Asynchronous responses of CO₂ fluxes to hydrological anomalies in a subalpine peatland: evidence from hysteresis and lagged responses

Abstract. Ecosystem-atmosphere CO₂ exchange in peatlands is highly sensitive to hydrological changes. However, continuous observations of how drought-induced hydrological anomalies and subsequent recovery affect the response pathways and timescales of CO₂ fluxes remain limited. The 2022 extreme drought in the Yangtze River Basin provided a valuable observational window for this study. Based on continuous eddy covariance (EC) measurements from 2021 to 2023, we characterized annual CO₂ source-sink status and, using the growing season (May–September) as the primary analysis window, compared variations in CO₂ flux components across different hydrological stages and analyzed their diel hysteresis and day-scale lagged responses to hydrological and temperature variables. The results showed that extreme drought shifted the Dajiuhu peatland from a CO₂ sink in 2021 (−353.20 g C m⁻² yr⁻¹) to a weak CO₂ source in 2022 (5.36 g C m⁻² yr⁻¹). During the growing season, gross primary productivity (GPP) declined more strongly than ecosystem respiration (Reco). Drought also altered diel hysteresis patterns in CO₂ flux responses. At the day scale, net ecosystem exchange (NEE) showed short lags to hydrological variables but relatively longer lags to temperature variables. This study provides continuous observational evidence of both asynchronous CO₂ flux responses to hydrological disturbances and drought legacy effects in a subalpine peatland.