Interactive effects of microplastics, heavy metals, and soil microecology under different irrigation water sources

Against the backdrop of increasing agricultural water scarcity, exploring the safe reuse of reclaimed water in farmland has become of great significance. In this study, maize fields within a river-irrigated area in Shanxi Province were selected as the research site. Four irrigation treatments were established: well water, river water, mixed water (river water: reclaimed water = 1:1), and reclaimed water. The microplastics, heavy metals, physicochemical properties, and microbial community composition of soil layers (0–80 cm) were analyzed. Results showed that irrigation water type significantly affected soil physicochemical characteristics ( p < 0.05). Under well-water irrigation, total nitrogen (0.98 mg/kg), organic matter (22.22 mg/kg), and available potassium (216.67 mg/kg) were the highest. River-water irrigation led to significant accumulation of available phosphorus (47.95 mg/kg), while reclaimed water irrigation caused marked enrichment of inorganic nitrogen (NO₃ − – N = 0.30 mg/kg). The mixed-water treatment maintained a balanced level of nitrogen, phosphorus, and potassium. Microplastic abundance was the highest under mixed-water irrigation (490 particles/kg dw), representing increases of approximately 57% and 44% compared to well and river water, respectively, and was significantly positively correlated with Pb and Cr ( r > 0.75, p < 0.01). Heavy metals were mainly enriched in the surface layer, with Pb and Cr concentrations of 6.0 and 35.1 mg/kg, respectively, under mixed-water irrigation. Microbial diversity was highest in the mixed-water treatment (Shannon = 7.11), accompanied by significantly enhanced sucrase (9.94 mg/g) and dehydrogenase activities (0.016 mg/g). Functional gene prediction revealed increased abundances of nitrogen-cycle-related genes (e.g., amoA , nirK ) under mixed and reclaimed water irrigation. Overall, mixed-water irrigation exhibited the most balanced performance in nutrient cycling, metabolic activity, and pollution control, providing a scientific basis for the safe and efficient agricultural reuse of reclaimed water. • Mixed irrigation improved soil fertility, nutrient balance, and stability. • Microplastics and organics jointly enhanced Pb and Cr immobilization. • Mixed irrigation increased enzyme activity and nitrogen cycle genes. • Microbial networks under mixed irrigation exhibited more redundancy and stability.