Sludge compost: a double-edged sword for depleted soil restoration revealed by integrated multi-omics analysis.

The prospective use of sludge compost for restoring depleted soils requires balancing its agronomic benefits against potential ecological risks. This study employed an integrated metagenomic and metabolomic approach to evaluate the dose-dependent effects of sludge compost on soil properties, maize growth, and rhizosphere microbial communities. Results showed that moderate compost application (≤15% w/w) enhanced soil nutrient availability, promoted root development, and enriched beneficial microbial taxa (Streptomyces, Mesorhizobium, Flavisolibacter), while upregulating plant stress-response metabolites (terpenoids, flavonoids). Conversely, excessive application (>20%) induced salinity stress, impaired root growth, and altered the microbial community, favoring thermophilic and xenobiotic-metabolizing taxa. Critically, high application rates led to the accumulation of residual pharmaceuticals (anti-neoplastic and anti-epileptic agents) and pesticides (insecticides and rodenticides), which correlated with the enrichment of microbial pathways associated with human diseases, highlighting a significant ecological risk. In addition, root integrity was the primary determinant of a sustainable plant-microbe feedback loop. These findings underscore the necessity for tailored application strategies to harness the soil-restorative potential of sludge compost while mitigating contaminant-driven risks, providing a framework for its safe use in sustainable agriculture.