Date of Award


Level of Access

Campus-Only Thesis

Degree Name

Master of Science (MS)


Ecology and Environmental Sciences


Ivan J. Fernandez

Second Committee Member

Stephen A. Norton

Third Committee Member

Kevin S. Simon


Chemical climate change in the form of elevated levels of atmospheric sulfur and nitrogen deposition is a continuing topic of concern for the health and stability of temperate forest ecosystems. The detrimental effects of acid deposition on soil base cation status are of particular interest, as they have a direct effect on tree health and runoff to associated surface waters. The Bear Brook Watershed in Maine (BBWM) is a long-term paired watershed experiment, where the experimental watershed has been treated with (NH4)2SO4 since 1989. The adjacent unmanipulated watershed has been subject to ambient deposition and serves as a biogeochemical reference. Soils were quantitatively sampled in the summer of 2010, analyzed for soil base cation status and physical properties, and statistically analyzed for evidence of treatment effects on soil condition after nearly two decades of experimental treatments. Soil exchangeable calcium (Ca) concentrations from the 2010 sampling were compared with soil exchangeable Ca concentrations from earlier studies, in order to examine soil chemical changes over the last decade of the experiment. A critical loads assessment was conducted for both watersheds, based on the 2010 data.

The results suggest that soil exchangeable base cation status at BBWM has remained relatively stable throughout the second decade of treatment. Few significant changes were present between Ca concentrations from 1998 to 2010. The bulk of the statistically significant differences present in soil base cation status in 2010 were attributable to differences in forest cover type, rather than differences in treatment. Base cation concentrations were highest in O horizon soils. Mean Ca concentration was 4.76 cmolc kg-1 in the O horizon, 0.33 cmolc kg-1 in the upper 5 cm of the B horizon soil, 0.15 cmolc kg-1 in the in the B horizon soil, and 0.08 cmolc kg-1 in the C horizon. Mean base saturation was 36.1% in the O horizon, 7.7 % in the upper 5 cm of the B horizon soil, 6.7% in the remaining B horizon soil, and 7.4% in the C horizon.

The reference watershed experienced acid inputs that are below the estimated critical load, and is projected to recover from earlier losses of base cations throughout the twenty-first century. The treated watershed experiences acid inputs that are above the estimated critical load, and is projected to maintain a relatively steady-state condition of depleted base cation availability throughout the next century - no further depletion of base cations is predicted. Both watersheds are experiencing N inputs that are above an estimated critical load for N, and are projected to export above background levels of NO3 to surface waters from soils throughout the twenty-first century.