Grazing enhances deep soil water conservation and runoff control on steep slopes via root-mediated eco-engineering

Balancing ecological restoration with agricultural production and water-related ecosystem services is a critical challenge for the sustainable management of dryland agroecosystems. However, the specific biophysical mechanisms governing hydrological regulation under different land-use regimes across complex terrain remain unclear. Based on a 25-year long-term controlled grazing experiment and a five-year period of continuous high-frequency monitoring on the Loess Plateau, this study investigated the divergent ecohydrological responses of moderate grazing (G) versus grazing exclusion (GE) across a continuous topographic gradient. We combined surface runoff monitoring with vegetation-root trait analysis to identify topographic thresholds and root-soil feedback mechanisms that dictate agroecosystem functioning. Results revealed a fundamental functional substitution: GE relied on a fragile physical barrier mode dominated by surface litter, whereas G shifted towards a resilient bio-engineering mode driven by deep roots. While GE effectively reduced evaporation on gentle slopes, its water conservation capacity diminished significantly on steep terrain due to gravitational dominance. Conversely, grazing stimulated a compensatory proliferation of deep fine roots, which facilitated deep soil water recharge and maintained high temporal stability. Crucially, on steep sunny slopes (>45°), the G treatment exhibited significantly lower runoff coefficients (5.48%) compared to GE (6.12%) and sustained higher deep soil moisture conservation. Partial least squares path modeling confirmed that deep root hydrology was the primary inhibitor of runoff under grazing, replacing the litter-dominated pathway of exclusion. These findings challenge the conventional agri-environmental policy that total exclusion is universally superior for soil and water conservation. We conclude that moderate grazing serves as an effective, topography-based agroecological management strategy, enhancing the provision of regulating ecosystem services on steep slopes through deep-root anchoring.