Rice and Azolla co-cultivation: An ancient nature-based solution for mitigating greenhouse gas emissions and increasing rice yield

Rice and Azolla co-cultivation, an ancient rice cropping pattern in China, is a promising nature-based solution for reducing CH 4 emissions while maintaining high yields. However, the reduction potential and underlying mechanisms remain unclear when considering water management and N 2 O emissions. This study investigated the comprehensive effects of rice and Azolla co-cultivation on both CH₄ and N 2 O emissions through their influence on soil physicochemical properties, enzymes, and related functional genes, using a series of pot experiments with three irrigation regimes (AWD, 2 cm, and 5 cm flooding depth). Rice and Azolla co-cultivation reduced CH 4 emissions by 2.9 %-13.2 % across all irrigation regimes, with the most significant reductions observed at 2 cm and 5 cm flooding depths. Higher water depths were associated with higher CH 4 emissions. This reduction can be clearly attributed to the decrease in the abundance of mcrA , mcrA / pmoA, and β -1,4-glucanase, as well as the increase in the abundance of pmoA , Eh, and DO. Meanwhile, rice and Azolla co-cultivation had a reduction of N 2 O emission by 1.7 %-8.6 %. Compared to AWD, 2 cm and 5 cm irrigation regimes greatly reduced N 2 O emissions by 22.0 %-26.5 % and 35.2 %-39.7 %, respectively. It was observed that rice and Azolla co-cultivation decreased the abundance of AOA , AOB , nirS , protease, and nitrite reductase, while enhancing the abundance of nosZ and denitrification enzyme. Additionally, rice and Azolla co-cultivation enhanced the rice yield by 6.2 %-8.3 % under all irrigation regimes. Results confirmed rice and Azolla co-cultivation as a nature-based solution for low-carbon and high-yield rice planting, especially under 5 cm irrigation regime. Note: The blue numbers represent the differences observed between treatments with Azolla and those without. The green numbers indicate the variations from the average values under AWD and 2 cm irrigation regimes in relation to the 5 cm irrigation regime. The red numbers illustrate the differences between the mean values under Azolla with 5 cm irrigation regime compared to other irrigation regimes without Azolla . • Azolla reduced CH 4 by 2.9 %-13.2 % via decreased mcrA , mcrA / pmoA , β -1,4-glucanase and increased pmoA , Eh, DO. • Azolla reduced N₂O by 1.7–8.6 % via lower AOA , AOB , nirS , nitrite reductase, and higher nosZ , denitrification enzyme. • Azolla increased rice yield by 6.2 %-8.3 % and decreased GHGI by 6.2 %-8.3 %. • Azolla achieved the win-wins in GHG mitigation and food security under different irrigation regimes.