Tord Ranheim Sveen

Understanding the processes that underpin the community assembly of bacteria is a key challenge in microbial ecology. We studied soil bacterial communities across a large‐scale successional gradient of managed and abandoned grasslands paired with mature forest sites to disentangle drivers of community turnover and assembly. Diversity partitioning and phylogenetic null‐modelling showed that bacterial communities in grasslands remain compositionally stable following abandonment and secondary succession but they differ markedly from fully afforested sites. Zeta diversity analyses revealed the persistence of core microbial taxa that both reflected and differed from whole‐scale community turnover patterns. Differences in soil pH and C:N were the main drivers of community turnover between paired grassland and forest sites and the variability of pH within successional stages was a key factor related to the relative dominance of deterministic assembly processes. Our results indicate that grassland microbiomes could be compositionally resilient to abandonment and secondary succession and that the major changes in microbial communities between grasslands and forests occur fairly late in the succession when trees have established as the dominant vegetation. We also show that core taxa may show contrasting responses to management and abandonment in grasslands.