Chlorpyrifos degradation and its impacts on phosphorus bioavailability in microplastic-contaminated soil

Pesticide residues and microplastics (MPs) in agricultural soils are two major concerns for soil health and food safety. The degradation of chlorpyrifos (CPF), an organophosphorus pesticide, releases phosphates. This process may be affected by the presence of MPs in the soil. The combination of CPF and MPs presence in the soil may thus produce interaction effects that alter the soil phosphorus (P) balance. This study explores the degradation pathways of CPF (6 mg kg⁻¹, 12 mg kg⁻¹ of CPF addition) in soils with different levels of polylactic acid MPs (PLA-MPs) (0.0 %, 0.1 %, 0.5 %, 1.0 % w/w), and analyzes soil P fractions and phosphatase enzyme activities to investigate soil P bioavailability under different treatments. Results show that the degradation of CPF fits to a first-order decay model, with half-lives (DT50) ranging from 11.0 to 14.8 d depending on PLA-MPs treatment. The concentration of its metabolite 3, 5, 6-trichloropyridine 2-phenol (TCP) reached a peak of 0.93–1.67 mg kg⁻¹ within 7–14 days. Similarly, the degradation of CPF led to a significant transient increase in P bioavailability within 3–7 days (p < 0.05), with a peak range of 22.55–26.01 mg kg⁻¹ for Olsen-P content and a peak range of 4.63–6.76 % for the proportions of available P fractions (H₂O-P+NaHCO₃-P+NaOH-P), before returning to prior levels (Olsen-P: 11.28–19.52 mg kg⁻¹; available soil P fractions: 4.15–5.61 %). CPF degradation (6 mg kg⁻¹) was significantly inhibited in soil with 1.0 % PLA-MPs addition. The effects of MPs and CPF on soil P fractions occur at different time frames, implying that their modes of action and interactions with soil microbes differ.