Impact of aggregate size and clay content on the bacterial community structure in soil microaggregates and macroaggregates of a clay catena

Aggregation of soil components is contributing to the formation ofdiverse niches for microbial life. Soil aggregates of different size are knownto promote that niche differentiation. Moreover, aggregates differ in physicaland chemical properties, therewith likely supporting biological diversity insoil. To evaluate whether different types of aggregates affect bacterialcolonization, we analyzed the impact of soil aggregate size in dependence of soilclay content on the bacterial community composition. A further distinction wasmade between microaggregates that occur as free structures in the soil matrix andthose that exist occluded within macroaggregates. Upon size fractionation ofsoil samples from a clay catena (19 – 35%), we analyzed community compositionin ten different aggregate size fractions by amplicon sequencing of the 16SrRNA gene. Both, the aggregate size fraction and the clay content showed aninfluence on the bacterial community, although this influence explained only aminor amount of the total observed variation (approx. 20%). The occurrence ofmicroaggregates as free or occluded structures did not contribute todiversification. Furthermore an increasing clay content led to more distinctcommunities in the individual aggregate size fractions, which we explain by an enhancedaggregate stability fostering the existence of more stable habitats over time,which support the development of more distinct bacterial communities. This isalso indicated by an increased occurrence of oligotrophic microbes in thesmallest and most stable aggregate fractions.