Can I use the relative abundance of gene copies of my microbial samples (16S rRNA: for bacteria or ITS: for fungi) as a proxy for biomass?

My thought process (I know there are limitations):

Microbial Biomass of Taxon X = Relative Abundance of 16S rRNA (or ITS) Copies of Taxon X

Sum Biomass Across Taxa: Sum the biomass estimates for all taxa to obtain an estimate of total microbial biomass in my soil sample.

Any thoughts?

Thank you!