A COMPARISON OF CHEMICAL WEATHERING IN SCHIST-DOMINATED VS. GRANITE-DOMINATED WATERSHEDS: IMPLICATIONS FOR FOREST HEALTH IN VERMONT AND NEW HAMPSHIRE

While much is known about the impacts of acid deposition, lingering questions remain regarding weathering rates and buffering capacities of different types of soils. Tills derived from schist and granite are two common parent materials for soils in northern New England, so we designed a study to examine soil parameters (e.g. pH, base saturation, Al:Ca, quantitative mineralogical assessment of weathering reactions) in a lithosequence where the primary difference in soil-forming factors is parent material—specifically, we studied till derived from schist (Cambrian Underhill Formation) in northern Vermont on Bolton Mountain and granite (Devonian New Hampshire Series) in central New Hampshire at the USFS Bartlett Experimental Forest. Other soil-forming factors are very similar between the two sites—for example, mean annual precipitation and mean annual temperature are 1300 mm and 6 oC at Bolton VT and 1270 mm and 4.5oC at Bartlett NH. The forest at both sites is dominated by mixed hardwood that transitions upward to spruce-fir. Topography consists of gently- to moderately-sloping till plains and soils at both sites began forming approximately 10 ka.

Notable mineralogical differences exist between the two sites. While both have 35 – 40% quartz and 20 – 25% plagioclase, Bolton VT has 15 % chlorite and 15% muscovite whereas Bartlett NH has 35% K-feldspar (no chlorite and <4% mica). Yet in spite of the mineralogical differences, exchangeable cation compositions and soil acidity are very similar at the two sites. For example, soil pH at Bolton VT ranges from 5.3 to 3.2 whereas in granite-derived soils at Bartlett NH, pH ranges from 5.1 to 2.6. At Bolton VT, Al:Ca ratios range from 0.7 to 17.3 (mean = 3.1); at Bartlett NH, 1.0 to 18.7 (mean = 4.5). Studies show that the threshold for 50% forest dieback risk is 2.0. At Bolton VT, base saturation ranges from 7.3% to 78.5% (mean = 32%); at Bartlett NH, from 9.0 to 64.0% (mean = 30%). BS <20% is considered harmful to forest health. Because of its abundance and reactivity, plagioclase weathering is the dominant mineralogical control on soil geochemistry, a finding that is consistent with work of Hyman et al (1998), who show that, even in soils with appreciable amounts of reactive phyllosilicates like chlorite and mica, plagioclase weathering controls response to acid deposition in New England soils.