Terrestrial carbon cycling plays an important role in land–atmosphere interactions, yet carbon dynamics in dryland ecosystems across different aridity levels remain poorly understood. Using multi-year eddy-covariance observations from four desert ecosystems along a regional precipitation gradient in the Hexi Corridor, northwestern China, we investigated growing-season carbon fluxes and their environmental controls. All sites acted as weak net carbon sinks during the growing season (−43.73 to −64.26 g C m⁻² growing season⁻¹), with carbon uptake generally increasing with long-term mean annual precipitation. Weekly carbon fluxes (7-day moving averages) showed nonlinear responses to soil water content (SWC), with site-specific thresholds ranging from 1.87% to 14.44%. Below these thresholds, gross primary productivity (GPP) and ecosystem respiration (Reco) increased rapidly with increasing SWC, whereas responses weakened above the thresholds. Environmental controls differed among sites: carbon fluxes at the Huangmo site (HMo) were primarily regulated by soil moisture and vegetation activity, whereas fluxes at the Bajitan site (BJT) and Shenshawo site (SSW) were jointly controlled by soil moisture, soil temperature, and normalized difference vegetation index (NDVI). At the Linze site (LZe), both air and soil temperatures showed significant relationships with carbon fluxes, indicating stronger thermal constraints at this site. Soil temperature generally exhibited stronger associations with carbon exchange than air temperature. These results highlight the coupled roles of water availability and temperature in regulating carbon exchange across dryland desert ecosystems and provide insights into a better representation of dryland carbon cycle processes in land-surface and Earth system models.