Road construction effects on estuarine wetland soil organic carbon and heavy metals: evidence for near-road disturbance thresholds and buffer zoning

While the impacts of road construction are well-known, quantitative data on SOC disturbance thresholds specifically for estuarine wetlands remain relatively limited. To provide evidence for this specific context, we analyzed the near-road disturbance in the Fujian Minjiang River Estuary Wetlands (Ramsar Site No. 2506, southeast China). We used 56 plots at 5–70 m from an expressway to examine soil responses to construction. Surface (0–10 cm) and selected profile (0–50 cm) soils were analyzed for soil organic carbon (SOC), bulk density (BD), moisture, pH, and heavy metals (Cr, Ni, Cu, Zn, Cd). Distance effects were tested using mixed-effects and segmented regression models, analysis of variance (ANOVA), and correlation analyses. Surface SOC increased significantly with distance from the road. When grouped into distance classes, mean SOC in the roadside zone (0–30 m) was about 12% lower than in the reference zone (> 50 m). Segmented regression further identified a SOC breakpoint at 29.9 m from the road (95% CI: 27.0–32.8 m), beyond which the SOC–distance relationship became much flatter (R² = 0.86). Near-road soils showed the highest BD and lowest moisture, while several metals were clearly enriched within 0–30 m. These patterns indicate that compaction, altered hydrological connectivity, and traffic-related pollution jointly redistribute wetland carbon. We recommend a three-zone buffer (0–30, 30–50, > 50 m) and ≥ 50 m road–wetland setbacks, expanded in high-risk settings, to better align expressway development with protection of estuarine wetland carbon sinks.