Forest conversion is more strongly associated with soil microbial functioning than chronic trace element exposure in an industrialized coastal Mediterranean ecosystem.

Abstract

Forest soils in industrialized coastal zones can experience chronic diffuse trace element (TE) exposure, yet how vegetation replacement influences soil microbial communities under these conditions remains poorly understood. This study compared soils from native sclerophyllous forests and exotic pine plantations in the Hualpén Peninsula Nature Sanctuary (Chile), located near an industrial complex characterized by oil refining, petrochemical, steel, and port activities. Twenty-two soil sites were evaluated for TE concentrations, contamination indices, physicochemical properties, microbial biomass, enzymatic activities, and bacterial community composition using 16S rRNA gene sequencing. Native forest soils contained 89% more organic matter (14.4% vs. 7.6%) and nearly twice the available N (53.6 vs. 26.2 mg kg-1) than plantation soils. These soils also exhibited greater respiration and microbial biomass carbon, whereas plantation soils showed higher metabolic quotients (qCO2), indicating lower microbial metabolic efficiency. Soil pH remained acidic in both systems (5.6-5.8). Mn and Zn were the most abundant elements, while contamination factors were highest for Pb, Cr, and Mn, indicating persistent trace element enrichment across the study area. Bacterial communities were dominated by Proteobacteria, Actinobacteriota, and Acidobacteriota, with native forest soils showing significantly higher Faith phylogenetic diversity (p = 0.041). Redundancy analysis revealed that bacterial community structure was more strongly associated with nutrient availability and microbial functional indicators than with trace element enrichment. However, Cr showed a significant secondary association (envfit r2 = 0.53, p = 0.032). Overall, the results indicate that variation in soil microbial functioning was more closely associated with vegetation type and related soil properties than with chronic trace element enrichment. These findings highlight the importance of considering land-use change, soil quality, and contamination together when evaluating microbial responses in industrially influenced forest ecosystems.