The responses of soil, soil solution, and stream water of a northern hardwood forest to experimental addition of wollastonite

Date of Award


Degree Type


Degree Name

Doctor of Philosophy (PhD)


Civil and Environmental Engineering


Charles T. Driscoll


Soil solution, Stream water, Hardwood forest, Wollastonite, Calcium

Subject Categories

Civil Engineering


In this dissertation, I examined the response of a forest watershed with acidic soil and surface waters to changes in calcium (Ca) supply. Watershed 1 (W1), that has been impacted by acidic deposition at the Hubbard Brook Experimental Forest in New Hampshire, U.S.A., was experimentally treated with calcium silicate (CaSiO 3 : wollastonite) in October, 1999.

Exchangeable Ca (Ex-Ca), soil pH s (in 0.01M CaCl 2 ), effective cation exchange capacity (CEC e ), and effective base saturation (BS e ) increased, while exchangeable acidity (Ex-Acid) decreased in organic soil horizons of W1 following wollastonite treatment. Mineral soil experienced either small increases in Ex-Ca, pHs, CEC e , BS e , small decreases in Ex-Acid, or no changes. Soil solutions draining the forest floor responded to the treatment by increases in concentrations of Ca, H 4 SiO 4 , pH, and acid neutralizing capacity (ANC), and decreases in inorganic monomeric Al (Al i ). Positive correlations between Ca/Al i in soil solution and Ex-Ca/Ex-Al ratios in soil indicated that changes in the chemistry of soils significantly influenced the chemistry of soil water, and that Ca derived from the dissolution of wollastonite mitigated the mobilization of Al. Wollastonite addition also significantly increased concentrations and fluxes of Ca 2+ , H 4 SiO 4 , pH, and ANC, and decreased concentrations and fluxes of

Al i in stream water at all elevations. Although mean values of Ca 2+ decreased slightly from 33.8 to 31.7 μmol/L with increasing stream discharge in W1, the mean value of ANC was positive (1.2 μeq/L ) during summer storm events, compared to negative values (-0.2 μeq/L) in an untreated reference watershed. This response was presumably due to enhanced Ca 2+ supply in W1 (20.7 to 29.0% of dissolved Ca 2+ derived from wollastonite) to stream water as a result of interflow along shallow flowpaths. Approximately 3% of the added Ca and 10% of the added Si were exported through the stream outlet from the spruce-fir-white birch (SFB), 2% and 6% from the high-elevation hardwood (HH), 2% and 7% from the low-elevation hardwood (LH) subcatchments, respectively over the six-year study period. Most of the added Ca remained in soil exchange site of the forest floor, but influenced the acid-base status of soil solutions and stream water.


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