Meta-analysis: effect of improved nitrogen management practices on nitrous oxide emissions in Canadian studies

Crop production in Canada demands consistent use of nitrogen (N) fertilizer, which produces substantial nitrous oxide (N 2 O) emissions. It may be reduced through scientifically supported N management practices, including applying right sources at right time, right rate, and right place (i.e., 4R practices). One promising 4R practice is use of enhanced efficiency fertilizers (EEFs). However, there is a lack of research synthesis quantifying their effectiveness. To address this gap, we conducted a Canada-wide meta-analysis of field research studies to measure the effectiveness of EEFs and their effects when compared to, or combined with other 4R management practices. Here we present the results of two analyses: (i) comparison of EEFs to conventional N fertilizers (21 studies, 291 observations) using the natural-log-of-the-response-ratio as the effect size, and (ii) the comparison of different 4R practices (31 studies, 561 observations) using emission factor (EF) as the effect size. Mean effect sizes were calculated using a generic inverse variance, random effect multilevel model. Overall, EEFs reduced N 2 O emissions by 11% compared to conventional fertilizers. EEF types had 23%, 15%, 6%, and −9% N 2 O reductions (with negative values implying an increase) from nitrification inhibitors, dual inhibitors, slow-release, and urease inhibitors, respectively. The most influential moderators for EEF performance were air temperature at fertilization and soil silt content. The best 4R practices were split application (EF = 0.78%) and EEFs (EF = 0.75%), followed by organic fertilizers (EF = 0.8%). The most influential moderators of EFs were annual relative humidity, percent of soil sand, and crop type.