The effects of biosolid microplastics on rhizosphere respiration of root exudates in Glycine max

Biosolids are an important nutrient source in agriculture, but their usage transfers microplastics that have side-effects on soil properties, the rhizosphere microbial community, and plant root exudation. However, little is known about the consumption rates of root exudates in the rhizosphere under exposure to biosolid microplastics and whether consumption profiles are sensitive to microplastic concentration, polymer type, or biosolids. Here, we studied the effect of two microplastic type-shape pairs (polyethylene film and polypropylene fragments) at two concentrations (2000 and 15,000 pcs/particles kg −1 dry soil) of the size fractions found in biosolids, plus biosolids naturally containing microplastics, on the respiration rates of isolated rhizosphere soil from 11 week old Glycine max. Respiration rates were quantified using the MicroRESP system with and without substrate induced respiration involving 15 root exudate compounds. Respiration rates without exudates were higher than the control for the polyethylene, polypropylene, and biosolid treatments, with the highest average respiration rate in the polypropylene at 15,000 pcs/particles kg −1 dry soil treatment. Substrate induced respiration of exudates revealed only the polypropylene at 15,000 pcs/particles kg −1 dry soil treatment had statistically significant first principal component scores. Respiration rates of amino acids were most responsive to microplastic concentration and type-shape pairs. Polypropylene at 15,000 pcs/particles kg −1 dry soil had uniquely high respiration rates for all amino acids, galactose, glucose, oxalic, and tartaric acid. Root exudate consumption rates were positively related/correlated to rhizosphere microplastic concentration and differed between type-shape pairs, yet could only be distinguished as a unique group at the concentration found in biosolids. • Microplastics were detected at similar rates in bulk and rhizosphere soil. • Microplastics in soybean rhizosphere soil increased microbial respiration rates. • Plastic type plus shape influenced microbial respiration rates. • Respiration rates and microplastic concentration were positively related. • Microbial consumption of exudates shifted to sugars plus amino acids.