Carbon Emissions of Submerged Plant Biomass Offset Its Carbon Sink Effect in Lake Hydro‐Fluctuation Belts

Abstract In hydro‐fluctuation belts (HFBs), plants thrive during dry seasons, sequestering atmospheric carbon dioxide (CO 2 ). When water levels rise in wet seasons, these plants become submerged, resulting in their die‐off and decomposition, which release CO 2 and methane (CH 4 ) into the atmosphere. To assess the climatic implications of this sequestration‐emission sequence, we conducted incubation experiments on HFB plants from Poyang Lake, China, to quantify changes in CO 2 ‐equivalent (CO 2 ‐eq) based on CH 4 global warming potentials and to elucidate the underlying microbial mechanisms using quantitative polymerase chain reaction. The results demonstrated that, after submersion, the aboveground parts die and partially decompose, creating anaerobic conditions that promoted methanogenesis for CH 4 production. This sequence from initial CO 2 sequestration to the subsequent release of CO 2 and CH 4 increased CO 2 ‐eq and contributed to the greenhouse effect. When CH 4 global warming potentials are considered over 20‐, 100‐, and 500‐year time horizons, the CO 2 ‐eq increases by 51.4, 15.9, and 2.7 g per gram of plant carbon, respectively. If plant desiccation induced by drought occurs prior to submersion, it may lead to a higher net CO 2 ‐eq following subsequent inundation. These findings offer new insights into the ecological impacts of plant carbon sequestration in HFBs.