Rhododendron maximum is a native evergreen shrub that has expanded in Appalachian forests following declines of Castanea dentata and Tsuga canadensis trees. Rhododendron is of concern to land managers and property owners because it suppresses hardwood establishment by limiting light and soil nutrient availability needed for tree seedling recruitment. We are testing Rhododendron canopy removal with and without organic soil (O-horizon) removal as a management strategy to promote forest recovery. We hypothesized that 1) Rhododendron canopy and O-horizon removal would increase soil inorganic nitrogen (N) and dissolved organic carbon (DOC), resulting in increased abundance of ammonia-oxidizing microorganisms and a shift towards a bacterially-dominated microbial community; and 2) increased N availability would increase microbial carbon demand, resulting in higher activities of extracellular enzymes associated with carbon acquisition. Rhododendron removal treatments were applied to 20 m x 20 m plots in a 2 x 2 factorial design, with two Rhododendron canopy removal levels (removed vs not) combined with two O-horizon removal levels (burned vs unburned), a total of four treatments. In April and July 2017, three 0 – 10cm soil cores were taken from each plot, cores were subsampled, and subsamples were composited by plot. Total extractable N, ammonium (NH4), nitrate (NO3) and DOC were measured following extractions with K2SO4, while microbial biomass was measured using chloroform extraction. Bacterial, fungal, and ammonia-oxidizing microorganism abundance were measured through quantitative PCR amplification of the 16s rRNA gene, ITS gene, and amoA gene, respectively. Activities of eight extracellular enzymes were measured using a fluorometric microplate method.
Results/Conclusions
Total extractable N, NH4, and NO3 increased with Rhododendron canopy + O-horizon removal, which is potentially related to variation in ammonia-oxidizer abundance. Extractable DOC and microbial biomass also increased with Rhododendron canopy + O-horizon removal. Contrary to our predictions, we did not see treatment effects on bacterial dominance, though microbial communities from all treatments shifted towards fungal dominance from spring to summer. Finally, we observed increases in carbon-acquisition enzymes involved in degrading cellulose (β-glucosidase) and hemicellulose (β-xylosidase) in the canopy + O-horizon removal plots relative to all other treatments. We did not find significant responses to Rhododendron canopy removal only. Our results show that Rhododendron + O-horizon removal stimulates microbial activity by increasing soil C and inorganic N availability and suggest that Rhododendron canopy combined with O-horizon removal will accelerate recovery of riparian forest structure and function by increasing N availability for tree seedling recruitment.