Soil microbial communities in dry and moist tropical forests exhibit distinct shifts in community composition but not diversity with succession.

Soil microbial communities are integral to ecosystem function but our understanding of how they respond to secondary succession in fragmented landscapes is limited, particularly in tropical dry forests. We used DNA metabarcoding to evaluate successional changes in soil bacteria and fungi, comparing land managed for cattle, young, and older secondary forests at moist and dry sites in the Republic of Panama. We highlight key functional groups of microbes that interact with plants, including arbuscular mycorrhizal fungi (AMF), nitrogen-fixing bacteria, and plant pathogenic fungi. Plant diversity was higher at the moist site and increased with succession as the plant communities changed at both sites. By contrast, bacterial diversity was similar across sites and successional stages, and while overall fungal diversity was higher at the moist site, it also showed no changes with succession at either site. However, microbial community composition did change, with pastures and older forests having distinct bacterial and fungal communities and young secondary forests often displaying transitional ones. Functional groups of microbes showed contrasting patterns between sites, with the dry forest having a higher diversity of Nitrogen-fixing bacteria despite lower densities of legumes, higher diversity and different communities of AMF, and a much lower incidence of putative fungal plant pathogens than the moist site. Our findings highlight the importance of looking at aboveground and belowground effects together and demonstrate that predictions generated for soil microbes in moist tropical forests may not apply to dry forests. These results may also inform the restoration of climate-resilient forests. IMPORTANCE: Secondary forests are important components of neotropical landscapes and soil microbes help to shape these forests and the ecosystem services that they provide. This study demonstrates that soil microbial communities in moist and dry tropical forests can recover and reassemble after only 20 years of natural succession following the removal of cattle. However, successional patterns that are seen in the plant community are not always seen belowground. These patterns were more predictable at the moist than the dry site where the patchiness of the landscape likely restricts dispersal of both plants and soil microbes. We highlight the importance of preserving remaining tropical dry forests as they host unique microbial biodiversity that may help forests respond to drought conditions. As community shifts in soil microbes influence plant establishment, forest productivity, and other aspects of ecosystem functioning during the succession of tropical forest communities, our results can inform the restoration of climate-resilient forests.