Hormetic responses of soil microbiota to exogenous Cd: A step toward linking community-level hormesis to ecological risk assessment.

We investigated hormetic responses of soil microbial communities to exogenous Cd by assessing microbial count, bacterial and fungal abundance, and microbial community diversity. We found that the bacterial count (BC) decreased (3-40%) by 0.2-40 mg Cdkg. Addition of 0.6-2.0 mgkg significantly increased fungal count (FC) by 7-42%, while addition of 4.0-40 mgkg Cd decreased FC by 29-51%, indicating a hormetic dose response. We also found that the FC/BC ratio increased by 0.6-2.0 mg Cdkg, with a maximum stimulation of 51%, and decreased (18-27%) by 4.0-40 mg Cdkg. Cd had no adverse effect on the α-diversity of bacterial or fungal communities. For relative abundances (RAs) of bacteria and fungi at phylum level, Bacteroidetes RA exhibited a biphasic dose-response curve, with an 18-24% increase at 0.6-4.0 mgkg and a 10% decrease at 40 mgkg compared with control. The results of FC, FC/BC, and Bacteroidetes RAs suggest that hormesis occurred at microbial community level, with positive effects occurring at 0.6-2.0 mgkg. This study can contribute to incorporating microbial community hormesis into the ecological risk assessments in the future.