Mitigation of yield-scaled greenhouse gas emissions from irrigated rice through Azolla, Blue-green algae, and plant growth–promoting bacteria

Irrigated transplanted flooded rice is a major source of methane (CH₄) emission. We carried out experiments for 2 years in irrigated flooded rice to study if interventions like methane-utilizing bacteria, Blue-green algae (BGA), and Azolla could mitigate the emission of CH₄ and nitrous oxide (N₂O) and lower the yield-scaled global warming potential (GWP). The experiment included nine treatments: T₁ (120 kg N ha⁻¹ urea), T₂ (90 kg N ha⁻¹ urea + 30 kg N ha⁻¹ fresh Azolla), T₃ (90 kg N ha⁻¹ urea + 30 kg N ha⁻¹ Blue-green algae (BGA), T₄ (60 kg N ha⁻¹ urea + 30 kg N ha⁻¹ BGA + 30 kg N ha⁻¹ Azolla, T₅ (120 kg N ha⁻¹ urea + Hyphomicrobium facile MaAL69), T₆ (120 kg N ha⁻¹ by urea + Burkholderia vietnamiensis AAAr40), T₇ (120 kg N ha⁻¹ by urea + Methylobacteruim oryzae MNL7), T₈ (120 kg N ha⁻¹ urea + combination of Burkholderia AAAr40, Hyphomicrobium facile MaAL69, Methylobacteruim oryzae MNL7), and T₉ (no N fertilizer). Maximum decrease in cumulative CH₄ emission was observed with the application of Methylobacteruim oryzae MNL7 in T₇ (19.9%), followed by Azolla + BGA in T₄ (13.2%) as compared to T₁ control. N₂O emissions were not significantly affected by the application of CH₄-oxidizing bacteria. However, significantly lower (P<0.01) cumulative N₂O emissions was observed in T₄ (40.7%) among the fertilized treatments. Highest yields were observed in Azolla treatment T₂ with 25% less urea N application. The reduction in yield-scaled GWP was at par in T₄ (Azolla and BGA) and T₇ (Methylobacteruim oryzae MNL7) treatments and reduced by 27.4% and 15.2% in T₄ and T₇, respectively, as compared to the T₁ (control). K-means clustering analysis showed that the application of Methylobacteruim oryzae MNL7, Azolla, and Azolla + BGA can be an effective mitigation option to reduce the global warming potential while increasing the yield.