Community structure and selected genes abundance shift of rhizosphere and endophyte bacteria from roots associated with the sludge application under reclaimed water irrigation

Reclaimed water and sewage sludge as renewable resources are urged to be used in agriculture, but their reuse poses potential chemical and microbiological risks. As a waste biomass resource, sludge combined with reclaimed water irrigation is a major way to develop and utilize renewable resources and control environmental pollution. However, whether reclaimed water irrigation and sludge application have adverse effects on the agricultural environment and human health remains a controversial point of discussion. A greenhouse pot experiment was conducted to investigate the effects of reclaimed water and sewage sludge on rhizosphere soil and root endophytic bacterial communities, the abundance of selected genes, and to evaluate the soil quality of sludge application. The results of this study demonstrated that the application of sewage sludge could result in the accumulation of nutrients in soil. The rhizosphere soil and root endophytic bacteria possessed common dominant groups at phylum level, including Proteobacteria, Bacteroidota, Actinobacteriota and Chloroflexi. Reclaimed water irrigation had less effect on rhizosphere soil and root endophytic bacterial communities than sludge application. Pseudomonas showed a decrease in relative abundance in both rhizosphere soil and root endophytes following sludge application, whereas beneficial bacteria like Bacillus, Stenotrophobacter, Cellvibrio and Altererythrobacter experienced an increase. Redundancy analysis (RDA) showed that the diversity and composition of bacterial communities in rhizosphere soil among treatment groups were closely related to soil organic matter, total nitrogen and total phosphate contents. The functional prediction highlighted the participation of functional groups in nitrogen and carbon cycling as well as degradation processes at varying sludge application rates. The abundance of selected genes was more affected by sludge application. A considerable amount of sludge application to the soil resulted in a significant increase in the abundance of pathogens and antibiotic resistance genes, as well as functional genes, compared to unamended soil with sludge treatment. Direct or excessive application of sludge might aggravate the dissemination and accumulation of deleterious genes in soil-crop systems irrigated with reclaimed water. Overall, our study results may provide valuable information on how sludge influences the microbial community characteristics and abundance of specific genes, guiding the assessment of biological quality and the appropriate use of sludge in agriculture irrigated with reclaimed water.