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Ion-exchange resins (IER) offer alternative approaches to measuring ionic movement in soils that may have advantages over traditional approaches in some settings, but more information is needed to understand how IER compare with traditional methods of measurement in forested ecosystems. At the Bear Brook Watershed in Maine (BBWM), one of two paired, forested watersheds is treated bi-monthly with S and N (28.8 and 25.2kgha−1yr−1 of S and N, respectively). Both IER and ceramic cup tension lysimeters were used to study soil solution responses after ∼11 years of treatment. Results from both methods showed treatments resulted in the mobilization of base cations and Al, and higher SO4—S and inorganic N in the treated watershed. Both methods indicated similar differences in results associated with forest type (hardwoods versus softwoods), a result of differences in litter quality and atmospheric aerosol interception capacity. The correlation between lysimeter and IER data for individual analytes varied greatly. Significant correlations were evident for Na ( r=0.75), Al ( r=0.65), Mn ( r=0.61), Fe ( r=0.57), Ca ( r=0.49), K ( r=0.41) and NO3—N ( r=0.59). No correlation was evident between IER and soil solution data for NH4—N and Pb. Both IER and soil solution techniques suggested similar interpretations of biogeochemical behavior in the watershed. [ABSTRACT FROM AUTHOR]

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]