Azolla planting reduces methane emission and nitrogen fertilizer application in double rice cropping system in southern China

Rice paddies are a major source of methane. How to reduce the methane emission in the paddy field without decreasing the yield has become a major concern of scientists, environmental groups, and agricultural policymakers worldwide. Azolla, used as a dual crop in rice cultivation, has multiple agronomic benefits. However, the effects of the dual cropping of Azolla on methane emissions of double rice cropping paddies have not yet been reported. Here, we conducted a 3-year field experiment to evaluate the impacts of rice + Azolla on methane emission and rice yield in a double rice cropping system. The results indicated that the rice + Azolla without N fertilizer and with moderate N fertilizer (200 kg N ha−1 a−1) significantly reduced methane emissions over the rice cycle by 12.3 and 25.3% compared with the conventional rice cropping with common N fertilizer (400 kg N ha−1 a−1), respectively. The reason for the trend was because the dual cropping of Azolla has significant effect on dissolved oxygen and soil redox potential, which are key factors for methane emission in this study. The rice yield under the rice + Azolla with moderate N fertilizer annually averaged 12.7 Mg ha−1, which was comparable with that of the conventional rice cropping with common N fertilizer. Moreover, the rice + Azolla with moderate N fertilizer had the lowest yield-scaled methane (25.2 kg Mg−1 grain yield). Here, we showed for the first time that Azolla planting allows sustainable rice production coupled with methane mitigation in double rice cropping systems.