Scaling microbial ecology to peatland biogeochemistry

Background/Question/Methods

Microbial activities are central for modeling soil carbon (C) and nitrogen (N) cycling because microorganisms release extracellular enzymes to decompose soil organic matter (SOM) and produce bioavailable C and N. Such modeling is crucial for boreal wetlands because of the large C stocks present and their vulnerability to climatic shifts. However, scaling microbial traits, such as biomass or enzyme potentials, is challenging in boreal wetlands because of the spatial complexity induced by micro-topographic features.  Characterizing spatial variability in microbial traits can improve our ability to detect and accurately scale microbial mechanisms that influence carbon cycling. To quantify the spatial variation and functional characteristics related to microbial decomposition of peat organic matter, we measured microbial traits along a micro-topographic gradient at the Spruce and Peatland Responses Under Climatic and Environmental Change Experimental (SPRUCE) site in the Marcell National Forest, Minnesota, USA. Fresh peat cores were collected from hummocks and hollows from 0 to 20 cm below the hollow surface at the beginning (June) and the end (September) of the 2013 growing season. The bacterial and fungal community composition, microbial biomass and potential extracellular enzyme activities were assayed on cores at 10 cm increments.

Results/Conclusions

Depth and topography were central drivers of differences in microbial abundance and activity across the peatland.  At equivalent depths for hummocks and hollows, microbial biomass was the highest in the hollow surface (0 to10 cm depth). Micro-topographic variation created by hummocks and hollows differentially affected potential enzyme activity. Polypeptide-degrading enzymes were consistently greater in hollows than hummocks, whereas carbon cycling enzymes were sensitive to topographic variation in September but not June. Overall enzyme activity was greatest in hollows in September, and correlated with greater microbial biomass. Fungal biomass in hollows and hummocks was similar at the beginning of the growing season and decreased significantly in hummocks but not hollows at the end of the growing season. Changes in biomass and activity may be related to significantly drier conditions in September, when hollows were 3% drier than the hummocks at equivalent depth. Our results suggest that topography plays a critical role in regulating decomposition within peatlands. These results illustrate that integrating microbial attributes into ecosystem models will require specific understanding of the sensitivity of microbial parameters to local conditions prior to developing more generalized representations at the ecosystem scale.