Earthworms-mediated the effects of polyethylene and polylactic acid microplastics on bacterial community in different soil aggregates

Microplastics (MPs) are viewed as an emerging pollutant in soil, with potential implications for soil microbial characteristics. Earthworms are vital to soil ecosystems, and may alleviate the negative effects of MPs on soil by improving their aggregates and nutrient conditions. Despite their pervasive presence, the mechanisms by which MPs and earthworms interact with soil microorganisms at the aggregates level remain largely unexplored. Here, we evaluated the effects of two contrasting types of MPs: conventional polyethylene (PE) and degradable polylactic acid (PLA), combined with earthworms, on bacterial communities, extracellular enzyme activities and microbial metabolic limitation across soil aggregates of two diameter ranges (0–0.25 mm and 0.25–2 mm) over 3 months. We found that both PE and PLA affected the diversity and composition of soil bacterial communities, reduced soil nutrient acquisition enzymes activities, and affected soil microbial metabolic limitation, which may be related to MPs changing the structure and composition of soil bacterial communities. Small aggregates had higher enzyme activity than large ones, with PE reducing β-1,4-glucosidase (BG) by 58.10 % and increasing alkaline phosphatase (AP) by 28.32 % in small aggregates, while in large aggregates, PE decreased BG by 25.36 % and increased β-D-1,4-cellulobiohydrolase (CB) by 82.76 %. Earthworms may partially mediate the negative impacts of MPs on bacterial networks and carbon metabolism by enhancing soil structure and improving nutrient utilization efficiency. These findings enhance understanding of soil microbial responses to MPs, which could offer valuable insights into the response of soil aggregate microorganisms to MPs with the involvement of soil microfauna.