Effects of aged and pristine microplastics on physical, chemical, and biological soil properties

Microplastics (MPs) can negatively affect soil microbial communities and biogeochemical processes. This study evaluated the effects of two polymers, polylactic acid (PLA) and low-density polyethylene (LDPE), at three concentrations (0.25%, 2.5%, 5% w/w) and two aging degrees (pristine and artificially UV-aged) on soil microbial community structure and functionality. A 12-week microcosm experiment was conducted to assess changes in soil physicochemical properties, enzyme activities, and microbial diversity. Sequencing of 16S rRNA gene and ITS region were used to analyse bacterial and fungal α- and β-diversity, and functional potential was inferred using FAPROTAX and FungalTraits. MPs generally increased total organic carbon (TOC) and reduced water holding capacity (WHC). Aged MPs, particularly PLA at 5% w/w, reduced intracellular and overall soil enzyme activity. Bacterial richness and evenness decreased at 0.25% w/w but increased at 5% w/w, likely due to the dominance of a few competitive taxa. Fungal communities were more sensitive, showing reduced α-diversity across treatments. High MP contents promoted saprophytic families and shifted predicted metabolic pathways from aerobic to anaerobic, consistent with reduced WHC and increased TOC.