Microbial resistance in rhizosphere hotspots under biodegradable and non-degradable microplastic amendment: Community and functional sensitivity

The annual global production of plastics is currently nearly 400 million tons, leading to widespread concern regarding the quantity of degradation-resistant plastics entering terrestrial environments. Farmland soils are a major sink for microplastics (MPs, size <500 μm) owing to the wide use of plastic film mulching. Although the influence of MPs on soil parameters has been investigated, the response of microbiomes to soil microenvironments with contrasting limiting factors, particularly in flooded soil environments such as rice paddies, remains unknown. Using zymography and high-throughput sequencing, we conducted an experiment with polylactide (PLA) and polyvinyl chloride (PVC) MPs to compare the effects of biodegradable and conventional MPs on rice growth, exoenzyme kinetics, and microbial communities. Both conventional and biodegradable MPs significantly inhibited rice growth, possibly by affecting nutrition. Compared with the control soils, both PLA- and PVC-amended soils exhibited higher enzyme activity in the hotspots. The enzymatic resistance to MPs was higher in ‘coldpots’ with PVC addition compared to that in PLA and control treatments. 
Bacterial biomass increased but diversity declined in PLA-amended soils, possibly because PLA particles act as carbon input inhabited the population of bacteria. Our findings suggest that co-occurrence networks among bacteria were strengthened by the addition of both MPs, with an increase in microbial functionality resilience and enhanced competition with neighboring roots for nutrient mining. This competition for nutrients may 
adversely affect plant growth.