Neutral assembly processes and high inter-annual turnover shape soil microbial communities in Mediterranean watersheds.. Koiliaris

In recent years there is a growing interest to improve our understanding on the ecological processes governing the assembly of soil microbial communities. Pertinent information is expected to provide valuable insights on ecosystem functioning and modeling, the system’s response to environmental shifts, and on ecosystem services. Here, using the Sloan neutral community model (SNCM), we provide support that neutral processes, i.e., homogenizing dispersal, govern the assembly of bacterial communities in a Mediterranean watershed, and that their contribution varies over the years. In specific, we found a decrease (from 14% to 3%) in the effect of soil variables during a 3-year period and we showed that this trend was independent of land use (natural ecosystems vs agricultural land) or soil depth. Despite their high inter-annual turnover, the sub-communities of the SNCM showed phylogenetic clustering implying common assembly mechanisms. Estimates βNTI indicated that selective processes dominated the assembly of the more-abundant community, but stochastic processes dominated the less-abundant. Network analysis of the sub-communities revealed limited non-random co-occurrence patterns that might suggest antagonistic, rather than mutualistic, interactions between equally fit microbial taxa. Furthermore, the increase in the relative contribution of neutral processes was accompanied by a deterioration of community network properties indicating potential impacts on ecosystems functioning and management. Our study sets a paradigm of a large-scale microbial metacommunity that is consistently assembled by stochastic mechanisms, and of the prevalent role of homogenizing dispersal in this system.