Mechanism of water-soil-carbon synergistic evolution and its ecological restoration thresholds in watershed

The Yellow River Basin While ecological restoration enhances the functions of terrestrial ecosystems, it can also trigger mutual interference among these functions. Understanding the underlying response mechanisms is crucial for scientifically guiding the restoration efforts in river basins. By using the Carnegie-Ames-Stanford Approach (CASA), the Soil and Water Assessment Tool (SWAT), and Landscape Ecology Indices, this study analyzed the evolution mechanisms and threshold effects of the net ecosystem productivity (NEP), water yield (WY), and soil conservation (SC) in the Yellow River Basin during the water-carbon cycle process over the past 20 years. During the ecological restoration period, NEP, WY, and SC maintained a synergistic growth trend. Among them, the synergies between NEP-WY and NEP-SC were enhanced, while the synergy between WY-SC was weakened, with a significant manifestation in the Loess Plateau. Precipitation dominated the synergistic sensitivity of NEP, vegetation coverage significantly drove the WY-SC synergy, and ecological restoration in the middle reaches of the basin had a positive impact. Model coupling revealed critical nonlinear thresholds for NEP, WY, and SC at 420 gCm⁻²a⁻¹ , 280 mm, and 25,000 t·km⁻², respectively. This indicates that while current ecological restoration actions are proving effective, it is essential to be vigilant about their threshold effects to avoid ecosystem service trade-offs. • Distributed physical modeling was employed to assess key watershed processes, including net ecosystem productivity, water yield, and soil conservation. • The synergistic patterns of water-soil-carbon and their underlying mechanisms were quantitatively analyzed by integrating landscape ecology metrics. • Threshold effects of these synergies under ecological restoration were identified, providing a scientific basis for guiding current watershed management strategies.