Effects of digestate soil amendment on the fate of trace metals and on the soil microbial community

This study evaluated, over a 28-day soil column experiment, (i) the fate of trace metals (Zn, Cu, Pb) in a loamy sand soil after amendment with a non-source-separated municipal solid waste (OFMSW) digestate; (ii) the impact of the OFMSW digestate on soil microbial community; and (iii) the effects of two pharmaceuticals (metformin and lamotrigine) on trace metal fate and microbial community dynamics. Three conditions were tested: natural digestate, digestate spiked with metformin, and digestate spiked with lamotrigine. Soil samples were collected over time to measure trace metal concentrations and fractionation and characterize the soil prokaryotic community using Illumina next generation sequencing technology. Results showed Pb mobility factor (MF) increased over time by 182% in amended soil and 126% in the other soil layers, while Zn MF increased by 85% and decreased by 36%, respectively. Total metal concentrations were, nevertheless, low, pointing out that the impact of these metals on the soil in this study should be low. Cu MF remained constant (ca. 1.75%). Digestate amendment increased soil microbial diversity, with Shannon Index rising from 4.9 to 5.6, and shifted its composition over time, promoting a more diverse community. Initially dominated by Firmicutes, it stabilized by day 14 with Proteobacteria and Bacteroidota becoming dominant. Pharmaceuticals at environmentally relevant concentrations did not significantly affect metal behaviour or prokaryotic community structure. These findings suggest digestate may immobilize contaminants, making it a promising resource for soil improvement practices. However, preliminary treatment and monitoring are crucial for its safe application within circular bioeconomy strategies.