Improved nitrogen fertilizer management practices that reduce growing season nitrous oxide emissions may increase non-growing season emissions

Field cropping practices in Canada include routine use of nitrogen (N) fertilizer, which produces substantial amounts of nitrous oxide (N₂O) emissions. Adoption of improved N management practices may reduce both the amount of N applied and these N₂O emissions. Using flux-tower field measurements, we investigated how dual inhibitors (urease and nitrification inhibitors with urea) reduced N fertilizer-induced N₂O emissions, compared with urea only, in eastern Canada across 7 years. We also used meta-analysis (of static chamber studies) to examine how inhibitors and other enhanced efficiency fertilizers (EEFs), along with other improved N management techniques, affected fertilizer-induced N₂O emissions from Canadian agricultural cropping systems. From the field study, the dual inhibitors reduced growing season N₂O emissions by 22% and annual N₂O emissions by 10% for high N application rates to corn (Zea mays), while N₂O emissions from lower N applications to wheat (Triticum aestivum) showed no differences between the EEF and urea. Crop yields for both the corn and wheat were similar between the different N fertilizer treatments. Across Canada, the meta-analysis showed that EEFs (which include coated slow-release fertilizers and both nitrification and urease inhibitors combined and on their own), on average, reduced N₂O emissions by 11%. Nitrification inhibitors (alone or in combination with urease inhibitors) averaged a 19% reduction in N₂O emissions. Most of the studies used in the meta-analysis had minimal sampling through the non-growing season though, so the total annual N₂O emission reductions were not evaluated and may actually be lower. The meta-analysis indicated that the most effective N management techniques for reducing N₂O emissions were the use of EEFs, split application of N fertilizers and the use of organic fertilizers, with the effectiveness of these practices all strongly influenced by soil and weather conditions. The meta-analysis also found that reductions with EEFs from studies that included year-round measurements, tended to be less than studies that included only the growing season. This suggests that when improved N management practices use the same N application rates as the regular practice, more residual N may be available for non-growing season losses. As a result, when no yield benefit is noted, these improved practices should be combined with N rate reductions.