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Some Observations on the Stoichiometry of Feldspar Hydrolysis in Granitic Soil

Forestry Science Lab., 316 East Myrtle St., Boise, ID 83702.

^* Corresponding author.

ABSTRACT

Weathering rates of orthoclase and plagioclase were computed from mass balances of Na, K, and Ca in three forested watersheds in the Idaho batholith. On the basis of stand conditions, two watersheds were assumed to have no net gains or losses of cations in biomass, and increases in biomass were measured in the third watershed. Balanced feldspar hydrolysis reactions were established based on kaolinite as a weathering product. Free silica (SiO₂) release predicted from reaction stoichiometries ranged from 94 to 99% of measured SiO₂ flux from the watersheds. These results suggest that the entire flux of Na, K, and Ca can be attributed to cation release from primary mineral hydrolysis without invoking net loss of cations from exchange sites in these watersheds. The acid neutralizing capacity (ANC) arising from hydrolysis is approximately 1500 to 1700 mol(c) ha^–1 yr^–1. Annual H⁺ input from bulk precipitation currently averages 70 mol ha^–1 yr^–1. Attempts by other researchers to check reaction stoichiometry by SiO₂ have not been particularly successful. Simple mineralogy of parent material and lack of anthropogenic sources of acid deposition may explain why predicted and actual SiO₂ fluxes were similar in this study.

Key Words: Feldspar weathering rate • Soil acid neutralization capacity • Reaction stoichiometry

Contribution from the Intermountain Res. Stn., Forest Serv., USDA, Ogden, UT 84401.

Received for publication April 1, 1987.