Chronic atmospheric N deposition alters the composition and function of saprotrophic soil bacteria

Background/Question/Methods

Future rates of atmospheric nitrogen (N) deposition can slow organic matter decay by repressing activities of saprotrophic soil microorganisms, which can result in greater soil carbon (C) storage.  Furthermore, litter decay rates are correlated with initial ratios of C:N, lignin:N, and interactions among N, lignin, and cellulose. Thus, our understanding of how microbial communities respond to future rates of atmospheric N deposition has implications for the cycling and storage of C and N in soil.  The goal of this study was to determine how saprotrophic bacterial communities have responded to long-term experimental N deposition utilizing high-throughput sequencing and a functional gene microarray.  Since 1994, the Michigan Gradient Experiment has experimentally increased NO₃ deposition in replicate stands of northern hardwood forest across a 500-km climactic gradient in the Great Lakes region of North America, resulting in: i) reduced fungal laccase (lcc) expression, and altered actinomycete, basidiomycete, and Actinobacteria community composition, and ii) increased soil C storage and phenolic DOC production. Here, we address the questions: has the reduced physiological capacity of saprotrophic fungi opened a niche for less-efficient lignolytic bacteria to occupy? Or conversely, has organic matter accumulated due to an overall reduction in the metabolic activities of saproprophic fungi and bacteria? 

Results/Conclusions

Experimental N deposition caused a decline in bacterial abundance, as well as a shift in community composition.  This effect was observed in a suite of bacterial and fungal genes mediating the metabolism of starch, hemicellulose, cellulose, chitin, and lignin. Furthermore, bacterial genes responsible for ammonification, assimilatory and dissimilatory N reduction, denitrification, and N fixation exhibited a similar decline in abundance and community composition shift.  Across all gene assemblages, a decline in diversity (H’) and richness occurred in conjunction with greater assemblage heterogeneity (increased β-diversity) under chronic N deposition. Our results indicate that chronic experimental N deposition produced an overall reduction in saprotrophic bacteria, as well as those bacteria involved in the assimilation and cycling of N.  Together, the observed compositional changes in soil bacterial communities appear to be a part of a broad-scale suppression of soil microbial communities by levels of atmospheric N deposition expected in the near future.