Suppression of N cycle functional genes in response to chronic atmospheric N deposition

Background/Question/Methods

Future rates of atmospheric nitrogen (N) deposition can slow organic matter decay and alter microbial community composition and function.  However, our understanding of how anthropogenic N enrichment may alter the physiological mechanisms by which soil microorganisms assimilate and cycle N in soil are largely unknown.  Since 1994, the Michigan Gradient Experiment (MGE) has experimentally increased NO₃ deposition in replicate stands of northern hardwood forest across a 500-km climactic gradient spanning the north-south geographic range of the sugar maple (Acer saccharum) dominated forests in the Great Lakes region of North America.  Previous work has demonstrated the effects of elevated N on soil carbon (C), and N dynamics: N has caused an increase in soil C storage and DOC production, despite no change in above-ground litter production, as well as increased leaching of inorganic and total N and increased leaf N concentrations.  Experimental N deposition has also altered community composition of saprotrophic fungal and actinobacterial communities.  The goal of this study was to examine how functional genes encoding N-cycle processes in soil microbial communities have responded to experimental atmospheric N deposition in the MGE using the GeoChip 4.0 microarray. 

Results/Conclusions

Experimental N deposition caused a decrease in overall abundance (signal intensity (SI)), and functional richness (proportion of probes detected) of archaeal and bacterial genes responsible for N fixation (nifH), ammonification (gdh, ureC), denitrification (narG, nirK, norB, nosZ) and assimilatory nitrate reduction (nasA, NiR, nirA), with additional bacterial genes responsible for nitrification (hao), and dissimilatory nitrate reduction (napA, nrfA).  Observed decreases in abundance and richness in response to experimental N deposition occurred uniformly across each N cycle component.  Mean annual temperature and A. saccharum biomass composed a significant amount of the total variation for bacterial and archaeal gene abundances, with pH being an additional significant predictor only for Bacteria.  These data suggest microbial community function may be repressed by experimental N deposition, and that this suppression is not driven by a disproportionate decrease of the aforementioned transformations within the N-cycle.  Taken together with our observed decline in functional richness, it appears experimental N deposition may fundamentally alter the physiological potential of soil microbial communities, and that bacterial and archaeal communities may respond differently to this environmental change.