Niche based or neutral processes? Bacterial community assembly in soil aggregates during primary and secondary succession

Abiotic and biotic filters shape microbial phylogenetic diversity. Here, we evaluated bacterial phylogenetic diversity in microand macro-aggregates across primary and secondary soil successions to assess the relative influence of niche-based and neutral processes. We reanalyzed a large amplicon dataset from soil aggregates of contrasting textures (sandy and clay) collected in Belgium and the Czech Republic. Phylogenetic diversity, estimated from the standardized effect size of mean pairwise distance, consistently showed negative values, ranging from −3 to −7, indicating strong phylogenetic clustering. Higher diversity in macroaggregates compared with micro-aggregates was only observed in sandy soils under secondary succession. No aggregate-size differences were detected in primary succession soils. Over time, bacterial phylogenetic diversity decreased in sandy soils but increased in clay soils during secondary succession, while no temporal trend was observed in sandy soils under primary succession. Together, these findings demonstrate that bacterial community assembly in soil aggregates is primarily driven by nichebased processes, modulated by soil texture, succession stage, and ecosystem age.