Abundant Rainfalls Modulates the Impact of Fertilizer Application Practices on Nitrous Oxide Emissions during Spring Wheat Growing Season.

The interannual and seasonal variations in N₂O emissions in response to N management practices and climate variations were examined using micrometeorological fluxes measured in spring wheat fields in eastern Canada during four growing seasons. The common practice of urea application at ~77 kg N ha^-1 was compared with alternative practices, either ammonium nitrate application or application of ~43 kg N ha^-1. Although N₂O emissions tended to increase with ammonium nitrate application in comparison with urea application, emissions were not significantly different, and thus the effect of N rate on N₂O emissions was studied regardless of the form. Over the four growing seasons, air and soil temperatures and cumulative rainfall were close to the climate normals. During the growing season, cumulative rainfall was relatively similar between years, whereas cumulative N₂O emissions ranged from 815 to 1474 g N₂O-N ha^-1. The variations in N₂O emissions arose mainly from the vegetative period, owing to the possibilities of high N₂O emissions following fertilization and abundant rainfall during tillering. In comparison with the whole growing season, N₂O emissions during the tillering stage were 69%, 72%, 49%, and 62% from the common practices in 2001, 2003, 2005, and 2011, respectively, and 67%, 37%, and 55% from the alternative practices in 2001, 2005, and 2011, respectively. During the tillering stage, enhanced N₂O emissions were strongly related to the associated rainfall amount, with 76 g N₂O-N ha^-1 emitted for each 10 mm of rainfall. Given that N₂O emissions over the growing season were affected more by the distribution of abundant rainfall (>10 mm) than by the cumulative rainfall or the reduction of the N rate by 40%, mitigation strategies in regions where rainfall is projected to be more frequent and intense in the early part of the growing season should go beyond reducing the common N rates.