Afforestation enhances soil-water-nutrient coupling in Benggang erosion zones: Species-specific restoration trajectories on Leizhouṇ Peninsula, South China

• Eucalyptus urophylla best improves soil structure & carbon in Benggang erosion zones. • Restoration duration outweighs tree species identity for soil hydrology-nutrient synergy. • Afforestation shifts soil hydrology-nutrient coupling from disorder to quality coordination. • Soil water potential drives organic carbon accrual; phosphorus limits Eucalyptus long-term. • Phase Eucalyptus pioneers into mixed plantations for sustainable Benggang restoration. Benggang erosion, a severe form of soil degradation in subtropical South China, devastates ecosystems. Afforestation is a key restoration strategy, yet the mechanisms underlying coupled soil hydrology-nutrient improvements remain unclear. This study quantified soil hydrology-nutrient dynamics across five afforested species ( Pinus elliottii , Eucalyptus urophylla , Koelreuteria bipinnata , Cinnamomum camphora , and Acacia auriculiformis ) and bare ground (control) on Leizhou Peninsula in 2019 and 2023. We analyzed soil physical properties (bulk density-BD, porosity-TPo, water content-SWC, potential-SWP, available water capacity-AWC, particle size, fractal dimension-D) and chemical properties (soil organic carbon-SOC, total nitrogen-TN, phosphorus-TP). Coupling coordination degree (CCD) and a Minimum Data Set-based Soil Quality Index (SQI) were employed. Afforestation significantly improved soil properties by 2023, with E. urophylla demonstrating the greatest enhancements: reduced BD (>6.8 %), increased TPo (>15.4 %), SWC (>33.2 %), AWC (>45.8 %), and SOC dominance (>50 %). Soil texture shifted towards finer loams. Correlations between hydrological (TPo, SWC, SWP, D) and nutrient (SOC, C/P) indicators strengthened post-afforestation. Structural equation modeling identified SWP and SWC as primary drivers of SOC accumulation (β = 0.32 and 0.29, P < 0.01). While restoration duration exerted a stronger overall influence than species identity, the selection of pioneer species (e.g., E. urophylla ) was critical for initiating rapid hydrological improvements that facilitated long-term recovery. CCD escalated from moderate disorder (bare soil) to quality coordination (0.97) in E. urophylla by 2023. SQI increased 150–400 %, peaking in E. urophylla (0.82, “Higher” grade). Afforestation effectively restores Benggang zones by enhancing the interaction between hydrological and nutrient processes. E. urophylla is an optimal pioneer species, rapidly enhancing soil structure and carbon sequestration. For long-term sustainability, transitioning initial E. urophylla stands towards mixed-species plantations is recommended to address phosphorus limitations.