Can inoculation with living soil standardize microbial communities in soilless potting substrates?

- Major drivers were identified of microbial composition in soilless potting substrates.
- Both fungal and bacterial communities varied substantially between substrate batches.
- Proximity of roots, but not addition of living soil shaped the microbial communities.
- The profiles of active fungi (RNA) were dominated by asco- and basidiomycetous yeasts.
- Substrate components were the main determinants of the microbial communities.

Soilless plant cultivation substrates are commercially produced at large scales, but can show considerable variation in their performance in terms of sustaining plant growth and/or nutrition. This variation may be due to varying composition of microbial communities present in the substrates, mainly when composted organic materials are used as their components. Here we analyzed the portion of variability in composition of microbial (mainly the fungal) communities due to identity of substrate batches and compared it with variability due to the addition of a living soil (inoculation) or the presence of plant root system (i.e., the rhizosphere effect). The analysis was based on profiling total (DNA-based) and active (RNA-based) fungal and total (DNA-based) bacterial communities by using cultivation-independent molecular approaches. Contrary to expected effect of inoculation and rather limited variation across the substrate batches, identity of substrate batches in fact turned to explain the largest portion of biological variability, followed by the rhizosphere effect. The inoculation was completely ineffective as a factor affecting the indigenous microbial communities. These results indicate that the microbial communities in the soilless substrates are particularly resilient to plant- or inoculation-induced changes, but still highly variable between the individual production batches. Active fungal communities were dominated by yeasts recruiting either from Asco- or Basidiomycota. Due to phylogenetically and functionally similar but mutually exclusive dominants (Galactomyces and Candida) of the microbial communities in the different substrate batches without obvious correlation with their physico-chemical properties, we assume functional redundancy to play an important role in microbial community assembly within the substrates. Our results thus demonstrate as yet undescribed variation in microbial community composition with possible functional impact on plant performance in soilless substrates deserving further experimental attention.