Does physical fractionation of SOM pools preserve information about microbial taxa distribution and ecological functions?

In a previous study (Zaccone et al., Appl. Soil Ecol., 2018), a preliminary evaluation of potential ecological partition of total, bacterial and plant DNA across soil organic matter (SOM) fractions linked to conceptual stabilization mechanisms was provided. Here, we investigated if different ecologically meaningful SOM fractions share the same microbial communities. To test this hypothesis, DNA was recovered from SOM pools that differ in quality and level of physical and chemical protection from decomposition: free SOM located between aggregates (FR), SOM occluded within macro (MA) and microagreggates (MI), and mineral-associated SOM (MIN) (Plaza et al., Soil Biol. Biochem., 2013). Bacterial communities were then assessed by amplification of V3-V4 region of 16S rDNA while fungal communities by amplification of ITS region. NGS was performed by Illumina Miseq platform and the sequences analysis carried out using QIIME2 v2018.6.0. Bacterial communities showed a clear separation among the different SOM fractions, independently from the kind of the amendment applied to the soil (i.e., biochar, municipal compost). In particular, FR and MIN fractions were dominated by Firmicutes (65-75%), followed by Actinobacteria (~13%). On the opposite, MA and MI had a lower level of Firmicutes (~50%) and higher level of Actinobacteria (20-30%). Interestingly, among Bacillales, Bacillus and Brevibacillus spp. (aerobic) were largely dominant in FR and MIN, whereas the facultative anaerobic Limnochordaceae were more represented in MA and MI, and the anaerobic sporeforming Clostridium spp. Mainly in MI. These data clearly depose for a ‘selection’ of the microbial taxa according to the level of physical and chemical protection of SOM, with O2 availability as one of the main drivers. Although at a less extent, also fungal population was related to the SOM pools rather than to the amendment, with MIN and FR differing from MA and MI. In particular, Sordariomycetes are a fungal class that can represent a proxy of SOM evolution. In conclusion, (i) the physical fractionation method caused no mixing over of bacterial DNA through all SOM pools, and (ii) different microbial taxa seem to be specifically associated to SOM fractions linked to conceptual stabilization mechanisms.