Distinct Influence of Conventional and Biodegradable Microplastics on Soil Microorganisms and Nitrogen Cycling: Emphasizing the Effect of Biodegradable Polylactic Acid

Biodegradable plastics are a feasible option to reduce plastic pollution, but the environmental effects of their release of microplastics (MPs) into the soil are still unclear. In this study, three typical types of MPs: polyethylene, polyvinyl chloride, and biodegradable polylactic acid (PLA), with two particle sizes, were selected to systematically investigate the effects of short-term (one month) exposure to MPs on soil physicochemical properties, enzyme activities, and bacterial community structure and functions. The results showed that MPs exposure apparently increased the total organic carbon (TOC) and total nitrogen contents (TN) by 27.08–48.58% and 4.5–20.9%, respectively, while decreasing the ammonium nitrogen (NH₄⁺-N) and nitrate nitrogen (NO₃⁻-N) contents by 56.5–68.8% and 3.2–24%, respectively, but had no significant effect on pH and total phosphorus (TP) content. Small particle size (50 μm) MPs and PLA-MP had greater effects on soil enzyme activity. 50 μm PLA significantly decreased soil bacterial diversity (decreased by 9%, P < 0.05) and caused greater changes in bacterial community compositions and functions than other MPs. Nocardioides and Bacillus were proposed as MPs-degraders due to they were specifically enriched by MPs. Short-term exposure to MPs inhibited the nitrification process and organic degradation and synthesis processes, but enhanced the denitrification and nitrogen fixation processes. Our study highlighted the effect of smaller particle sizes of biodegradable PLA-MP on soil microbial community structure and function.