Search

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. [Copyright &y& Elsevier]

There is concern that changes in atmospheric deposition, climate, or land use have altered the biogeochemistry of forests causing soil base-cation depletion, particularly Ca. The Bear Brook Watershed in Maine (BBWM) is a paired watershed experiment with one watershed subjected to elevated N and S deposition through bimonthly additions of (NH[sub 4])[sub 2]SO[sub 4]. Quantitative soil excavations in 1998 measured soil pools of exchangeable base cations 9 yr after treatments began. Stream sampling at the weirs on a weekly and event basis, and weekly precipitation sampling, were used for input-output estimates. The treated watershed had lower concentrations of exchangeable Ca and Mg in all horizons, with evidence for the greater depletion in the O horizon compared to underlying mineral soils, and in softwoods compared to hardwoods. This difference between watersheds is interpreted to be treatment-induced base-cation depletion, which was reinforced by model simulations. The difference between watersheds was 66 and 27 kg ha[sup -1] of exchangeable Ca and Mg, respectively, after accounting for soil mass differences between watersheds. This was comparable with the total cumulative excess stream Ca and Mg export in West Bear after 9 yr of treatment of 55 and 11 kg ha[sup -1], respectively. Model simulations of watershed response to treatments predicted excess soil exchangeable Ca and Mg losses in the treated watershed of 47 and 9 kg ha[sup -1], respectively. These results indicate that the response to a step-increase in N and S deposition during the first decade of treatments in this experimental forested watershed was to invoke cationexchange buffering, resulting in a net decline in soil exchangeable base cations. [ABSTRACT FROM AUTHOR]