Exploring the influence of microplastics on soil biogeochemical traits using an artificial soil approach

Terrestrial microplastics are increasing in abundance and are an emerging contaminant of concern, particularly in agroecosystems where plastic mulch use and subsequent soil contamination are prevalent. Microplastics can impact soil physical, chemical, and biological properties; however, the mechanistic drivers of these soil-microplastic relationships are largely unknown. Notably, it is not fully understood how soil properties, like clay and organic matter content, interact with microplastic particle size, shape, or composition to influence microplastics impacts on soils. This 2-month incubation study investigated the effects of three different microplastics at a concentration of 1% (w/w), including polyethylene mulch film, polypropylene flat fragment, and polypropylene rope fragment microplastics, on a suite of soil health proxies. Laboratory-prepared artificial soils were also used to assess the effect of clay type and organic matter content on observed soil-microplastic interactions under controlled conditions. The microplastic treatments consistently reduced soil moisture and microbial respiration-processes that respond to altered soil conditions over relatively short periods of time compared to the other soil functions assessed (water-stable aggregates, nutrient mineralization, and microbial biomass). Notably, both clay mineralogy and organic matter content mediated select soil-microplastic interactions, highlighting a relationship that requires further research to fully understand. Additionally, the flexible polyethylene mulch film microplastics appeared more readily incorporated into the soil matrix, with less influence overall on soil moisture and basal respiration, than the more rigid polypropylene fragment microplastics. These findings suggest that a) short-term microplastic effects on soil properties may be largely driven by physical changes to soil structure, coupled with the hydrophobic nature of plastic polymers; b) the magnitude and direction of microplastic influence depends on innate soil properties; and c) short-term microbiological processes may be relatively more resilient to microplastic influence than abiotic edaphic properties.