Reducing nitrous oxide emissions

Much of the N₂O emitted from farmland is produced when excess NO₃- in soil undergoes denitrification, either on farmland or after it is leached away. Farmers can reduce these emissions by preventing build-up of NO₃- or avoiding soil conditions that favor denitrification. Some N₂O is also emitted when NH₄+ is converted to NO₃- (nitrification). Adding less NH₄+ or slowing the rate of nitrification can reduce emissions from this source. The best way to reduce N₂O losses is to manage the N cycle more efficiently, thereby avoiding the buildup of excessive NH₄+ or NO₃-.

Specific ways of reducing N₂O emission vary for farming systems across Canada, but examples include the following:

Match fertilizer additions to plant needs: The best way to reduce N₂O emission may be to apply just enough N so that crops can reach maximum yield without leaving behind any available N. A perfect match is rarely achievable, but the synchrony can often be improved by basing fertilizer rates on soil tests and estimates of N release from residues and organic matter. In fields where fertility needs vary, applying N at different rates across the landscape ("precision farming") may also improve the match between amount applied and the amount taken up by crops.

Avoid excessive manure application: Heavily manured land can emit a lot of N₂O because the manure adds N and available C, both of which promote denitrification. Moreover, manure is often applied to land as a means of disposal, so that rates can be excessive. Applying the manure at rates that just supply plant demands can greatly reduce N₂O emissions from this source.

Optimize timing of nitrogen application: When the N is applied is as important as the rate of addition. Ideally, farmers should apply N just prior to the time of maximum uptake by the crop. Wherever possible, they should avoid applying fertilizer and manure in fall. Similarly, they should time the plough-down of N-rich crops, like legumes, so that N release from the residues coincides with subsequent crop demands.

Improve soil aeration: Denitrification, and hence N₂O emission, is favored by the low oxygen levels that usually occur in saturated soil. As a result, farmers can reduce emission of N₂O by managing soil water-draining soils prone to water-logging, avoiding over-application of irrigation water, and using tillage practices that improve soil structure.

Use improved fertilizer formulations: Some research suggests that certain forms of fertilizer emit more N₂O than others. Highest emissions may occur from anhydrous ammonia; lowest from forms containing NO₃-. This finding suggests that, by selecting appropriate fertilizers, farmers could reduce N₂O release. However, the differences among forms of fertilizer have not yet been widely verified in Canada. Another option is to use slow-release fertilizers, such as sulfur-coated urea. These forms release available N gradually; they feed the crop yet prevent available N from accumulating. Though effective in reducing N₂O emissions, slow-release forms may only be economical for high-value crops.

Use appropriate fertilizer placement: Placing fertilizer in close proximity to crop roots can improve the efficiency of nutrient use, allowing the farmer to achieve high yields with lower rates of application. On the other hand, placing the fertilizer too deep in the soil, or concentrating forms like urea in bands, may increase N₂O emissions.

Use nitrification inhibitors: Certain chemicals, applied with fertilizers or manures, inhibit the formation of NO₃- from NH₄+. Their use may suppress N₂O formation in several ways: it reduces N₂O formation during nitrification, it prevents denitrification of accumulated NO₃-, and, because NH₄+ does not leach easily, it prevents loss of N into groundwater where denitrification could occur.

Use cover crops: Where the growing season is long enough, farmers can sow crops after harvest to extract excess soil NO₃-, which prevents it from leaching or converting to N₂O.

Lime acid soils: Because it is favored by acidity, N₂O emission can be suppressed by applying neutralizing lime to acid soils.

Reduce tillage intensity: Though results are still inconsistent, some studies in Canada suggest that N₂O emission may be lower in no-till than in conventional tillage. If confirmed, this observation may point to no-till as a method of reducing emissions, at least in some soils.

These practices can help reduce N₂O emissions in many settings. Because N₂O fluxes are so sporadic, however, all these practices cannot yet be recommended with confidence across Canadian soils and cropping systems. But those that improve the efficiency of N use are often already justified for reasons quite apart from reduced N₂O emission. Fertilizers account for about 9% of production costs on farms, and any method that reduces N losses has economic benefits.