Microbial communities in soil macro-aggregates respire less, are more diverse and stable across successional and geographical gradients

The stability of soil organic carbon is empirically believed to relate to the location of soil microorganisms inside or between aggregates. However, there are knowledge gaps about how micro-niches shape the microbial community composition and activity and how these effects vary between various soils. Here, we investigate fungal and bacterial community structures (composition and biomass), networks, and respiration in individual micro-niches between and inside soil aggregates using seven different chronosequences (both primary and secondary successions covering sites from pioneer stages to well-developed ecosystems) from a maritime climate in Belgium to a more continental climate in Hungary. We show that while the sampling site is the most crucial factor in shaping microbial community structures, soil aggregates are often more important than succession age and vegetation in differentiating major microbial taxa. Soil fractions are also the dominant factor affecting microbial biomass along the individual chronosequences. Specifically, macro-aggregates often have more variable α-diversities and high microbial community stability, accompanied by low microbial respiration rates. Although the other isolated soil fractions have similar microbial diversities as macro-aggregates, they feature unstable microbial communities with a higher respiration rate. The isolated primary particles have more stable bacterial communities in secondary than primary successions. We, thus, provide a mechanism for interpreting the links between soil microsite heterogeneity, microbial community stability, and microbial respiration.