Linking phytoavailability of heavy metals with microbial community dynamics during municipal sludge composting

Abstract

Municipal sludge compost contains residual heavy metals (HMs), which could limit the utilization of compost as a resource. Studies have revealed that the availabilities of HMs in composting system are affected by microbial-driven organic matter and physicochemical indices. However, how the microbial community dynamics affect the availability of HMs during municipal sludge composting remains unclear. In this study, high-throughput sequencing technology was applied to investigate the response relationship between microbial dynamics and different availability of HMs. A total of 31 main genera within 5 phyla were identified. The concentrations of the most available HMs increased, and the increasing rates varied from 102% to 240%. Proteobacteria, Firmicutes, and Chloroflexi could affect organic matter to control the phytoavailability of Cr and Pb, whereas Proteobacteria, Bacteroidetes, and Actinobacteria could affect the phytoavailability of Cu and Zn. The bioaccessible HMs are only affected by organic functional groups, whereas the leachable HMs are not affected by organic matter or microorganisms. The “physicochemical index-microorganisms-available heavy metal” response pathway and principal component analysis showed that the increase in total C and N and the decreasing content of NO3−–N reduce the phytoavailable HMs. We finally proposed a regulation method to improve the resource utilization of municipal sludge composting and provide theoretical support for the harmlessness of municipal sludge composting.