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 CH4 emissions while maintaining high yields. However, the reduction potential and underlying mechanisms remain unclear when considering water management and N2O emissions. This study investigated the comprehensive effects of rice and Azolla co-cultivation on both CH₄ and N2O 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 CH4 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 CH4 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 N2O emission by 1.7 %-8.6 %. Compared to AWD, 2 cm and 5 cm irrigation regimes greatly reduced N2O 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.