitrogen, but Farrell said even a winter thaw could cause the product to swell and crack.

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