Microplastics in agricultural soil: Polystyrene fragments inhibit soil microbial and enzymatic activities but promote nutrient concentration of Cowpea (Vigna unguiculata)

The disintegration and fragmentation of plastics into microplastics (MPs) and their transport from aquatic and terrestrial ecosystems into agricultural soils have been reported to be hazardous to soil health and crop growth. However, it is unknown how biological soil quality indicators and agronomic indices of a C3 plant respond to MPs' ecotoxicity after short-term exposure. Here, we conducted two experiments to define the eco-toxicity impact of polystyrene-MPs (PS-MPs) exposure at concentrations of (i) non-amended control, (ii) 0.5% PS-MP (w/w), (iii) 1% PS-MP (w/w), (iv) 2% PS-MP (w/w), and (v) 4% PS-MP (w/w) on the soil microbial biomass, respiration, and enzymatic activity—indicators of soil quality, and cowpea (model C3 plant) growth and nutrient concentrations. Our results demonstrate that PS-MPs, generally considered nontoxic, negatively inhibit microbial biomass and extracellular enzymatic activities after 30 days of soil incubation. On the other hand, increasing concentrations of PS-MPs increased soil basal respiration and the microbial metabolic quotient (qCO2), indicating microbial stress. Cowpea growth, irrespective of PS-MPs concentration and exposure time, showed neutral or non-inhibitory responses after 60 days. However, PS-MPs at low concentration (0.5% w/w) promoted an increase in cowpea leghemoglobin content, total chlorophyll pigment, and shoot and root nutrient concentrations (N, P, Zn, and Fe) over the control. Overall, our findings affirm the short-term ecotoxicity effect of PS-MPs on soil biological indicators of soil quality but also indicate that PS-MP exposure may not always have negative impacts on the agronomic performance of certain C3 plant species, such as cowpea.