4.2. Experimental approaches to address compositionality

The challenge of interpreting relative abundance data as illustrated in Figure 3 indicates the advantages of adding quantitative information to current amplicon sequencing approaches. Knowledge on absolute values (e.g., total microbial biomass) can help to make more robust inferences about the nature of observed shifts in microbial community structure (Fig. 3, t2d and t2e; \citep{Barlow_2020,Wang_2021}.  In the following, we discuss some approaches ranging from molecular techniques to classic soil microbiology that could help improve our interpretation of amplicon sequencing data.

4.2.1. Quantitative PCR approaches 

One relatively affordable and well-established quantitative method is quantitative real-time PCR (qPCR). qPCR enables to assess copy numbers of a marker gene which may be multiplied by the relative abundance data of the same sample obtained by amplicon sequencing. This approach benefits strongly from using the same primers in both qPCR and sequencing to reduce bias stemming from PCR (see section 2) and from correcting for the copy numbers of said marker gene in the genome of target organisms.