Effect of dispersal by inundation on soil bacterial communities depends on soil developmental stage

Dispersal is crucial for the dynamics and assembly of bacterial communities during ecological succession. However, we still lack the knowledge on this as the relative importance of dispersal is often not directly measured. Here, we performed a microcosm experiment to directly investigate the extent to which soil bacterial communities from different successional stages respond in terms of community difference to changes in inundation frequency. In the microcosm experiment, artificial application of natural sea water was used to simulate four levels of inundation frequency, i.e. 2x per day, 1x per day, 1x per 3 days, and 1x per 7 days. In addition, sterile sea water was used as a control to enable distinguishing between the influence of biotic (dispersal of microbial cells) and abiotic (environmental changes) factors in community responses. Soils collected from early (0-year) and late (70-year) successional stages were used, as these two successional stages that are naturally subjected to different inundation regimes on the island of Schiermonnikoog, the Netherlands. Soil bacterial communities were profiled by sequencing 16S rRNA genes at 6 time points in a treatment period of 20 days. Overall, our results showed that frequency of inundation did not affect the temporal turnover in bacterial species diversity and composition, for soil originating from both stages of succession. The effect of inundation was clearly observed in early stage soils, with sampling time being the most important factor driving the variation of bacterial communities. The higher temporal turnover of bacterial communities in soil from the early stage was mostly governed by changes in members of the rare biosphere. This suggests that low abundance bacteria in the early successional stage of salt marsh are sensitive to inundation and can be vulnerable to the changes of inundation, such as accelerated sea-level rise. Comparisons between inundation treatments using natural or sterile sea water suggested that the impact of inundation to soil bacterial communities happened mainly through changes of soil physicochemical conditions, but to a lesser extent through dispersal, although these differences were not significant. Taken together, our results highlight that bacterial communities in the early successional stage of salt marsh are sensitive to inundation and vulnerable to environmental change. Considering that changes in the structure of soil bacterial communities in response to inundation at early successional stages may directly impact the functioning of these soils as well as induce cascading, long-term effects on soil development, future conservation attempts should put more focus on early stages of soil succession.