Seasonal variation in carbon flux and the driving mechanisms in the grassland ecosystem in a mountain region of Northwest China

• Grassland ecosystem in summer had the greatest carbon sequestration capacity and acted as a carbon sink. • The trade-off between the impacts of influencing factors on GPP and ER explained the seasonal variation of grassland NEE. • LAI positively affected spring NEE, soil moisture negatively affected summer NEE and positively affected autumn NEE. • Atmospheric aridity conditions directly affect GPP and ER, water supply conditions indirectly affect GPP and ER through LAI. Carbon flux is generally influenced by climate, soil, and vegetation factors, with intrinsic relationships existing among these factors. However, few studies have analyzed the effects of these influencing factors on seasonal variations in carbon flux. In the current study, the regional carbon flux (gross primary productivity (GPP), ecosystem respiration (ER), and net ecosystem carbon exchange (NEE)) of the grassland ecosystem was upscaled using a long-term dataset of carbon flux sites in the Qilian Mountains based on a random forest model. Furthermore, correlation analysis and a piecewise structural equation model were used to identify the driving factors of seasonal carbon fluxes. Our findings indicated that GPP and ER of the grassland ecosystem were highest in the summer and showed an increasing trend from 2000 to 2022. The grassland ecosystem acted as a carbon sink in the summer, with an increase in carbon sequestration capacity, while it acted as a carbon source in the spring and autumn (NEE > 0). The trade-off between the different impacts of climate, soil, and vegetation factors on GPP and ER explained the seasonal variation in grassland NEE. Leaf area index had a positive effect on spring NEE. Soil moisture was the most important factor regulating variations in NEE during summer and autumn, having a negative effect on summer NEE and a positive effect on autumn NEE. The influence of atmospheric aridity conditions on NEE was greater than that of water supply conditions, and their impact paths on NEE changes were different. This study underscores the urgent need for analysis of factors influencing grassland carbon flux and its components at the seasonal scale, while also providing important information on the variations in carbon fluxes in the grassland ecosystem of the Qilian Mountains.