Linking microbial communities, functional genes and nitrogen-cycling processes in forest floors under four tree species

Tree species can influence rates of soil N transformations, but the question remains whether differences in N cycling rates are mirrored by the abundance of relevant functional genes. We studied whether the influence of tree species on soil N transformation processes and abundance of functional genes exist across two sites in British Columbia with different N availability. We used the 15N pool-dilution method to estimate gross rates of ammonification and nitrification in forest floors of four conifers in a common garden experiment. The abundances of bacteria, fungi, nitrification (AOA amoA, AOB amoA) and denitrification (nirS, nirK) genes were determined by qPCR. Western red cedar (Thuja plicata) had the highest rates of gross ammonification and NH4+ consumption, followed by Sitka spruce (Picea sitchensis), hemlock (Tsuga heterophylla), and Douglas-fir (Pseudotsuga menziesii); all species showed net nitrate immobilization. Western red cedar forest floors had the greatest abundance of bacterial 16S genes and ammonia-oxidizing archaea amoA genes. This suggests that tree species foster different abundances of ammonification and denitrification functional groups. Differences in N transformation rates between the sites were related to site N status, as reflected in C:N ratios of the forest floor and microbial biomass, and were more closely tied to rates of N consumption rather than gross mineralization. Rates of most N transformation processes were related to microbial C:N ratio, indicating that the N status of microbes rather than their biomass or activity level determined the rates of N cycling. Ammonification rates were associated with forest floor and microbial biomass C:N ratio as well as bacterial and fungal abundances. Nitrification rates and denitrification gene abundance were associated with microbial biomass C:N ratios and AOA amoA gene abundance. The forest floor's genetic potential for denitrification was positively correlated with its nitrification potential as indicated by ammonia-oxidizer abundance. We conclude that tree species influenced forest floor N cycling and soil microbial gene abundances, and that functional genetics can be useful for exploring mechanistic links between tree species and nitrogen cycling processes.