Intensive agricultural practices have led to progressive soil degradation, in terms of organic matter exacerbated by the co-occurrence of heavy metals and pharmaceutical residues. This study aimed to evaluate the effectiveness of a green compost (GC), derived from vegetal waste, in improving the quality of an agricultural soil with low organic carbon and reduced microbial activity. A microcosm experiment was conducted using lettuce (Lactuca sativa) seedlings grown in the treated soil. A contaminant mixture consisting of three antibiotics: sulfamethoxazole, ciprofloxacin, and chlortetracycline (7 mg kg⁻¹ each) and copper (as copper phosphate, 30 mg kg⁻¹) was added to simulate worst case scenario of co-contamination. Control treatments included soil with and without compost and/or contaminants.

Soil microbial communities were assessed in terms of structure (microbial abundance and diversity, NGS) and activity (dehydrogenase, phosphatase, and β-glucosidase activities). A Soil Quality Index (SQI), integrating both biotic and abiotic indicators (e.g., pH, temperature, contaminant and organic carbon presence), was applied to provide a holistic view of soil health.

Application of GC improved the SQI, enhancing soil physical and chemical properties, such as increasing pH and reducing compaction, which in turn supported microbial activity and plant growth. Antibiotic resistance genes (ARGs) were promoted in antibiotics and copper presence with the highest values in the bulk soil. Interestingly, GC promoted not only the highest plant biomass, but also the lowest ARG values. Moreover, the soil microbial community showed shifts in some populations under compost and lettuce treatments.

Although the contaminant mixture inhibited lettuce development, the presence of compost partially mitigated these adverse effects.

These findings highlight the potential of a vegetal waste compost as a sustainable strategy for the ecological restoration of degraded and co-contaminated agricultural soils. The use of green compost not only promotes microbial functionality and plant productivity but also contributes to soil resilience in contaminated environments.