Organic Carbon Fractions in Extracts of O and B Horizons from a New England Spodosol: Effects of Acid Treatment

We examined the effects of H₂SO₄ and HNO₃ on dissolved organic carbon (DOC), pH, and DOC fractions in extracts from Spodosol O and B horizon samples of a forest soil from eastern Maine. Preliminary work with HNO₃ at p[H⁺] = 3 (1 to 24 h shaking, ionic strength adjustment with NaNO₃ (I = 0.1 to 0.001) indicated rapid equilibration (1 h) of DOC in treatment solutions with B horizon samples, but that DOC levels in extracts from the O horizon increased continuously over time. In the remaining work, H₂SO₄ and HNO₃ and their Na salts were added at a constant I of 0.01 and solutions were recovered after shaking for 16 h. The DOC in O horizon extracts decreased with increasing acidity, reaching a minimum at p[H⁺] = 2 (from ca. 5 mmol C L⁻¹ with no acid addition to ca. 2 mmol C L⁻¹). For the B horizon extracts, DOC increased with increasing acidity. Percent neutralization of added H⁺ ranged from 18 to 99% in the O horizon for acid treatments ranging from p[H⁺] = 1 to 4 respectively; B horizon values were 16 to 85% for H₂SO₄ and 2 to 70% for HNO₃. The DOC in O horizon solutions was dominated by hydrophobic and hydrophilic acids (ca. 45 and 20% of DOC, respectively), which were altered by acidic inputs; primarily by a decrease in hydrophobic acids (45 to ca. 15–18% of DOC) and an increase in hydrophilic acids (ca. 20 to 35–40% of DOC) as acidity increased. For the B horizon, similar trends were observed, although hydrophobic/hydrophilic acid ratios were lower, indicating that hydrophobic acids have a lower solubility with increasing acidity. From an environmental perspective, increasing acidic inputs through wet and dry deposition on forest Spodosols of New England may have significant consequences in the solubilization and transport of metals and acidity because of differences in DOC fractions.