From Forests to Fens: Linking the Nitrogen Cycle to Microorganisms in Multiple Boreal Ecosystems

Background/Question/Methods

The composition and functioning of boreal ecosystems are vulnerable to changes in climate, including such effects as longer growing periods, changes in the fire regime, drier soils, and changes in the moisture regime brought about by permafrost thaw. To assess the influence of vegetation and soil moisture on nitrogen cycling in boreal ecosystems, we sampled surface soils along an elevation-driven hydrologic gradient that corresponds with five plant communities typical of interior Alaska. The dominant organisms in the five communities, from the driest upland site to the lowland fen, were 1) black spruce, 2) willow and birch, 3) tussock grasses, 4) Equisetum and Carex, and 5) Drepanocladus mosses. We examined the chemical composition of the surface organic moss and soil, measured gross nitrification and N-mineralization, and potential rates of nitrification and denitrification from soil cores collected in mid summer. We used quantitative PCR to assess the abundance of ammonia oxidizers, dentrifiers, and nitrous oxide producers using a functional gene approach. Results/Conclusions Rates of N cycling increased dramatically with increasing soil moisture along the gradient, from 2 μg/g soil/d in the willow/birch site to very high rates (~ 20 μg/g soil/d) in the Equisetum and Carex dominated fen. Rates of N cycling were linearly related to abundance of organisms responsible for the process (e.g. denitrifiers and denitrification). In addition, N cycling process rates were better explained by organism abundance than by other biophysical data (e.g. soil moisture, soil temperature, soil C, soil N). This study demonstrates that rates of N cycling are strongly affected by the moisture regime, and qPCR methodologies are a strong tool for understanding linkages between microbial populations and biogeochemical process rates. Quantitative analysis of different microbial functional groups across landscapes could be established as a sensitive indicator of changes in nitrogen processing.