Nitrous oxide emission related to ammonia-oxidizing bacteria and mitigation options from N fertilization in a tropical soil. Nitrification inhibitors in tropical soils

Nitrous oxide emissions from nitrogen fertilizers applied to sugarcane are a main target for reducing the environmental impact of ethanol production. The aims of this study were to determine the main microbial processes responsible for the N2O emissions from a tropical soil fertilized with different N sources, to identify options to mitigate cumulated greenhouse gas (GHG) emissions, and to determine the impacts of the N sources on the soil microbiome. To address these objectives, we conducted a 278 -day field experiment in which nitrogen was applied as calcium nitrate, urea (UR), urea with the nitrification inhibitors (NIs) dicyandiamide (DCD) or 3,4 dimethylpyrazone phosphate (DMPP), and urea coated with polymer and sulfur (PSCU). A control treatment without N (Control) was also included. Greenhouse gases (CO2, CH4, N2O) were extensively sampled over the experimental duration, and soil samples were taken for molecular analyses, including real-time PCR of the 16S rRNA, 18S rRNA, key nitrification and key denitrification genes as well as 16S rRNA amplicon library sequencing. Urea treatment caused the highest soil N2O emissions (1.7% of N applied) and PSCU treatment did not reduce cumulative N2O emissions compared to UR. Nitrification inhibitor treatments reduced N2O emissions by 95% compared to UR and had emissions comparable those of the Control. Similarly, calcium nitrate treatment resulted in very low N2O emissions. Interestingly, N2O emissions were significantly correlated only with bacterial amoA across all treatments, but not with denitrification gene (nirK, nirS, nosZ) abundances, suggesting that ammonia-oxidizing bacteria (AOB), via the nitrification pathway, were the main contributors to nitrous oxide emissions. While some microbial taxa were indicators of each treatment, the treatments had little short-term effect on the overall microbial (bacterial and archaeal) composition or diversity. Based on this study, we suggest nitrate-based fertilizers or the addition of NIs in NH4+-N based fertilizers as viable options for reducing N2O emissions in tropical soils and lessening the environmental impact of biofuels produced from sugarcane.