Can fertigation reduce nitrous oxide emissions from wheat and canola fields?

Increasing nitrogen fertilization and irrigation can contribute to nitrous oxide (N₂O) emissions from agriculture. Relative to the conventional practice of one-pass fertilization with all N applied at crop seeding, this study examined how splitting the total N fertilization into seeding time and in-crop fertigation impacts N₂O emission factors (EF) in irrigated wheat (Triticum aestivum) and canola (Brassica napus) in Southern Alberta, Canada during two growing seasons (May to Oct. in 2015 and 2016). With all the N applied at crop seeding, the growing-season N₂O EF of irrigated wheat and canola was in average 0.23 ± 0.03%. Conversely, implementing N fertigation lowered the magnitudes of N₂O EF in each of the four crop-years, averaging 0.16 ± 0.04%. Most of the reductions in N₂O emissions due to fertigation occurred with low and intermediate N rates (total rates of 60 and 90 kg N ha⁻¹) and in the second year of the study. This second year had recurrent, early-season rainfalls following seeding (and prior to fertigation) that triggered differences in the daily and cumulative N₂O fluxes. Within this year, fertigation on wheat consistently lowered the growing-season N₂O EF from a high of 0.27% to only 0.11% (P < 0.001). Also, at the intermediate rate of 90 kg N ha⁻¹, fertigation synergistically reduced the N₂O EF of canola by half, from 0.13% to 0.06% (P < 0.01). However, the mitigating effects of fertigation vanished with the highest N rate in the study (120 kg N ha⁻¹). Even with fertigation, this highest N rate resulted in high emissions in wheat, and lesser so in canola in part due to the higher N uptake of canola. Moreover, canola often manifested narrower ratios of N₂O emission-to-grain yield (EFyᵢₑₗd) than wheat. This interplay of crop species, rainfall and N management suggests that implementing fertigation with reduced N rates can proactively mitigates N₂O.