Influence of Azolla incorporation and/or dual cropping on CH4 and N2O emissions from flooded rice paddy systems

Azolla, a common aquatic fern has been used successfully as a dual crop with lowland rice. It grows rapidly and fixes atmospheric nitrogen for rice paddy. However, its ecological significance especially on greenhouse gases emissions remains unclear. Three independent experiments -two pot (2016 and 2017), and one field in 2019- were conducted to investigate the effects Azolla (A. filiculoides Lam.) either or both as dual cropping and green manure along with rice plant on simultaneous methane (CH4) and nitrous oxide (N2O) emissions from constantly flooded paddy soil. Under pot setups, dual cropping Azolla as a cover with rice plant significantly decreased seasonal CH4 emission by 34.7%, with no effect on N2O emissions. Suppressed CH4 emission was likely due to an increase in dissolved oxygen concentration and redox potential at the soil-water interface simulating CH4 oxidation. However, incorporation of Azolla as green manure into the soil plus dual crop in conjunction with chemical fertilizers significantly increased CH4 emission by 37.5% but decreased N2O emission by 74.5%. The significantly higher CH4 and lower N2O emissions were attributed to the readily decomposable incorporated Azolla, acting both as a source of CH4 production and N2O reduction. Contrary to the pot observations, application of Azolla as a dual crop in conjunction with chemical fertilizer or incorporated as green manure plus dual cropping in the field did not significantly affect seasonal CH4 emissions, but significantly increased cumulative N2O emissions at the middle rice growth stages by 645%—816%, and the total seasonal emission 3.4-fold. The higher N2O emissions were partly attributed to large quantities of exogenous organic carbon resulting from the accelerated growth and subsequent senescence of Azolla cover applied as a dual crop as impacted by higher summer air temperatures. Our observations suggest that dual cropping of Azolla with rice has the potential to reduce CH4 emissions from flooded rice paddies. Conversely, incorporation of Azolla as green manure into the paddy soil plus dual cropping in conjunction with or without chemical fertilizers indicates an inconsistent relationship between CH4 and N2O emissions. Long-term studies are needed to evaluate the relationship between leguminous cover crops and their effects on factors influencing CH4 and N2O emissions from continuously flooded rice paddies.