Biodegradable polylactic acid microplastics affect nutrient cycling during the entire crop growth cycle: Implications...

While microplastics (MPs) have been extensively studied for their effects on soil nutrient cycling, their influence on ecosystem multifunctionality (EMF) across the entire crop growth cycle remains poorly understood. This study systematically investigated the impacts of a model biodegradable MP, polylactic acid (PLA), on soil microbiomes and EMF across different maize incubation periods. Results of 16S rRNA amplicon sequencing and metagenomic analysis revealed that PLA-MPs decreased bacterial community α-diversity, co-occurrence network complexity, and stability throughout the 120-day incubation period. Particularly, PLA-MPs exerted more pronounced effects at early incubation stages (30 and 60 days), and these effects were intensified with increasing PLA-MP concentrations. PLA-MPs suppressed anaerobic carbon fixation (porA, porB, frda) and pyruvate metabolism (ppdk), while promoting fermentation (L-lactate dehydrogenase), nitrogen fixation (nifD, nifH, nifK, anfG), and microbial phosphorus (P) acquisition (phoD, phn cluster). Over the entire incubation period, PLA-MP-induced shifts in nutrient cycling enhanced soil carbon (C) function by 37.6-569%, while decreasing nitrogen (N) and P functions by 8.40-22.4% and 16.8-56.2%, respectively. Path analysis revealed that PLA-MPs altered soil properties and bacterial community diversity, which in turn regulated functional genes and these individual soil functions, thereby reducing EMF by 2.05-27.0% (R² = 0.923), with bacterial community diversity as the primary driver of EMF (standardized path coefficient of 0.978). These findings underscore the impacts of PLA-MPs on EMF in the soil-crop system throughout the entire maize growth cycle, advancing the understanding of the agroecological safety of biodegradable MPs.