Impacts of the 2024 Extreme Flooding Disaster on Greenhouse Gases Exchanges in Rice Floodplains of Southern Brazil

Extreme hydrometeorological events have become more frequent and intense, with direct implications for carbon, water, and energy exchanges between the biosphere and the atmosphere. Decades ago, extensive areas of natural wetlands in southern Brazil were converted into flooded rice paddies. The extreme floods that affected the state of Rio Grande do Sul in 2024 highlighted the critical regulatory role of these areas. Even when used for agriculture, wetlands acted as natural hydrological buffers, attenuating flood peaks. However, prolonged inundation can substantially alter greenhouse gas exchanges, particularly methane (CH₄), under anaerobic soil conditions. This study quantified the impacts of the extreme floods of 2024 on ecosystem–atmosphere exchanges of CO₂, CH₄, and H₂O, based on continuous eddy covariance measurements conducted in an irrigated rice lowland in southern Brazil. The site is managed under a typical intensive regional system and followed the crop rotation sequence: flood-irrigated rice (December 2023 to April 2024); winter fallow due to prolonged flooding (May to October 2024); rainfed soybean (December 2024 to April 2025); and forage crops with cattle grazing (May to October 2025). The study compared the May–October period of 2024 (flood year) with the same period in 2025 (non-flood year). During the prolonged inundation period in 2024, the system exhibited higher CO₂ and CH₄ emissions compared to the corresponding non-flooded period in 2025, while Evapotranspiration was similar. The absence of flooding and the cultivation of forage crops in 2025 resulted in reductions of up to 20% in CO₂ emissions and 60% in CH₄ emissions relative to the flooded fallow period of 2024. These results demonstrate that extreme hydrological disturbances can induce short but intense pulses of greenhouse gas emissions, with persistent effects on annual carbon balances. At the same time, adaptive management practices, such as crop rotation and the reduction of fallow periods, show potential to mitigate these effects and enhance agroecosystem resilience. The findings contribute to the derivation of local emission factors, the development of climate-adaptive agricultural strategies, and integrated assessments of extreme events at local and regional scales.