Response of Soil CO2 Diurnal Flux to Precipitation Changes in a Desert Steppe

Desert steppe is a typical ecosystem in arid and semi-arid regions and an important component of the global carbon cycle. Under the background of global climate change, the increasing frequency of extreme precipitation events and changes in precipitation patterns can significantly affect water- and heat-sensitive desert steppe ecosystems, thereby regulating soil CO2 flux; however, the underlying mechanisms remain unclear. To investigate the effects of precipitation changes on soil CO2 flux and their roles in carbon cycling and ecological sustainability, this study was conducted in a desert steppe. Seven precipitation treatments were established, including a control (CK) and ±15%, ±30%, and ±45% precipitation gradients. Based on the static chamber-gas chromatography method, combined with principal component analysis (PCA), correlation analysis, random forest modeling, and stepwise regression, the main influencing factors and their diurnal variation patterns of soil CO2 flux were analyzed over 24 h periods from June to August. The results show that CO2 flux ranged from −68.33 to 77.59 mg·m−2·h−1. During the study period, CO2 flux exhibited a diurnal pattern characterized by daytime emissions and weak nighttime emissions or uptake, along with clear seasonal variation. The ±30% precipitation treatment showed the largest fluctuation in CO2 flux. Soil hydrothermal factors were identified as the key drivers of CO2 flux. With changes in precipitation intensity, the combined effects of multiple factors increased ecosystem complexity, and the controlling factors showed clear seasonal differences. The results from different analytical methods were generally consistent, providing a reference for predicting CO2 flux, developing carbon sink strategies, and supporting sustainable ecological management in desert steppe regions.