Decadal responses in soil N dynamics at the Bear Brook Watershed in Maine, USA

Atmospheric nitrogen deposition to forested ecosystems is a concern because of both geochemical and biological consequences for ecosystem integrity. High levels of prolonged N deposition can lead to “N saturation” of the ecosystem. The Bear Brook Watershed in Maine is a long-term, paired forested watershed experiment with over a decade of experimental N additions (∼ 34 kg ha −1 per year = ambient+ treatment) to investigate the biogeochemical consequences of N saturation. Both in situ and laboratory studies of N mineralization and nitrification were carried out to evaluate the changes in N cycling brought about by the long-term N additions. Consistent with hypotheses set forth in the literature (sensu [BioScience 39 (1989) 378]), the treated watershed had higher rates of N cycling compared to the reference watershed. In addition, we report important differences in N cycling rates as a function of forest cover type and soil horizon. Higher rates of net N mineralization occurred in hardwood O horizons compared to softwoods, but the opposite was true in the mineral soils suggesting an important link between litter type and N mineralization that varies with depth in the pedon. Nitrification showed the greatest response to N treatments, with the majority of mineralized N subsequently oxidized to nitrate in the mineral soils. By comparing the data herein with that previously reported for the Bear Brook experiment, it appears that the ecosystem response to N treatment continues to evolve on a decadal time scale and inherent differences in forest cover types and their underlying soils alter the fate of depositional N.