Tree species and nitrogen additions alter forest floor microbial communities and extracellular enzyme activities

Background/Question/Methods

Forest nitrogen (N) retention and soil carbon (C) storage are influenced by tree species and their associated soil microbial communities. As global change factors (e.g. pollution, climate, pathogens, invasive species) alter forest composition, predicting long-term C and N dynamics will require understanding microbial community structure and function at the tree species level. Because atmospheric N deposition is increasing N inputs to forested ecosystems in the northeastern US, it is also important to understand how microbial communities respond to added N.  Based on prior studies, we hypothesized that N additions stimulate extracellular enzyme activities in relatively labile litters, but suppress oxidative enzyme activities in recalcitrant litters. We measured enzyme activities and microbial community composition (using phospholipid fatty acid analysis) of the forest floor in single-species plots dominated by sugar maple (Acer saccharum), yellow birch (Betula alleghaniensis), red oak (Quercus rubra), American beech (Fagus grandifolia) and eastern hemlock (Tsuga canadensis) in the Catskill Mountains, NY, species whose litters range from labile to recalcitrant. Half the plots were fertilized with N by adding NH₄NO₃ (50 kg ha^-1 yr^-1) from 1997 to 2009. Non-metric multidimensional scaling (NMS) and multi-response permutation procedures (MRPP) were used to examine microbial community structure and relationship to enzyme activities.

Results/Conclusions

We found that the soils under different tree species have generally distinct microbial communities and enzyme activity patterns. For example, microbial communities of sugar maple were significantly different (p < 0.003) from those of beech (MRPP effect size A=0.15), hemlock (A=0.24), and oak (A=0.09).  In response to N additions, both microbial community composition and enzyme activities changed; however changes were tree species-specific and the direction of these changes was sometimes surprising.  For example, in contrast to other studies, we found that N additions caused an overall increase in fungal biomass (F=6.0, p=0.03) that was strongest for yellow birch (24% increase) and weakest for sugar maple (1% increase). As with other studies, we found that N additions increased β-glucosidase activity for maple, birch and oak plots. Unexpectedly, N additions reduced hydrolytic enzyme activities in hemlock plots and reduced oxidative enzyme activity in birch plots, a species with relatively labile litter. These unexpected responses suggest that our understanding of the interactions between microbial community composition, enzyme activity, substrate chemistry, and nutrient availability is incomplete. Enhancing our understanding of microbial responses to added N in single and mixed-species substrates may be needed for accurate predictions of future soil C and N dynamics.