Impact of high-density polyethylene (HDPE) microparticles on soil physical-chemical properties, CO2 emissions, and microbial community in a two-year field trial

Contamination by microplastics (MPs) is a serious problem affecting both aquatic and terrestrial ecosystems, but despite the large number of papers published in recent years, the impact of microplastics (MPs) on soil is still debated. This work aims to evaluate the effects of different amounts (0, 1, 2 % v/v) of high-density polyethylene (HDPE) microparticles (1-0.25 mm in size) on soil properties over time. Specifically, in a field plot experiment lasting about 2 years, treated and control soils were periodically sampled and analysed for their physical (aggregate stability and distribution), chemical (total N, soluble C and N, available P), and soil biochemical (basal respiration, microbial biomass C, enzyme activities, and fatty acid methyl ester - EL-FAME) properties. In addition, CO₂ fluxes from soil to atmosphere were measured throughout the experiment. The physical and chemical parameters of the treated soils did not differ significantly from the control soil, whereas specific changes occurred in the biochemical characteristics during the experiment, particularly in the soil treated with the higher dose of MPs. In the early period (21 to 46 days after the treatment), some changes in the microbial community structure were observed for the soil treated with 2 % MPs, suggesting the occurrence of stress conditions for the microbial biomass, likely due to nutrient limitation. After 166 g from the start of the experiment, the 2 % MPs-treated soil showed, other than a lower CO₂ flux than the control soil, a reduction of basal respiration together with an increase in actinomycetes and total fungi (both saprophytes and AMF). There was also an increase in C-related enzyme activities one year after treatment with MPs. These latter results suggest that the soil microbial community may be adapting to the new conditions and available energy substrates, which may also indicate the onset of HDPE degradation processes.