Abstract
Chemical weathering of minerals is the principal mechanism by which base cations (Ca2+, Mg2+, K+, and Na+) are released and acidity is neutralized in soils, bedrock, and drainage waters. Quantifying the release of base cations from watershed soils is therefore crucial for the calculation of “critical loads” of atmospheric acidity to forest ecosystems. We used a mass-balance approach to estimate the rate of release of base cations in 25 headwater catchments in the Catskill region of New York, an area historically subject to high inputs of acid deposition. In 2010–2013, total net base cation release via geochemical processes averaged 1,704 eq ha–1 yr–1 (range: 928–2,622). Calcium accounted for 58% of this total, averaging 498 mol ha–1 yr–1 (range: 209–815). Mass balance estimates of net geochemical release of base cations were most strongly driven by stream export and biomass uptake fluxes, with only minor contributions from precipitation. Documented rates of base cation depletion from soil exchange sites in the region were also small relative to the net geochemical release rates. We observed a significant influence of bedrock type on net base cation release rates (P = 0.002), and a weak but significant negative correlation with watershed elevation (r = −0.51). Relationships with other geographic factors such as aspect and watershed size were not significant. Net base cation release was 4.5 times higher than precipitation inputs of SO42– and NO3–, suggesting that sources of acidity internal to the watershed are now more important drivers of weathering than acid deposition. Our data suggest that release of base cations from most Catskill forest soils is sufficient to neutralize existing inputs of acidity.
Highlights
Because of their proximity to midwestern industries and power plants, as well as east coast urban areas, the Catskill Mountains of New York receive among the highest rates of atmospheric deposition of SO2 and NOx in the United States (Lovett et al, 2000)
Using the mass-balance approach, we estimated that the average net geochemical release rate of Ca for the period of 2010–2013 for all of the watersheds was 498 mol ha−1 yr−1 (Table 4)
The results of this study will aid in estimating critical loads of acid deposition for the Catskills
Summary
Because of their proximity to midwestern industries and power plants, as well as east coast urban areas, the Catskill Mountains of New York receive among the highest rates of atmospheric deposition of SO2 and NOx in the United States (Lovett et al, 2000). Between 1998 and 2000, for example, wet deposition in the Catskill region had an average pH of 4.4, approximately 10 times more acidic than the background conditions (Driscoll et al, 2003). Cation Mobilization in Catskills Watersheds of New York, such as the Catskills, acidic deposition has impacted soils, the health of sensitive tree species, fish and other aquatic organisms, and has negatively affected water quality. Even though there have been substantial reductions in acid deposition in the northeastern United States as a result of the CAAA, the acid neutralizing capacity (ANC) of surface waters in the region has been slow to recover. While there has been some recovery of ANC and pH in regional surface waters, base cation concentrations have declined (calcium [Ca2+], magnesium [Mg2+], sodium [Na+], and potassium [K+]), and the soils have continued to acidify (Likens et al, 1996; Warby et al, 2009)
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