Polyethylene microplastics drive contrasting organic carbon pool–microbe–buckwheat response across acidic, neutral, and alkaline purple soils

Microplastics (MPs) are increasingly recognized as a novel carbon input in terrestrial ecosystems, yet their effects on soil organic carbon (SOC) pools across purple soils with different pH remain poorly understood. Here, acidic, neutral, and alkaline purple soils were collected from the Sichuan, China, and a pot experiment was conducted to investigate the responses of SOC fractions, microbial communities, and buckwheat growth to polyethylene (PE)-MPs. The addition of PE-MPs increased soil total organic carbon across all soils (p < 0.05), while its effects on labile pools differed by pH: microbial biomass carbon (MBC) increased in neutral, and alkaline soils, whereas particulate organic carbon (POC) increased in acidic soil (p < 0.05). However, microbial necromass (MNC) remained stable under PE-MPs addition in all soils (p > 0.05). PE-MPs inhibited buckwheat height in neutral but promoted the biomass in alkaline soil (p < 0.05). For the microbial communities, PE-MPs reduced bacterial diversity in acidic and neutral soils, but increased fungal richness in neutral and alkaline soils (p < 0.05). Gemmatimonas and Sphingomonas were significantly enriched by PE-MPs in neutral and alkaline soils, while Fusarium was reduced in alkaline soil under PE-MPs treatment. Mantel tests revealed stronger bacterial associations with SOC and MBC in neutral and alkaline soils, whereas fungal genera were primarily linked to MNC in neutral soil. Partial least squares path modeling suggested weaker associations between PE-induced carbon gains and microbial properties in acidic soil, but tighter linkages among SOC pools, microbial diversity, and buckwheat performance in neutral and alkaline soils.