Methane emissions responding to Azolla inoculation combined with midseason aeration and N fertilization in a double-rice cropping system

Methane (CH₄) is an important greenhouse gas (GHG), and paddy fields are major sources of CH₄ emissions. This pot experiment was conducted to investigate the integrated effects of Azolla inoculation combined with water management and N fertilization on CH₄ emissions in a double-rice cropping system of Southern China. Results indicated that midseason aeration reduced total CH₄ emissions by 46.9%, 38.6%, and 42.4%, followed by N fertilization with 32.5%, 17.0%, and 29.5% and Azolla inoculation with 32.5%, 17.0%, and 29.5%, on average, during the early, late, and annual rice growing seasons, respectively. The CH₄ flux peaks and total CH₄ emissions observed in the late rice growing season were significantly higher than those in the early rice growing season. Additionally, CH₄ fluxes correlated negatively to soil redox potential (Eh) and dissolved oxygen (DO) concentration. Azolla inoculation and N fertilization greatly increased the rice grain yields, whereas midseason aeration had distinct effects on grain yields in both rice seasons. The highest annual rice grain yields of approximately 110 g pot⁻¹ were obtained in the Azolla inoculation and N fertilization treatments. In terms of yield-scaled CH₄ emission, Azolla inoculation combined with midseason aeration and N fertilization generated the lowest yield-scaled CH₄ emissions both in the early and in the late rice growing seasons, as well as during the annual rice cycle. In contrast, the highest yield-scaled CH₄ emission was obtained in the treatment employed continuous flooding, without Azolla and no N application. Our results demonstrated that Azolla inoculation, midseason aeration, and N fertilization practices mitigated total CH₄ emissions by 18.5–42.4% during the annual rice cycle. We recommend that the combination of Azolla inoculation, midseason aeration, and appropriate N fertilization can achieve lower CH₄ emissions and yield-scaled CH₄ emissions in the double-rice growing system.