Long-term NEE at ATTO (2014–2024): drought legacy effects and seasonal controls on Amazon carbon uptake

The Amazon rainforest stores ~150–200 Pg of carbon and plays a central role in the global carbon cycle, yet it is highly vulnerable to land-use change, fire, and climate change, leaving its future carbon balance uncertain. Net Ecosystem Exchange (NEE) quantifies the balance between ecosystem carbon uptake and release, but long-term NEE assessments in Central Amazonia remain scarce due to limited observational coverage from eddy covariance towers and CO₂ profile measurements. The Amazon Tall Tower Observatory (ATTO; https://www.attoproject.org) helps overcome this limitation by providing more than a decade of continuous measurements and by capturing two extreme drought events (2015/2016 and 2023/2024). In this work, we analyzed 11 years of NEE estimates at ATTO (2014-2024) and we propose a methodology for selecting the friction velocity threshold (u*), based on the identification of a plateau in the NEE-u* relationship, where NEE becomes independent of increasing turbulence, and all periods with u below the threshold are filtered due to insufficient turbulence*. We estimate monthly u* thresholds ranging from 0.18 to 0.21 m s⁻¹. We found that, on average, the forest in the ATTO flux footprint generally acted as a net carbon sink. However, in years following severe droughts, such as 2016 and 2024, we detect a temporary reversal, with the ecosystem becoming a CO₂ source during the wet season. We also quantified the effect of environmental drivers modulating NEE across seasons. We find that higher air temperature reduces carbon uptake during the wet season. In contrast, soil moisture shows opposite relationships depending on season: during the dry season, increasing soil moisture (10 cm depth) reduces net carbon uptake, whereas during the wet season, increasing it enhances net carbon uptake. Our findings deliver critical observational evidence to refine model parameterizations of tropical carbon-water interactions and to reduce uncertainty in predictions of the Amazon carbon balance under future climate scenarios.