Diagnostic strategy for the combined effects of microplastics and potentially toxic elements on microbial communities in catchment scale.

Combined effects of potentially toxic materials (PTMs) released from production activities on microbial communities in environmental flimsy area are poorly recognised. Microplastics (MPs) and potentially toxic elements (PTEs) were investigated in soils and river sediments in a headwater catchment from the Qinghai-Tibet Plateau. Their co-effects on microbial communities and the controlling factors affecting communities were further explored. Results showed that MPs and PTEs significantly accumulated in soils and sediments. Among which fragment-shaped MPs and copper (Cu) dominated, with mean contents of 1.11 × 10 and 1.81 × 10 items kg and 13.80 and 7.33 mg kg in soils and sediments, respectively. Distribution index (0.54) suggested that fiber-shaped MPs preferred to transport into rivers and deposited in sediments. The film mulching contributed significantly to the occurrence of fragment-shaped MPs, while Cu may be derived from industrial wastewater. The antagonistic effect between fiber-shaped MPs and zinc (Zn) on soil microbial structure was found based on their obtuse angle in canonical correlation analysis. While the synergistic effect between total phosphorus (TP) and Cu on diversity was detected by interaction detector model (q(TP ∩ Cu) >q(TP) >q(Cu), p < 0.05). Soil TP and Cu were identified as controlling factors influencing diversity through random forest model and factor detector (q(TP) = 0.49, q(Cu) = 0.36, p < 0.05), which may be related to direct nutrient supply and microbial resistance, respectively. The negative effects of MPs on structure might be counteracted by increasing Zn content, while the co-existence of TP and Cu further increased diversity. A diagnostic framework, which involves background data collection, sampling analysis, characterisation and relationship investigation, was proposed to explore the co-effects of complex pollution and factors on communities. This study may provide strategies to mitigate the negative effects on microorganisms in the environment.