EFFECTS OF SOIL COMPACTION AND COMPOST AMENDMENT ON VEGETATION ESTABLISHMENT, STORMWATER QUALITY, AND CONTAMINANT RETENTION IN ROADSIDE ENVIRONMENTS

Abstract

Soil compaction and the use of low-organic-matter subsoil during roadside construction can restrict vegetation establishment, reduce infiltration, and increase sediment and contaminant transport. This study evaluated the interacting effects of soil compaction, yard-waste compost amendment, and vegetation on roadside soil restoration, hydrologic performance, runoff water quality, and heavy-metal fate. A 12-week mesocosm study evaluated topsoil, subsoil, and subsoil amended with 2% yard-waste compost under contrasting compaction and vegetation conditions. Soil type, compaction, and vegetation jointly influenced roadside soil performance. Topsoil supported the greatest vegetation growth and biomass, whereas compost amendment improved the physical and hydrologic performance of subsoil. Incorporating 2% compost increased organic matter, improved infiltration, and reduced sediment export relative to unamended subsoil, without causing prolonged nutrient release. Vegetation establishment further influenced runoff quality: fertilized cool-season turfgrass achieved rapid canopy closure and shortened the period of elevated sediment loss, whereas native vegetation established more slowly. Compaction reduced saturated hydraulic conductivity by approximately one order of magnitude, with the greatest adverse effects occurring in unamended subsoil.A 20-week greenhouse pot study examined topsoil, subsoil, and compost-amended subsoil with 3%, 6%, and 10% organic matter under high-density (HD) and low-density (LD) conditions. Established turfgrass received weekly applications of synthetic roadway runoff containing Ni, Cd, Pb, Zn, Cu, nitrate, and orthophosphate. All treatments attenuated dissolved-metal concentrations, but contaminant export varied with hydraulic throughput. HD treatments transmitted less water and achieved higher mean metal retention (93.3%) than LD treatments (79.3%). Across density conditions, Pb and Cu were retained more, with mean retention efficiencies of 98% and 95% under HD conditions and 91% and 84% under LD conditions, respectively. Under HD conditions, the 3% amendment achieved the highest mean retention across the five metals (95.4%), whereas the 6% amendment performed best under LD conditions (84.9%). Increasing compost to 10% provided no additional improvement in overall metal retention. Mass-balance analysis further showed that retained metals accumulated predominantly in the soil rather than in plant biomass, indicating that soil-mediated immobilization was the principal retention mechanism. Higher compost rates increased soil organic matter but also increased nutrient and dissolved organic carbon export, demonstrating a trade-off between soil improvement and water-quality protection. Overall, the findings support moderate compost incorporation (3 to 6 %) combined with appropriate compaction management and vegetation establishment as a practical nature-based strategy for restoring roadside soils while reducing sediment, nutrient, and heavy-metal export from transportation corridors.