Small-sized biodegradable PLA microplastics inhibit plant nitrogen uptake by reshaping soil microbial communities and stimulating microbial metabolism.

The effects of microplastics (MPs) varying in polymer type and size on soil microbial community composition, metabolic functions, and nutrient cycling remain insufficiently understood. Here, we conducted a pot experiment using MPs differing in polymer type (non-biodegradable polyethylene [PE], and biodegradable polylactic acid [PLA]) and four particle sizes (1200-1400, 600-700, 120-150, and 25-38 μm), with amplicon sequencing, shotgun metagenomics, and nitrogen-15 (15N) tracing model. Our results showed that small-sized PLA-MPs (25-38 μm) reduced bacterial diversity, destabilized microbial networks, and shifted community assembly toward deterministic processes, whereas PE-MPs and larger-sized PLA-MPs exerted minimal effects. This shift was associated with enhanced depolymerization-related enzymatic potential, accompanied by greater dissolved organic carbon (DOC) availability. The resulting increase in C availability stimulated central C metabolism, promoting microbial resource acquisition and biomass synthesis. To maintain microbial C:N homeostasis, microbial N assimilation was stimulated through ammonium (NH4+) assimilation mediated by the glutamate dehydrogenase (GDH) and glutamine synthetase-glutamate synthase (GS-GOGAT) pathways and nitrate (NO3⁻) assimilation via assimilatory nitrate reduction to ammonium (ANRA). Consistently, the 15N tracing model revealed that microbial assimilation rates of NH4+-N and NO3⁻-N increased by 10.5-fold and 12.7-fold, respectively, exceeding gross N mineralization rates, thereby depleting soil inorganic N pools and suppressing plant N uptake. Overall, our findings provide mechanistic insights into how PLA-MPs reshape soil functioning by reprogramming microbial communities and metabolism, thereby altering plant-microbe competition for N. These results highlight the potential risks of increasing biodegradable plastic inputs for cropland nutrient cycling and plant N acquisition.