Influences of Hemlock Woolly Adelgid-Induced Stand-Level Mortality on Nitrogen Cycling and Stream Water Nitrogen Concentrations in Southern Pennsylvania

Hemlock woolly adelgid, Adelges tsugae, is spreading throughout the northeastern United States, causing large-scale dieback of the eastern hemlock, Tsuga canadensis. This study examined soil nitrogen transformations and stream water nitrogen content at five hemlock-dominated stands in southern Pennsylvania, spanning a spectrum of hemlock mortality, from no mortality to total A. tsugae-induced mortality with subsequent regrowth. Organic content, extractable nitrate, and net nitrification and mineralization rates, as well as nitrate and ammonium movement through the soil were significantly higher at sites with high mortality. Our results suggest that stand-level hemlock mortality is leading to increased nitrogen inputs to stream water. Stream water nitrate concentrations were lowest at the site with little to no mortality (0.035 mg/L NO3-) and highest at the site with intermediate mortality (0.69 mg/L NO3-). In the sites with the longest time since infestation and highest mineralization and nitrification rates, regenerative seedling growth (Betula spp. in particular) is leading to increased uptake of nitrate, likely reducing nitrate leaching rates once substantial regeneration occurs. However, for a period of several years after infestation, hemlock dieback may be a significant source of nitrate in headwater streams, potentially altering headwater aquatic community composition and increasing downstream nutrient pollution.