Succession of soil microbial communities and enzyme activities in artificial soils

Soil microorganisms are frequently attached to mineral surfaces or organo-mineral complexes, yet little is known about the microbial colonization of different soil minerals. The use of artificial soils that differ only in their mineral composition (illite, montmorillonite, ferrihydrite, boehmite) and the presence of charcoal, but not in soil texture and organic composition, offered a unique opportunity to study composition, function and succession of soil microorganisms colonizing newly exposed organo-mineral surfaces. Artificial soils were incubated with a microbial inoculum from an arable topsoil at constant temperature (20 °C) and moisture conditions for up to 18 months. The succession of enzyme activities involved in C-, N- and P-cycling gave clear evidence that nutrient limitation drove microbial community structure during the incubation independent of mineral composition. Discriminant analyses of principal components of PLFAs showed that microbial community structure changed over a period of 18 months toward similar communities for all artificial soils at the end of incubation. This was supported by a shift in the soil microbial community from dominance of specific phyla like Betaproteobacteria, which is often referred to as copiotrophic organisms, during the first 6 months of the incubation, toward systems with a higher dominance of e.g. Acidobacteria, which are suggested to follow the oligotrophic life-strategy. The effect of mineral surface properties on enzyme activities was pronounced during the first 6 months of incubation. Microbial colonization and succession on mineral surfaces was likely affected by mineral properties such as surface charge and, at the end of incubation, availability of beneficial nutrients. Charcoal affected the microbial community only during the first 6 months of incubation with slightly increased colonization by bacteria which are often described as oligotrophic organisms. In contrast, illite and montmorillonite probably provided nutrient rich environments with montmorillonite supplying more exchangeable cations. The artificial soils experiment clearly showed that changes in substrate availability as well as mineral properties are important drivers for the development of microbial communities.