Assessing the persistence, potential risks, and driving mechanisms of microbial risk factors in soil following biogas slurry irrigation

Manure biogas slurry is a widely used liquid fertilizer in crop cultivation, but its associated microbial risks have been underexplored. To address this knowledge gap, we conducted a 120-day field study, systematically investigating the effects of varying dosages and frequencies of biogas slurry irrigation on the spatial and temporal dynamics of exogenous microorganisms, antibiotic-resistant genes (ARGs), virulence factor genes (VFGs), and pathogenic antibiotic-resistant bacteria (PARB). Our findings revealed an initial enrichment of exogenous microorganisms (1.88 %-2.17 %) and three microbial risk factors (ARGs: 0.14–0.35 copies/cell, VFGs: 0.32–0.88 copies/cell, PARB: 0.001–0.02 copies /cell) during the first 20 days of irrigation, followed by a subsequent decline, with a slight enrichment observed on the 60th day compared to the initial value, but ultimately stabilizing towards initial levels within a maximum of fifteen months. Interestingly, soils at depths of 30–90 cm exhibited greater susceptibility to microbial invasion compared to the surface soil. Furthermore, repeated irrigation at a dosage of 450 t/ha led to a substantial increase in ARG (by 4.87 × 10⁷ copies/g dry soil) and PARB (by 3.35 × 10⁶ copies/g dry soil) densities from single irrigation, thereby elevating potential microbial risk of soil. Notably, soil properties and antibiotics, especially tetracyclines, played pivotal roles, explaining 53.5 % and 42.0 % of the variance in PARB abundance, respectively. This comprehensive study underscores the importance of prudent irrigation practices to mitigate potential microbial hazards, promoting environmentally friendly agricultural practices.