as a general marker of bacterial activity (Couradeau paper).
A direct use of absolute abundances of microbial cells to "standardize" sequencing data has not been reported to the best of our knowlegde as one would have to assume that all microbial groups (regardless of the taxonomic level) show a consistent abundance in all soils, a prerequisite that is not reflected very well (REFs metagenomics, qPCR?).
FISH approaches are helpful in estimating actual abundances of taxa of interest as identified by sequencing. Example CARD-FISH Piwosz, shorten.
In order to obtain better estimates of group-specific cell numbers, Catalyzed Reporter Deposition coupled to Fluorescence In Situ Hybridization (CARD-FISH) provides the opportunity to stain and count target soil microorganisms of interest on a phylum but also species level (Eickhorst and Tippkötter, 2008; Schmidt and Eickhorst, 2013). Recently, Piwosz et al. \cite{Piwosz2020} used CARD-FISH to count the abundances of bacterial taxa in aquatic samples and to compare these to relative abundances generated by amplicon sequencing. The authors concluded that relative abundance data obtained through amplicon sequencing was robust enough for ecological interpretation on a community level. For specific taxonomic groups, however, the correlation of abundances obtained with both techniques disagreed in large parts, suggesting that care has to be taken when interpreting relative abundance data of single taxa derived from amplicon sequencing. Such technical comparisons for soil samples are rare (e.g. \cite{Ushio_2014}) but given the diversity of soil microbiomes we suggest to use amplicon sequencing data for screenings of soil microbiomes on a community scale (e.g. phylum/class/order level). If certain phylogenetic groups are of interest we suggest to follow up their quantification with direct microscopic counts that are not underlying PCR-bias.