Moderate Hydrological Droughts Maximized CO₂ Sink in China's Largest Floodplain Lake

Carbon dynamics in floodplain lakes are critical to gaining a full understanding of the global carbon budget. Here, we constructed a spatially explicit carbon dioxide (CO₂) flux data set covering 2003-2022 for China's largest floodplain lake (R² = 0.86, RMSE = 0.49 gC m^-2 d^-1). The annual fluxes varied from 52.57 ± 4.71 gC m^-2 in 2010 to -186.36 ± 7.27 gC m^-2 in 2011. Temporal variations in CO₂ flux were primarily driven by changes in the hydrological regime and wetland vegetation conditions. Specifically, water rise onset and recession onset emerged as the two most influential factors. A 10-day delay in lake water rise enhanced CO₂ uptake by 19.20 gC m^-2, whereas a 10-day advance in lake water recession increased uptake by 11.63 gC m^-2. However, the enhancement of the CO₂ sink can be impaired in the case of excessively early or rapid lake water level decline. For example, the extreme drought in 2022 reduced CO₂ uptake by over 20% compared to moderate drought years due to plant water stress and increased ecosystem respiration. The findings offer insights into fully evaluating the ecological consequences of lake and water resource management from the perspective of carbon neutrality.