Effects of enhanced-efficiency nitrogen fertilizers on CH4 and CO2 emissions in a global perspective

It is widely recommended that enhanced efficiency nitrogen fertilizers (EENFs; urease inhibitors, nitrification inhibitors, urease and nitrification inhibitors combined, coated controlled-release urea) be applied to croplands to improve N use efficiency and crop yield via regulating N transformations. However, EENFs may inevitably affect soil C dynamics for the coupled relationship between soil carbon (C) and N biogeochemical cycles. Yet, a comprehensive assessment of the effects of EENFs on soil C dynamics is lacking. Here, we conducted a global meta-analysis using 67 publications to assess the overall effects of EENFs on soil CH₄ production, CO₂ emission, organic C (SOC) content, dissolved organic C (DOC) content, microbial biomass C (MBC) content under different environmental and management conditions (climate conditions, soil properties and fertilizer management practices). Our results showed that on average, compared to conventional N fertilizer, EENFs with the same amount of N fertilizer have a non-significant impact on CH₄ emission, which further depended on environmental and management conditions. Best scenarios for CH₄ reduction included: paddy field (25.8%), urease inhibitor (26.9%) and medium N application rate (150–300 kgN ha⁻¹; 23.6%), mainly due to the decreased stimulation of NH₄⁺ on CH₄ production; acid soil (33.7%), which was attributed to the enhanced methanotrophic communities' activities. The positive effect was also amplified by the increased mean annual precipitation and soil clay content that facilitated CH₄ production. Contrarily, EENFs significantly reduced CO₂ emission by 9.3%. The greater reduction was observed for the conditions producing more CO₂ after fertilization, i.e. field experiment, alkaline soils, low soil inorganic N content and high N application rate (≥300 kgN ha⁻¹). Nitrification inhibitors and coated N fertilizer were the best available options as they can reduce inorganic C dissolution in calcareous soil and SOC mineralization, respectively. Besides, EENFs application did not significantly alter SOC, DOC and MBC contents. Our findings highlighted the role of EENFs played in decreasing soil C emission in agroecosystems.