Excessive Wetness Suppresses Carbon Sink of Amazon Forest Under Seasonal Water Surplus

The hydrological cycle in the Amazon basin has been shifted to a "wet season get wetter while dry season get drier" pattern. How the shifted hydrological pattern has been influencing the carbon budget of the Amazon forest remains unknown. Based on multiple observationbased carbon fluxes and climate datasets, we found that the carbon sink of the Amazon forest decreased over the past two decades, with 60.3% of the decreases occurring outside of the area of direct anthropogenic disturbances and approximately 52.6% of the decreases occurring in the wet season. The decreased carbon sink was dominated by the declined photosynthetic carbon uptake inferred by gross primary production (GPP). During the wet season, the entire Amazon basin is characterized by water surplus with climatically low photosynthetically active radiation (PAR), and the western to southern Amazon basin showed wetting trends with increased precipitation and decreased vapor pressure deficit (VPD), which enhanced the limitation of PAR, thereby reducing the GPP and thus the carbon sink. During the dry season, although the decreased GPP and carbon sink were mainly attributed to water stress associated with decreased precipitation and increased VPD, the excessive wetness with PAR limitation suppressed the GPP in the northwestern water surplus regions. We highlight the negative impacts of excessive seasonal wetness other than increased aridity on the carbon sink of the Amazon forest.