Simulated Atmospheric Nitrogen Deposition Alters Actinobacterial Community Composition in Forest Soils

Anthropogenic N deposition can slow the decay of plant detritus, leading to an accumulation of soil organic matter and the production of phenolic dissolved organic C (DOG), which can leach from soil to ground and surface waters. Actinobacteria are one of the few groups of saprotrophic microorganisms that oxidatively depolymerize lignin, producing substantial soluble polyphenolics in the process. In combination, these observations present the possibility that lignolytic Actinobacteria may become more important agents of lignin decay as atmospheric N deposition continues to increase during the next decade. To test this idea, we quantified actinobacterial abundance and community composition in a well-replicated field study in which atmospheric N deposition has been experimentally increased since 1994. Actinobacterial abundance was assessed using quantitative polymerase chain reaction of 16S rRNA and community composition was evaluated using clone libraries and phylogenetic community analyses (i.e., LIBSHUFF and UniFrac). Contrary to our expectation, experimental atmospheric N deposition had no effect on actinobacterial abundance in the forest floor (similar to 10(10) gene copies kg(-1)); however, it significantly decreased actinobacterial abundance by 47% and total DNA by 31% in surface soil. Our analyses revealed that experimental N deposition further elicited a significant membership change in forest floor and surface soil communities, as well as significant differences in the phylogenetic diversity of forest floor Actinobacteria. This shift in community composition occurred in concert with a slowing of plant litter decay, accumulation of soil organic matter, and a greater production of phenolic DOG. These observations are consistent with the idea that changes in actinobacterial community composition may underlie biogeochemical responses to experimental N deposition.