Disentangling the effects of OM quality and soil texture on microbially mediated structure formation in artificial model soils

The interaction between mineral particles and soil organic matter (SOM) is an important and complex process during soil structure formation, in which the effects of soil texture and OM properties are intertwined. Within the SOM, there are residues of particulate organic matter (POM) of various sizes, as well as microbial necromass co-existing. These OM residues undergo microbial decay whose products stabilize particle connections and thus induce aggregate formation. We developed an experimental set-up to study early soil structure formation within a controlled lab environment. Artificial soil microcosms with a mineral mixture of different textures (clay loam, loam, and sandy loam) were used to perform a short-term incubation for 30 days under constant water tension. OM was added individually either as POM of two different size classes (milled hay litter, 0.63–2 mm and <63 µm, respectively) or bacterial necromass (Bacillus subtilis). The dry mixing process and incubation of the mineral mixtures led to particle–particle interactions and fine particle coatings of the sand grains as shown by a reduction in the specific surface area (N₂-BET). The OM residues were quickly accessed and degraded by microbes (peak in CO₂-release within the first 10 days of incubation), which induced the formation of water-stable aggregates. The POM of both sizes induced predominantly the formation of macroaggregates (0.63–30 mm) with a mass contribution of 72% to 91%, irrespective of the mixtures’ texture. The bacterial necromass induced a texture-dependent formation of macro- and microaggregates (63–200 µm), with larger aggregates in sand-rich mixtures. The different aggregate sizes were related to differences in the developed microbial community, as obtained by PLFA analysis. The bacterial necromass induced a microbial community dominated by bacteria, whereas the POM fostered a high relative abundance of fungi, whose hyphae could enmesh and stabilize large aggregates in all textures. The formed aggregates are water-stable but have a very low mechanical stability. Dry crushing with a mechanical loading frame revealed that very low forces (<4 N) were sufficient for breaking the aggregates down. Microbial growth and degradation of the OM residues led to OM patches occupying <17% of the mineral surfaces after the incubation, suggesting that the aggregates are loosely connected structures, bound together by some distinct spots of processed OM acting as gluing joints. This initially formed scaffold holds particles in place for further stabilization processes and resists immersion in water but exhibits no stability toward mechanical forces.