Tree species effects on nitrogen cycling processes and functional communities in soil at a common garden experiment

Background/Question/Methods

The understanding of how nitrogen (N) is cycled in forests is important for improving models of global biogeochemical cycles, as forests can serve as important sinks and sources of greenhouse gases and tree species may influence N cycling pathways and microbial communities. To determine the role of tree species identity on N cycling we used two sites (one N rich the other N poor) within a common garden experimental forest on Vancouver Island, British Columbia, Canada. We used the ¹⁵N pool dilution method to ascertain gross and net rates of N mineralization and nitrification in soils taken from pure stands of four tree species (western red cedar, western hemlock, Douglas-fir, and Sitka spruce). We also measured soil microbial biomass carbon (C) and N to determine if the microbial pool was storing the ¹⁵N by the end of the pool-dilution experiment. We used qPCR to determine the abundance of total bacterial and fungal communities associated with different tree species and also the abundance of functional genes associated with nitrification (AOA amoA, AOB amoA) and denitrification (nirS, nirK) in the soil communities. 

Results/Conclusions

Tree species identity influenced soil communities among species grown on the same site for AOA amoA and nirS, but not for AOB amoA and nirK. These results identify that tree species foster different abundances of nitrification and denitrification functional groups, and the pools and fluxes of N those communities regulate. Site strongly influenced nitrifying microbial communities with differences observed between the two sites and among the same tree species for AOA amoA, but not with AOB amoA. Denitrifying communities differed within a site, for nirS for cedar and spruce, but not for fir or hemlock, where nirK did not differ among the tree species, or between the two sites. This is consistent with previous studies of N mineralization at our study sites, which suggested that microclimate features may be responsible for the strong differences in mineralization rates between sites.