Long-Term Preservation of Humid Earthen Sites: Shelter Efficacy, Essential Oil Dynamics, and Microbial Adaptation.

This study evaluates the long-term conservation of humid earthen archaeological sites using protective shelters and plant essential oil treatments at the Laohuling Dam (Liangzhu, China), a UNESCO World Heritage site. Over seven years (2017–2024), structural deterioration, biological colonization, and microbial community dynamics were monitored through field surveys, amplicon sequencing (16 S rRNA and ITS), and shotgun metagenomics. Protective shelters effectively reduced large-scale structural damage and higher-plant colonization; however, enclosed and climate-controlled conditions promoted persistent microbial biofilms in high-humidity zones. Oregano essential oil treatments rapidly eliminated visible biofilms and suppressed recolonization for approximately 6–8 months, but did not prevent long-term microbial recovery. Post-treatment communities shifted from phototrophic and biofilm-forming taxa toward fast-growing, opportunistic heterotrophs, predominantly affiliated with Pseudomonadota. Metagenomic analyses revealed a stable resistome across consecutive treatment years. The high abundance of multidrug resistance genes (e.g., adeF, β-lactam- and CAMP-associated genes) primarily reflected the dominance of Pseudomonadota-related taxa rather than evidence of resistance evolution driven by essential oil application. No significant increase in resistance gene diversity or abundance was detected. These findings demonstrate that sheltering and essential oil treatments are effective short-term conservation tools but reshape microbial succession rather than eliminating biological risks. Long-term preservation of humid earthen sites therefore requires integrated strategies combining microclimate control, low-bioreceptivity materials, and continuous microbial monitoring.