Compost from decentralized composting models used for improving soil quality and plant-rhizosphere microbial...

The new EU Soil Monitoring Directive establishes the regulatory framework for assessing soil health and mitigating critical threats like salinity and contamination in degraded soils. While it promotes nature-based solutions-such as compost amendment-for soil restoration, compost efficacy remains strictly dependent on feedstock origin and processing technologies, which influence agronomic performance and potential risks of introducing emerging contaminants into soil. This study evaluated, for the first time within a holistic One Health framework, the efficiency of various composts in mitigating plant salinity stress while considering the possible spread of emerging contaminants, such as antibiotic resistance genes (ARGs) and the mobile genetic element intI1. The salinity tolerance of the Rosmarinus officinalis-microbiome system was assessed in pot experiments using degraded soil amended with four composts from various production systems: community (CC) and decentralized (DUC) urban composts, derived from the organic fraction of municipal solid waste and yard trimmings, and two agro-composts based on cattle manure and barley straw (SA) and on olive mill waste, leaves, poultry manure, and pruning residues (MA). Plant biomass, soil microbial activity and abundance were evaluated, alongside ARG dynamics. A Soil Quality Index was also calculated. Data were analysed using non-parametric ANOVA (p < 0.05). Salt stress significantly reduced plant biomass, while CC and DUC composts mitigated this effect, enhancing plant growth under this stress. Interestingly, the rhizosphere microbial community responded to salinity by increasing its activity, presumably owing to a pre-existent defence osmotic mechanism, not linked to the compost presence. However, compost type influenced microbial abundance and ARG dynamics. Notably, although salinity and composts promoted an ARG presence in soil, CC resulted more effective in improving soil quality and salt plant tolerance, including in a One Health prospective, confirming the importance of the compost production process for the use of these improvers.