Contrasting responses of carbon fluxes to hydrothermal drivers in dry grassland and wet meadow ecosystems of the temperate steppe

This study focused on dry grassland and wet meadow ecosystems, two dominant ecosystem types within the temperate steppe. CO2 flux dynamics and key hydrothermal drivers were comparatively analyzed using eddy covariance observations, with a view to revealing their threshold effects and lag responses. Results revealed that the wet meadow ecosystem (WME) acted as a significantly stronger CO2 sink with greater flux variability compared to the dry grassland ecosystem (DGE). While both ecosystems shared similar seasonal trends, they operated under fundamentally different regulatory mechanisms. In the DGE, net ecosystem exchange (NEE) was primarily regulated by latent heat flux (LE) and net radiation (Rn), leading to the system being energy-limited and prone to saturation. Net CO₂ uptake increased rapidly only under low-energy conditions, but a clear physiological saturation threshold was identified (LE > 74.7 W m−2 and Rn > 146.8 W m−2), suggesting that limited soil moisture restricted the ability of DGE to utilize excess energy for photosynthesis. In contrast, the WME, benefiting from abundant soil moisture, experienced a continuous intensification of net CO₂ uptake with increasing LE, Rn, and VPD. This response pattern successfully surpassed the energy saturation limits observed in the DGE. Nevertheless, a hydraulic limitation mechanism emerged under extreme atmospheric dryness, where CO₂ uptake plateaued when VPD exceeded 39.50 hPa, indicating stomatal closure to prevent water loss. At the diurnal scale, CO₂ fluxes closely tracked energy-related drivers, showing minimal lag effects. In the DGE, lag relationships with hydrothermal factors remained stable throughout the growing season, while they fluctuated significantly in the WME, particularly with soil temperature and VPD. These results offer key insights for assessing CO₂ sink potential and inform ecosystem conservation strategies.