The Effect of Microplastics on Soil Microbial Activity, Biomass, and Microbial Community Structure in Three Types of Temperate Forest

Microplastic pollution is a problem of global concern, but its effects on forest soils are largely overlooked. This study is based on a laboratory experiment where the effects of soil-added polyethylene microplastic particles (MP-) of two sizes (60 μm and 140 μm) (Cospheric LLC, USA) were measured to examine their effects on three types of temperate forest: dry pine forest, beech-dominated forest, and ash-dominated riparian forest that differ greatly in several physicochemical and biological soil properties. The addition of MP- did not significantly alter the respiration rate of any of the forest soils studied (p = 0.6303), as shown by ANOVA. Soil microbial biomass, as measured by the phospholipid fatty acid (PLFA) method, decreased under 60 µm MP treatment but not under 140 µm MP treatment (p = 0.0094). MP- did affect microbial community structure, especially increasing the proportion of bacteria in the community under 60 µm MP treatment (p = 0.0023). MP- affected the PLFA pattern, as shown by PERMANOVA analysis along with NMDS ordination; the effect was similar in the three studied forest types. As shown by SIMPER analysis, there was a relative decrease in fatty acid 16:1ω7 and a simultaneous increase in 16:0 and 18:0 under both MP treatments. This may potentially serve as an indication of MP pollution in temperate forest soils. Our results suggest that forest soil bacteria, as a group, may benefit from MPs at the expense of fungi, which provides a new perspective on how soil microorganisms interact under globally common MP pollution.