GHGMP Feature Articles

Matching crop nutrient needs helps minimize nitrous oxide emissions

Improve soil nitrogen test is an important start

All producers want the biggest bang for the buck in terms of getting optimum crop growth for their fertilizer or nutrient dollar. But moisture and temperature patterns across the country create regional differences in fertilizer management issues.

That makes for a bit of an east/west split when it comes to managing fertilizer that is not only effective, but has the least negative affect on the environment.

High moisture conditions found in parts of Manitoba and through to the East Coast, not only increase the risk of leaching nitrogen and other nutrients from the soil, but under high water content conditions, the fertilizer also releases nitrogen to the atmosphere. The process known as denitrification creates a direct economic loss of fertilizer dollars. As well, part of the nitrogen is lost in the form of nitrous oxide, which is one of the most serious greenhouse gasses.

Denitrification is the risk

Denitrification is the conversion of soil nitrate to nitrogen gases, whether the soil nitrate comes from commercial fertilizer, manure, or fixed by legumes. Conditions needed to cause denitrification and production of nitrous oxide include a nitrogen source, limited oxygen supply as you would find in wet soil, and a good carbon source such as manure or crop residue, which stimulates soil microbe activity. Reduced soil oxygen can also occur when soil microbial activity is very high. For example when manure is applied, the added carbon stimulates very rapid microbial respiration. This respiration depletes the soil oxygen supply.

While not unheard of, denitrification is a lesser known issue over the extensive grain and oilseed growing regions of Alberta and Saskatchewan through to B.C.'s Peace River Region. In many growing seasons, Prairie producers worry whether there's adequate moisture to make nutrients available to growing crops. Crop fertility strategies not only consider the nutrient needs during the growing season, but often look for ways to bank surplus nitrogen for next year's crop.

Producers in higher moisture areas worry about too much moisture, and in many parts of Eastern Canada with milder winter temperatures, consider residual nitrogen a disadvantage. The challenge is to use all available soil nitrogen during the growing season and avoid surplus nutrients, which would likely be lost through leaching due to runoff or heavy rainfall events. Similar challenges face B.C.'s Lower Mainland producers.

Nitrous oxide - a potent greenhouse gas

Not all emissions produced through denitifrication are harmful. Most of it is di-nitrogen gas, which makes up about 80 percent of the earth's atmosphere. But a percentage is N20 or nitrous oxide. Although relatively small amounts of nitrous oxide are released, it is a particularly potent gas in the greenhouse gas complex of emissions, having 300 times the effect of carbon dioxide.

For producers, dentrification represents an economic loss of as much as two to four pounds of nitrate per acre per day while the soil is in the saturated state. Nitrous oxide emissions can range from 0.5 to four pounds of nitrogen per acre per year depending on moisture and temperature. This range applies to non-manured fields and is likely higher on manured fields.

However, denitrification can be managed, say soil fertility specialists. While farmers can't control the weather they can use management techniques to protect nitrogen and reduce production of nitrous oxide.

Matching crop requirements

A major challenge in Eastern Canada is better matching nutrient levels to crop requirements, says Dr. Bernie Zebarth, a research scientist with Agriculture and Agri-Food Canada's Potato Research Centre in Fredericton, NB. Again some of the climatic differences between Eastern and Western Canada come into play. He regards nitrous oxide emissions as more of an issue in the humid environment of Eastern Canada, than the semi-arid environment of Western Canada. "Nitrous oxide is more of an issue on a unit basis in the east, meaning more nitrous oxide is produced per acre," he says. "But with the larger land area in the west, overall nitrous oxide emissions may in fact be larger."

Complicating the nutrient balance issue in Eastern Canada is the lack of reliable soil tests for nitrogen. Unlike producers in Western Canada who can generally rely on fall or early spring soil test recommendations to advise on crop requirements, Eastern farmers can only follow traditional soil fertility practices.

"With high moisture levels, nitrogen testing isn't reliable," says Zebarth. "A test done in the fall wouldn't be relevant by the time the crop is seeded in the spring." Producers apply traditional nitrogen levels and often error on the side of too much to ensure crops have adequate nitrogen. But, surplus nitrogen can leach through the soil or be subject to denitrification in wet soils.

To better manage crop nitrogen requirements, Zebarth hopes to develop a four-stage testing system to determine how much nitrogen is in the soil just prior to planting; the soil's potential for mineralizing nitrogen during the growing season; tissue testing to determine the crops actual needs half with through the season; and a post-harvest soil test "report card" that measures how much nitrogen is left after the crop is harvested.

"It's a bit involved, but hopefully it will allow producers to better manage fertilizer requirements not only to save themselves money, but to avoid over application which can harm the environment," says Zebarth.

Management strategies

Dr. Phillppe Rochette, a research scientist specializing in greenhouse gasses, based at Agriculture and Agri-Food Canada's Sainte-Foy Research Farm in Quebec, notes several other management options.

In the absence of accurate nitrogen tests, Rochette recommends banding fertilizer close to the crop root zone just prior to seeding to get maximum uptake once crops start growing. Also, precision farming techniques which apply variable rate fertilizer according to soil types, or split applications of fertilizer at planting and half way through the growing season, can help prevent over application. Nutrient testing of manure will also help target match crop nutrient requirements. And to remove residual nitrogen, he suggests planting a winter cereal or other cover crop after harvest. Since a key element of denitrification is waterlogged soils, any measures to improve field drainage, such as ditching or tile drainage, will help soils dry faster.

A good word about winter

A cold climate might be the best friend of producers in Manitoba, says John Heard, a soil fertility specialist with Manitoba Agriculture Food and Rural Initiatives. As a transition province between the humid East and semi-arid West, much of Manitoba's crop land has good moisture.

"Waterlogged or saturated soils are common at times of the year in most areas," says Heard. "Even in drier areas, there are always some spots in fields that are wet."

While denitrification can occur during the year, Manitoba producers don't have to worry about residual nitrogen leaching away over winter. "With five to seven months of frozen soil nothing is lost," says Heard.

Ideally applying fertilizer in the spring will reduce the risk of denitrification, but he says, producers also have valid reasons for banding fertilizer in the fall. To reduce losses after fall application he offers the following advise.

Choose an ammonium form of fertilizer as opposed to a nitrate form. Microbes are unable to release greenhouse gases such as nitrous oxide until the nitrogen converts from ammonia to nitrates. He recommends either anhydrous ammonia, urea or ammonium sulphate forms of fertilizer.

Second, apply the fertilizer late in the season when the soil temperature has cooled to at least 5 C. The cooler temperature slows the conversion of ammonia to nitrate. And lastly, banding the ammonia form of nitrogen further slows this conversion compared to broadcast applications.

"We're fortunate to have cold winters to keep the nitrogen in place," says Heard. "For us, residual nitrogen is still money in the bank."