Microbial taxa distribution in soil organic matter fractions with ecologically different functions

In a previous study (Zaccone et al., 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., 2012). 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 show a clear separation among the different SOM fractions, independently from the soil amendment (i.e., biochar, municipal compost). In particular, FR fractions were dominated by Firmicutes (70-80%), followed by Actinobacteria (10-20%) and Proteobacteria (5%). On the opposite, MA had a lower level of Firmicutes (ca. 40%) and higher level of Actinobacteria (30%) and Proteobacteria (20%). The dominance of spore forming taxa in all fractions is of interest and might be due to the better preservation of microbial DNA in the spore structure. Interestingly, the anaerobic spore-forming Clostridium spp. were mainly found in MA and MI and less in the MIN, whereas the most abundant taxa in FR were aerobic spore-formers (Bacillus spp., Brevibacillus spp.). 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 possible drivers. Although at a less extent, also fungal population was related to the SOM pool, with MIN and FR differing from MA and MI. In conclusion, different microbial taxa (mostly bacterial) seem to be specifically associated to SOM fractions linked to conceptual stabilization mechanisms.