Insights into soil microbial assemblages and nitrogen cycling function responses to conventional and biodegradable microplastics

Abstract

Biodegradable microplastics (MPs) are proposed as sustainable alternatives to conventional MPs, yet their distinct effects on soil microbial communities and ecological functions remain insufficiently understood. This study compares the impacts of biodegradable polylactic acid (PLA) and conventional polyvinyl chloride (PVC) MPs on soil microbial assemblages and nitrogen cycling. Fluorescein diacetate hydrolase (FDAse) activity was temporarily stimulated by 2% (w/w) PLA and PVC MPs, while 7% (w/w) PVC MPs initially inhibited FDAse activity before promoting it. PLA MPs (2% and 7%, w/w) dramatically reduced bacterial diversity and altered community structure, enriching genera such as Nocardioides, Arthrobacter, Agromyces, Amycolatopsis, Saccharothrix, and Ramlibacter, known for degrading complex compounds. Conversely, PVC MPs (2% and 7%, w/w) showed minimal influence on bacterial diversity, with only temporary structural shifts at high concentrations (7% w/w). Network analysis revealed greater microbial complexity with PLA MPs, where MPs-degrading taxa emerged as keystone species. PLA MPs at both concentrations notably increased the abundance of nitrogenase iron protein subunit H gene (nifH) and nitrogen-fixing bacteria, such as Bradyrhizobium, while also sustaining ammonia monooxygenase subunit A gene (AOB amoA) effects up to day 90. At higher doses (7% w/w), PLA MPs enriched copper-containing nitrite reductase gene (nirK) and cytochrome cd1 nitrite reductase gene (nirS) abundance, boosting denitrifiers like Cupriavidus, Pseudarthrobacter, and Ensifer. In contrast, PVC MPs showed short-term effects on nitrogen cycling function. These findings have important implications for promoting sustainable agriculture and managing the environmental risks posed by MPs in soil ecosystems.