Polylactic acid microplastics regulate organophosphorus pesticide impacts on phosphorus bioavailability in soil-plant systems.

Organophosphorus pesticides are known to potentially disrupt soil phosphorus (P) cycling. Microplastics often co-occur with organophosphorus pesticides in agricultural soils, with largely unknown combined effects. This work examined chlorpyrifos (CPF, 6 and 12 mg kg-1) fate in soils with and without co-occurring biodegradable polylactic acid microplastics (PLA-MPs, 1%), and their combined impacts on plant growth, P distribution, P fractions and P-related enzyme activities in maize-cultivated soils. Results showed that CPF alone enhanced the transformation of NaHCO3-Po (-54%) to NaHCO3-Pi (53%) and decreased occluded-P (-2%) at the seedling stage, and increased H2O-P (13%-40%) at the heading and harvesting stages. PLA-MPs significantly increased CPF residues by 24-45% under 12 mg kg-1 application, but increased and decreased its metabolite (3,5,6-trichloro-2-pyridinol) under 6 and 12 mg kg-1 application, respectively. Effects of 6 mg kg-1 CPF on plant biomass were statistically insignificant, but negative effects of CPF+PLA-MPs were observed at the seedling stage (12-36%). However, PLA-MPs mitigated the effects of CPF on plant P distribution at the seedling and heading stage. Compared to CPF alone, CPF+PLA-MPs increased H2O-P at the seedling stage and facilitated the transformation of NaHCO3-Po to NaHCO3-Pi at the heading stage. CPF alone only affected rhizosphere alkaline phosphatase activity (ALP), while CPF+PLA-MPs exerted effects on non-rhizosphere ALP activity depending on CPF dose and growth stages. These findings indicate that PLA-MPs regulate CPF fate and induce combined effects on plant growth and P cycling in soil-plant system, providing new insights into the ecological risks of multi-contaminants and environmental implications of biodegradable plastics.