Synergistic effect of microplastics and cadmium on microbial community and functional taxa in wheat rhizosphere soil

Microplastics and heavy metal contamination poses major threats to soil function and food security; however, their synergistic effects remain largely unclear. This study investigated the effects of single or combined addition of polyethylene (PE) microplastic (1% w/w) and cadmium (Cd; 1.5 and 5 mg kg−1) on functional microbial communities in the wheat rhizosphere soil. We observed that the biomass of wheat increased by 142.44% under high doses of Cd addition. The bacterial alpha diversity in wheat bulk soil reduced by 37.34%–37.83% with the combined addition of microplastic and Cd. The addition of microplastic reduced the relative abundance of Proteus involved in nitrogen fixation by 19.93%, while the relative abundance of Proteus and Actinobacteria involved in nitrogen cycling increased with the increase of Cd concentration, increasing by 27.96%–37.37% and 51.14%–55.04%, respectively. FAPROTAX analysis revealed that increasing Cd concentration promoted the abundance of functional bacterial communities involved in nitrification/denitrification and nitrate/nitrite respiration in rhizosphere soil. A FunGuild analysis showed that the synergy of PE-microplastics and Cd increased the abundance of saprophytic fungi, suggesting an enhanced degradation function. Our findings provide new knowledge on the effects of microplastics and heavy metals on soil microorganisms and functional microbial communities in agricultural soil.