Changes of the soil ecosystem along a receding glacier: Testing the correlation between environmental factors and bacterial community structure

As a glacier retreats, it leaves behind it a forefield that has a natural gradient of soil formation age. We systematically sampled the Damma glacier forefield (Switzerland) along a temporal gradient of soils deglaciated between 1956 and 2002. A significant change in organic carbon content, sulfate concentration, pH, water and nucleic acid content was observed along the forefield chronosequence. Based on 16S rRNA gene based fingerprinting, the structure of the bacterial community also shifted along the forefield. Shifts in the structure of the bacterial community were significantly correlated to changes of pH, soil water content and soil age. To test the impacts of an array of environmental variables including soil age, soil water content, and different anions on the structure of the bacterial community we incubated different glacier forefield soils under specific treatment conditions for 30 days at 25 °C. The incubation of recently deglaciated soils resulted into an increase of bacteria from the family Methylocystaceae and from the class Betaproteobacteria while the community composition from matured soil changed to a lesser extent. The total nitrogen concentration in matured soil doubled during incubation, whereas the nitrogen concentrations in recently deglaciated soil stayed constant. This suggested that the microbial ecosystem functioned differently in the mature versus the recently deglaciated soils. Only soil age and soil water content could be singled out as having significant effects on the structure and composition of the bacterial community, despite the fact that bacterial communities in glacier forefields are exposed to other steep environmental gradients.