Search

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 [(N[H.sub.4]).sub.2]S[O.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 cation-exchange buffering, resulting in a net decline in soil exchangeable base cations.

Increased rates of litter decay due to projected global warming could substantially alter the balance between C assimilation and release in forest soils, with consequent feedbacks to climate change. This study was conducted to investigate the effects of soil warming on the decomposition of red spruce (Picea rubens Sarg.) and red maple (Acer rubrum L.) foliar litter at Howland, ME. Experimentally increased Oa horizon soil temperatures (increase of 4-5 [degrees] C) were maintained during the snow-free season from 1993 through 1995 in replicated 15 by 15 m plots using heat-resistance cables. For red maple litter, significant treatment effects included greater loss of mass (27%) and C (33%), and greater accumulation of Zn (54%) during the first 6 mo of decay in the heated plots than the control plots. After 30 mo of decay, significant treatment effects were no longer evident for red maple litter. Few treatment effects were observed for red spruce litter during the initial 18 mo of decay. However, after 30 mo of decay, significant treatment effects included greater loss of mass (19%), C (19%), N (24%), Ca (27%), Mg (12%), K (4%), Zn (60%), and cellulose (40%) in red spruce litter in the heated plots than the control plots. We conclude that a modest increase in Oa horizon soil temperature (4-5 [degrees] C) can significantly increase litter decay rates and alter litter decay dynamics in this coniferous forest stand, and that these changes exhibit variations in their temporal development as a function of species and litter quality attributes.

The soil CO2 evolution and soil air CO2 concentration in the Howland Integrated study (HIFS) site, the Bear Brook Watershed in Maine (BBWM) site and the Letter E Township (Letter E) site, all in Maine, were estimated to characterize soil CO2 in various forest types, many years after the harvest. The major yearly changes in temporal patterns of these characteristics are a function of short-term climatic factors.

Northern forest soils represent large reservoirs of C and N that may be altered by ecosystem perturbations. Soils at three paired watershed in Maine were investigated as case studies of experimentally elevated N deposition, wildfire, and whole-tree harvesting. Eight years of experimental [([NH.sub.4]).sub.2][SO.sub.4] additions at the Bear Brook Watershed in Maine significantly reduced forest-floor C/N ratios from 30.6 to 23.4. Forest-floor C and N pools were lower in the treated watershed (38 Mg C [ha.sup.-l], 1612 kg N [ha.sup.-1]) compared with the reference (75 Mg C [ha.sup.-1], 2372 kg N [ha.sup.1]). Fifty years after wildfire at Acadia National Park, the burned watershed with hardwood regeneration had significantly lower forest-floor C and N concentrations (208 g C [kg.sup.-1] soil, 9.9 g N [kg.sup.-1] soil) than the reference watershed dominated by a softwoods (437 g C [kg.sup.-1] soil, 12.8 g N [kg.sup.-1] soil). Forest-floor C and N pools were lower in the burned watershed (27 Mg C [ha.sup.-1] 1323 kg N [ha.sup.-1]) compared with the reference (71 Mg C [ha.sup.-1], 2088 kg N [ha.sup.-1]). At the Weymouth Point, the harvested watershed regenerated to spruce-fir, the dominant stand type that existed before the harvest, and it had significantly lower forest-floor C concentrations and pools (406 g C [kg.sup.-1] soil, 24 Mg C [ha.sup.-1]) than the reference (442 g C [kg.sup.-1] soil, 39 Mg C [ha.sup.-1]) after 17 yr. All perturbations studied were associated with lower forest-floor C pools.