Microbial community succession and biogeochemical cycling: A comparison across glacial forelands

Background/Question/Methods

Models of primary succession often describe the directional change in plant community structure and function following initial plant colonization. However, a growing body of work from recently deglaciated sites has demonstrated microorganisms - not plants - are biological pioneers at the earliest stages of ecosystem development. These studies emphasize the importance of early microbial communities in setting the stage for plant colonization by fixing nitrogen, adding organic matter, and mineralizing nutrients. Thus, recent efforts have focused on incorporating microbial communities into the study of succession, with particular emphasis on community structure and ecosystem function relationships. These observations have led us to ask the following question: Are there generalizable patterns in the development of microbial community function and biogeochemical cycles during early ecosystem succession? To address this question, we used a suite of biogeochemical tools, including pyrolysis GC/MS, to track changes in soil nutrient pools and soil organic matter (SOM) chemistry and measured enzyme activities to assess microbial function over ecosystem succession. We hypothesized that early ecosystem development follows a predictable biogeochemical trajectory for soil nutrients and carbon resources. Additionally, we anticipated that despite known phylogenetic differences between microbial communities, function develops in a predictable manner in response to shifting resources.

Results/Conclusions

Across the three study sites, C and nutrient concentrations in soils (NH₄⁺-N, NO₃^--N, total N, organic C) increased with time-since-exposure. In addition, results indicate that as succession proceeds, soil carbon resources change in a predictable way. For example, we found that SOM becomes increasingly diverse with time-since-deglaciation. Along the same gradient, the activity of extracellular enzymes used in the biological acquisition of C, N, and P increased for all sites. This was particularly true in later stages of succession where plants were more abundant, though relationships were site specific. Our previous investigations have revealed that microbial C utilization follows consistent patterns across these same geographically and climatically distinct sites. Taken together, our findings point to the interrelatedness between the biogeochemical trajectory of developing soils and the succession of microbial communities. Moreover, our data support the development of a conceptual model to describe patterns of element accumulation, shifting resources, and microbial activity along primary successional gradients. Finally, our results reinforce previous suggestions that initial increases in microbial community functional diversity are consistent and biogeochemically important in early ecosystems prior to plant establishment.