I would say not evidence but indication. If the microbiome of furtile soil is characterized and/or any marker community or pathway is already or in future will be identified, it could definitely be used as an indicator of soil fertility. Since soil fertility id directly proportional to the organic carbon, nitrogen etc so healthy soil will have not very different microbial community or dynamics.
Lets not overlook the contribution of soil microbes in functionality of OM. How do different components of OM operate in energy web of soil..?? OM contributes towards soil fertility via different soil microbial communities. ..