The Rate of Weathering-Derived Calcium Input to a Base-Poor Forest Ecosystem in the Northeastern USA

Over the past 50 years,acid deposition in the northeastern USA has leached calcium and other base cations (K, Na, and Mg) from the soil exchange complex, creating a nutrient-poor environment for trees. In order to quantify losses from the soil exchange pool and estimate ecosystem recovery from acid deposition, inputs from atmospheric deposition and mineral weathering must be known. Atmospheric deposition can be measured directly, but weathering inputs are difficult to distinguish from cation exchange pool depletion in stream fluxes. The objectives of this study were to determine the variability of long-term weathering rates across a small watershed at Hubbard Brook Experimental Forest (HBEF), NH, and to compare these rates to the current flux of cations from the watershed. The HBEF is a northern hardwood forest last logged in the early 1900s; at higher elevations, conifers are dominant. Soils have developed on relatively homogeneous, glacial till deposited about 14,000 years ago and derived predominantly from granodiorite and pelitic schist. Soils were sampled by horizon from 45 soil pits within a 12-hectare watershed. Long-term weathering rates were calculated as the loss of elements from soils relative to the parent material using Ti as an immobile element. The average long-term chemical weathering rate in the watershed (40 meq m^-2 yr^-1) is similar to rates in other 10 to 15 ka old soils developed on granitic till in temperate climates. The soil parent material is chemically homogeneous, yet the weathering rates range from 21 to 65 meq m^-2 yr^-1 across the watershed, generally increasing with elevation due to changes in vegetation, soil water flow paths, and depth to impermeable bedrock. Feldspar weathering rates are greater at higher elevations at HBEF. Plagioclase weathering accounts for a significant amount (56-84%) of the calcium lost from the soils over the last 14,000 years, while the dissolution of apatite accounts for 15 to 32%. The present-day loss of base cations from the watershed greatly exceeds the long-term weathering rate providing additional support that the pool of exchangeable base cations in the soil continues to be diminished.