Polyethylene microplastics enhanced the toxicity of diisobutyl phthalate in saline soil microorganisms

The widespread use of diisobutyl phthalate (DiBP) and plastic products in agriculture has led to new characteristics of soil pollution in which DiBP coexists with microplastics, but their combined effects on soil ecosystems, especially saline soil microorganisms, are unclear. This study evaluated DiBP (1 and 10 mg/kg) and polyethylene microplastics (PE-MPs, 0.25w/w) on saline soil microorganisms' effects (60 days) from bacterial and fungal community structure and nitrogen-carbon cycle. DiBP exposure changed bacterial and fungal community structures, and the changes were more significant after increasing DiBP concentration or adding PE-MPs. Nitrospira, MND1, and Lecanicillium abundance decreased, while Sphingomonas and Mortierella abundance increased. Aromatic compound and hydrocarbon degradation functions were activated (61.47 %-177.96 %), while nitrate reduction was inhibited (0.16 %-14.18 %). The increase in DiBP concentration inhibited the nitrogen cycle by reducing denitrification genes nirK and nirS abundance, and promoted the carbon cycle by enhancing β-glucosidase activity. The combined exposure of DiBP and PE-MPs inhibited the carbon cycle by reducing carbon fixation genes cbbLG and cbbLR abundance, and promoted the nitrogen cycle by enhancing urease activity. DiBP and DiBP + PE-MPs exposure activated enzyme activity through hydrophobic forces. Prolonged exposure had a significant impact on dehydrogenase activity and cbbLR gene abundance, while dehydrogenase activity may serve as an early warning indicator of DiBP and DiBP + PE-MPs effect to soil microorganisms. DiBP toxicity to microorganisms increased with increasing concentration (8.06 %-17.78 %). PE-MPs increased DiBP toxicity to microorganisms (8.27 %-22.56 %). This study suggests that combined contamination of DiBP and MPs requires particular attention in saline agricultural soils.