Effects of low-density polyethylene and polyamide microplastics on the microbiological and chemical characteristics of an Andisol

Microplastics (MPs) are a significant threat to soils. However, there is scarce information on the impact of MPs on soil properties, particularly in volcanic ash-derived soils. The objective of this study was to evaluate the impact of polyamide (PA) and low-density polyethylene (LDPE) MPs on the biological and chemical characteristics of an Andisol from central Chile. Twenty-one parameters were evaluated, including pH, electrical conductivity (EC), total organic carbon (TOC), dissolved organic carbon (DOC), total phosphorus (TP), available phosphorous (AP), available nitrogen (AN), inorganic nitrogen forms (NH4+, NO2- and NO3), carbohydrates (CHO), polyphenols (POLs), humic substances, soil basal respiration (SBR) and activities of soil enzymes such as dehydrogenase, beta-glucosidase, phosphatase and urease. For this, a microcosm was set up in clay pots in an incubation chamber at 21 degrees C and 60% soil moisture, with the addition of PA and LDPE at doses of 1% and 3% w/w; a control treatment consisting of microcosm without MPs was also included. After 6 weeks of incubation, PA addition resulted in an increase in TOC and NH4+ by up to 32% and 26%, respectively, and a decrease in NO3- by 22%. AP decreased by 15%-19% with the addition of PA and LDPE. Similarly, acid phosphatase and beta-glucosidase activities decreased by 15% and 26% with PA and LDPE, respectively. The distance-based linear model (DistLM) was used to analyse relationships in chemical and biological datasets. The analysis revealed that TOC and TP were primary components in the best model for predicting microbiological changes (R2 = .469, AICs = 16.026), indicating that MPs accumulation affects soil carbon cycling and P content. Overall, the results show that MPs addition alters soil chemical and microbiological properties in Andisols, with varying effects depending on the type and dose of MPs, with the highest dose (3%) producing the most marked negative effects.