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 (CH) 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 CH and nitrous oxide (NO) and lower the yield-scaled global warming potential (GWP). The experiment included nine treatments: T (120 kg N ha urea), T (90 kg N ha urea + 30 kg N ha fresh Azolla), T (90 kg N ha urea + 30 kg N ha Blue-green algae (BGA), T (60 kg N ha urea + 30 kg N ha BGA + 30 kg N haAzolla, T (120 kg N ha urea + Hyphomicrobium facile MaAL69), T (120 kg N ha by urea + Burkholderia vietnamiensis AAAr40), T (120 kg N ha by urea + Methylobacteruim oryzae MNL7), T (120 kg N ha urea + combination of Burkholderia AAAr40, Hyphomicrobium facile MaAL69, Methylobacteruim oryzae MNL7), and T (no N fertilizer). Maximum decrease in cumulative CH emission was observed with the application of Methylobacteruim oryzae MNL7 in T (19.9%), followed by Azolla + BGA in T (13.2%) as compared to T control. NO emissions were not significantly affected by the application of CH-oxidizing bacteria. However, significantly lower (P<0.01) cumulative NO emissions was observed in T (40.7%) among the fertilized treatments. Highest yields were observed in Azolla treatment T with 25% less urea N application. The reduction in yield-scaled GWP was at par in T (Azolla and BGA) and T (Methylobacteruim oryzae MNL7) treatments and reduced by 27.4% and 15.2% in T and T, respectively, as compared to the T (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.