Bacterial Communities Are Stable Across Seasons but Dynamic with Soil and Ecosystem Development.

Organism succession during ecosystem development has been relatively well studied for aboveground communities while the associated pattern of change  belowground in microbial communities remains largely unknown. We initiated a study along a developmental sand-dune chronosequence bordering northern Lake Michigan at Wilderness State Park with the hypothesis that the soil bacterial communities will follow a pattern of change that is associated with soil, plant, and ecosystem development. This study site included 5 replicate sites along 9 dunes ranging in age from 95 to 4000 years since deposition.  The microbial composition and diversity in the soil was studied using bacterial tag-encoded FLX amplicon pyrosequencing of the 16S rRNA gene. As hypothesized Bray-Curtis ordination indicated that bacterial community assembly changed along the developmental gradient. However, the changes were not affected by seasonal differences, despite likely differences in plant root C (e.g. exudates), temperature, and water availability. Soil Ca, Mg levels and pH showed a significant log-linear correlation with the soil development (r ~ 0.83, 0.84 and 0.81).  Bacterial diversity represented by Simpson’s reciprocal index (Simpson’s 1/D) showed a steady decline from youngest to the oldest dunes with largest decline (212 to 58) during the initial stages of soil development (95 to 440 years). The change in plant species abundance was higher in the youngest sites than the older sites. This change was significantly correlated with the change microbial community distribution (p<0.000001; r=0.56). Overall change during soil and ecosystem development occurred quickly and then was consistent when soil communities matured (~800 to 4000 y). The results suggest that gradients in soil pH, soil nutrients and plant community could be a strong driving force in shaping bacterial community assembly across a developing soil ecosystem. Implications of a stable community hypothesis in mature soils have have potential ramifications for planetary estimates of biodiversity, plant-bacterial interactions, and biogeochemical functioning.