Watershed-Level Responses to Calcium Silicate Treatment in a Northern Hardwood Forest

Abstract

Watershed 1 (W1) at the Hubbard Brook Experimental Forest in New Hampshire, with chronically low pH and acid neutralizing capacity (ANC) in surface water, was experimentally treated with calcium silicate (CaSiO3; wollastonite) in October 1999 to assess the role of calcium (Ca) supply in the structure and function of base-poor forest ecosystems. Wollastonite addition significantly increased the concentrations and fluxes of Ca, dissolved silica (Si), and ANC and decreased the concentrations and fluxes of inorganic monomeric Al (Ali) and hydrogen ion (H+) in both soil solution and stream water in all sub-watersheds of W1. Mass balances indicate that 54% of the added Ca remained undissolved or was retained by vegetation during the first 6 years after treatment. Of the remaining added Ca, 44% was retained on O horizon cation exchange sites. The Ca:Si ratio in the dissolution products was greater than 2.0, more than twice the molar ratio in the applied wollastonite. This suggests that Ca was preferentially leached from the applied wollastonite and/or Si was immobilized by secondary mineral formation. Approximately 2% of the added Ca and 7% of the added Si were exported from W1 in streamwater in the first 6 years after treatment. Watershed-scale Ca amendment with wollastonite appears to be an effective approach to mitigating effects of acidic deposition. Not only does it appear to alleviate acidification stress to forest vegetation, but it also provides for the long-term supply of ANC to acid-impacted rivers and lakes downstream.