Urea is the most widely used nitrogen (N) fertilizer, with a projected increase in annual demand of 1.5% in the coming years. After its application to soil, urea undergoes hydrolysis via the urease enzyme, causing increases in the soil pH in the surrounding area of the granules and resulting in NH3 losses that average 16% of N applied worldwide and can reach 40% or more in hot and humid conditions. The use of urease inhibitors is an effective way to reduce NH3 losses. Several compounds act as urease inhibitors, but only N-(n-butyl) thiophosphoric triamide (NBPT) has been used worldwide, being the most successful in a market that has grown 16% per year in the past 10 years. Only in the past three years other compounds are being commercially launched. In comparison to urea, NBPT-treated urea reduces NH3 loss by around 53%. Yield gain by NBPT usage is of the order of 6.0% and varies from -0.8 to 10.2% depending on crop species. Nitrification inhibitors usually increase NH3 volatilization and mixing them with urease inhibitors partially offsets the benefits of the latter in reducing NH3 loss. The efficacy of NBPT to reduce NH3 loss is well documented, but there is a need for further improvement to increase the period of inhibition and the shelf life of NBPT-treated urea.

Nitrous oxide (N₂O) from nitrogen fertilizers applied to sugarcane has high environmental impact on ethanol production. This study aimed to determine the main microbial processes responsible for the N₂O emissions from soil fertilized with different N sources, to identify options to mitigate N₂O emissions, and to determine the impacts of the N sources on the soil microbiome. In a field experiment, nitrogen was applied as calcium nitrate, urea, urea with dicyandiamide or 3,4 dimethylpyrazone phosphate nitrification inhibitors (NIs), and urea coated with polymer and sulfur (PSCU). Urea caused the highest N₂O emissions (1.7% of N applied) and PSCU did not reduce cumulative N₂O emissions compared to urea. NIs reduced N₂O emissions (95%) compared to urea and had emissions comparable to those of the control (no N). Similarly, calcium nitrate resulted in very low N₂O emissions. Interestingly, N₂O emissions were significantly correlated only with bacterial amoA, but not with denitrification gene (nirK, nirS, nosZ) abundances, suggesting that ammonia-oxidizing bacteria, via the nitrification pathway, were the main contributors to N₂O emissions. Moreover, the treatments had little effect on microbial composition or diversity. 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 biofuel produced from sugarcane.

The environmental benefits of producing biofuels from sugarcane have been questioned due to greenhouse gas emissions during the biomass production stage, especially nitrous oxide (N₂O) associated with nitrogen (N) fertilization. The objective of this work was to evaluate the use of nitrification inhibitors (NIs) dicyandiamide (DCD) and 3,4 dimethylpyrazole phosphate (DMPP) and a controlled-release fertilizer (CRF) to reduce N₂O emissions from urea, applied at a rate of 120 kg ha⁻¹ of N. Two field experiments in ratoon cycle sugarcane were performed in Brazil. The treatments were (i) no N (control), (ii) urea, (iii) urea+DCD, (iv) urea+DMPP, and (v) CRF. Measurements of N₂O fluxes were performed using static chambers with four replications. The measurements were conducted three times per week during the first 3 mo and biweekly afterward for a total of 217 and 382 d in the first and second seasons, respectively. The cumulative N₂O–N emissions in the first ratoon cycle were 1098 g ha⁻¹ in the control treatment and 1924 g ha⁻¹ with urea (0.7% of the total N applied). Addition of NIs to urea reduced N₂O emissions by more than 90%, which did not differ from those of the plots without N. The CRF treatment showed N₂O emissions no different from those of urea. The results were similar in the second ratoon: the treatment with urea showed N₂O emissions of 0.75% of N applied N. Application of NIs resulted in a strong reduction in N₂O emissions, but CRF increased emissions compared with urea. We therefore conclude that both NIs can be options for mitigation of greenhouse gas emission in sugarcane used for bioenergy. Copyright © 2014. . Copyright © by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, Inc.