Conventional and Biodegradable Microplastics Both Impair Soil Phosphorus Cycling and Availability via Microbial Suppression

Microplastics (MPs) are emerging soil pollutants that can disrupt essential biogeochemical processes, yet their effects on phosphorus (P) cycling remain underexplored. Here, we conducted a 150-day incubation experiment using agricultural soil amended with either polyethylene (PE, conventional) or polylactic acid (PLA, biodegradable) MPs to investigate their impact on microbially mediated P cycling. MPs altered soil P cycling and decreased available phosphorus (AP) by ∼15% after 90 days. Fourier transform infrared spectroscopy revealed weakened AP-associated functional groups (P–O–P, P–O, and P═O), most pronounced under PLA treatment. These shifts were accompanied by reduced abundances of key P-cycling taxa ( Bacillus, Paenibacillus, and Sphingomonas ) and downregulation of phosphatase gene abundance ( phoA/D/X: −65.4% in PE, −59.8% in PLA). Correspondingly, the activities of acid, neutral, and alkaline phosphatases were all suppressed, with alkaline phosphatase in PE-treated soil reduced by 34.1%. Together, these results demonstrate that MPs disturb biotic transformation pathways, leading to subsequent alterations in the chemical speciation of soil P and decreased AP content. Notably, significant disruption was observed for both conventional and biodegradable types. Our findings challenge the prevailing assumption of environmental benignity for biodegradable plastics and underscore the urgent need for mechanistic assessments of their byproducts. Such disruption may hinder microbial P mobilization and decrease fertilizer use efficiency, ultimately threatening soil health and agricultural sustainability.