Effects of conventional and biodegradable microplastics at comparable environmental levels on pesticide degradation in soil

Abstract

The co-existence of microplastics (MPs) and pesticides in agricultural environments, especially under greenhouse conditions, is a common occurrence. However, there is limited information on how MPs affect pesticide degradation under comparable environmental concentrations. In this study, we conducted a laboratory experiment to evaluate the impact of two types of MPs—biodegradable polylactic acid (PLA) and conventional polyethylene (PE)—on the degradation of 2 pesticides, metolachlor (MET) and imidacloprid (IMI), in soil at low (0.1 %) and high (1.0 %) concentrations. Over a 30-day period, we assessed pesticide degradation rates and metabolic products, along with soil properties and the aging status of MPs. Our results show that PLA reduced the degradation rate of IMI with concentration variability — 1 % PLA (w/w) significantly reduced the degradation rate of IMI by a 6.7 % decrease after 30 days compared to the treatment of CK, but had no effect on MET. In contrast, PE did not influence the degradation of either pesticide. PLA significantly inhibited IMI degradation by reducing the soil pH from 7.28 ± 0.01 to 7.12 ± 0.05, diminishing bacterial diversity, and altering the composition of soil bacterial communities. Furthermore, the accelerated degradation of PLA, compared to PE, resulted in the release of a greater quantity of microplastic particles, exacerbating its impact on soil microbial functions and IMI degradation efficiency. These findings suggest that biodegradable MPs, such as PLA, can hinder the degradation of the two pesticides, posing ecological risks to agricultural environments. Our results provide insights for developing policies to prevent and control farmland pollution.