Hemlock woolly adelgid, Adelges tsugae, an introduced aphid-like insect from Asia, is spreading through the northeastern United States, causing large-scale dieback of the native Eastern Hemlock, Tsuga canadensis. The eastern hemlock is a late successional species, known to thrive primarily along the banks of mountain streams and facilitate slow nutrient cycling within forest soils. Hemlock dieback has the potential speed up soil nitrification and mineralization rates and to increase nitrogen leaching from forest soils into mountain streams, contributing to downstream nutrient loading. We examined soil nitrogen transformations and streamwater nitrogen content at five hemlock-dominated stands in south-central Pennsylvania, spanning the spectrum from no hemlock mortality to total adelgid-induced hemlock mortality with successive regrowth.

Results/Conclusions

Net nitrification and nitrogen mineralization rates were found to be significantly higher at sites with high mortality. The mean net nitrification rate at the two sites most affected by hemlock woolly adelgid was 7.5 ± 1.0 ?g N/g dry soil/day, while the mean net nitrification rate at the three least adelgid affected sites was 0.86 ± 0.19 ?g N/g dry soil/day. The mean net nitrogen mineralization rate was 8.7 ± 1.2 ?g N/g dry soil/day for the two most affected sites and 2.6 ± 0.3 ?g N/g dry soil/day at the three least affected sites. Extractable nitrate levels and nitrate leaching rates were highest in the site with the second highest mortality. Stream water nitrate levels were lowest at the site with little to no mortality (0.035 ppm NO3) and highest in the site with intermediate mortality (0.69 ppm NO3). These results suggest that in the sites with the longest time since infestation and the highest mineralization and nitrification rates, regenerative seedling growth (Betula in particular) is leading to increased uptake of nitrate, creating a net nitrogen sink.  However, for a period of several years after infestation, hemlock dieback may be a significant source of nitrate input to mountain streams, increasing the risk of downstream eutrophication.