Temporal variability and environmental drivers of net ecosystem CO_(2)exchange in terrestrial ecosystems of the Yellow River BasinOA

The Yellow River Basin(YRB),located in the mid-latitude region of China,encompasses diverse ecosystem types and is highly sensitive to climate change.However,the temporal patterns and environmental drivers of net ecosystem CO_(2)exchange(NEE)across multiple time scales remain poorly understood.Using eddy covariance observations from the ChinaFLUX network collected between 2003 and 2020,this study investigated the temporal dynamics of NEE and its primary environmental controls in five representative ecosystem types within the YRB and its adjacent 100-km buffer zone:cropland,forest,grassland,shrubland,and wetland ecosystems.The results showed that all five ecosystems exhibited a generally U-shaped diurnal pattern from May to September,characterized by net CO_(2)uptake during the daytime and net CO_(2)release at night.At the daily scale,cropland displayed a typical bimodal carbon uptake pattern,whereas forest ecosystem exhibited the greatest day-to-day variability in NEE.In contrast,grassland,shrubland,and wetland ecosystems showed relatively smooth daily fluctuations.The net CO_(2)source-sink functions derived from NEE differed substantially among ecosystem types.Forest ecosystems acted as the most stable and persistent carbon sinks,whereas croplands exhibited short-term but high-intensity carbon uptake.Wetlands showed pronounced interannual variability,including an extreme net CO_(2)release event at the Haibei wetland site in 2007.Grassland and shrubland ecosystems were more susceptible to environmental stress and could shift from net CO_(2)sinks to net CO_(2)sources during drought years.The environmental controls on NEE exhibited clear time-scale dependence.At the half-hourly scale,photosynthetically active radiation(PAR)was the dominant driver of NEE variability,the influence of temperature increased progressively from the daily to monthly scales.These findings improve the understanding of regional carbon dynamics in the YRB,provide insights into the net CO_(2)source-sink status of different ecosystem types,and elucidate the mechanisms regulating ecosystem CO_(2)exchange across multiple temporal scales.