Organic matter type and soil texture shape prokaryotic communities during early-stage soil structure formation

Background: Organic matter (OM) serves as substrate for heterotrophic microbial growth. Soil structure supports microbial life by providing various niches for colonization. Microorganisms in turn contribute to soil structure formation. Aims: We aim to understand how OM of different origin and soil texture affect prokaryotic community structure and the implications on early-stage soil structure formation. Methods: An artificial soil incubation experiment was conducted with different types of OM, including bacterial necromass and particulate organic matter (POM) of larger or smaller size (sPOM). The mineral composition was modified to obtain a clay loam, loam, and sandy loam texture. The abundance and composition of a natural microbial inoculum were determined after 30 days of incubation by real-time PCR and 16S rRNA gene sequencing, respectively. Results: The different OM types had a stronger effect on the prokaryotic community structure and abundance than texture. The necromass treatment supported the most distinct prokaryotic community with the highest abundance and lowest diversity, as well as the most intense formation of water-stable microaggregates in comparison to POM and sPOM treatments. Abundant bacterial taxa in all treatments are known to include extracellular polymeric substance producers, indicating that functional redundancy warrants aggregation by gluing agents. Texture-related effects were most consistent in the POM treatment, where larger prokaryotic populations were observed in the coarser-textured soils with fewer but larger soil pores and lower soil water content. Conclusions: Differences in prokaryotic community structure and abundance due to OM source indicate that aggregation is dependent on different ecological strategists, a POM-degrading population that promotes aggregation and contributes to necromass formation, and a necromass-degrading consortium in which bacteria play a major role.