Dissolved organic carbon and sulfate sorption by spodosol mineral horizons

Dissolved organic carbon (DOC) can influence the mobility and sorption of inorganic solutes in soils. Soil exchange sites can be occupied by DOC thus preventing the retention of SO₄²⁻, which enters the soil from acidic deposition. To determine the importance of DOC and SO₄²⁻interactions with soil, we examined the sorption of DOC and SO₄²⁻by several mineral horizons of three Spodosols. In general, sorption of DOC was found to be comparable between spodic horizons within each Spodosol, although sorption characteristics indicated some differences among soils. Due to the release of large amounts of DOC, analysis of sorption isotherms was characterized by the initial mass isotherm method instead of Langmuir or Freundlich techniques. Using the initial mass isotherm, we found that the carbon reactive soil pools (RSP) corresponded closely to total soil carbon (C_t) levels (e.g., RSP = 5.67 C_t− 8.44, r²= 0.856, n = 10). Temperature (276 and 294 K) and pH (equilibrium solution pH ranged from 4.2 to 5.2), however, had little influence on DOC sorption. Sulfate sorption, evaluated on the horizon of each soil with the highest extractable SO₄²⁻, was found to be distinctly different for each of the horizons. In the presence of DOC (5.6 mmol C L⁻¹), SO₄²⁻sorption decreased (e.g., 5–22% reduction at 10 mmol(−) kg⁻¹SO₄²⁻addition). Although DOC reduced the amount of SO₄²⁻adsorbed by each soil, the great affinity that mineral horizons have for DOC can rapidly reduce DOC concentration in soil leachates percolating through the soil profile. This would decrease the competitive DOC effect on SO₄²⁻sorption, and SO₄²⁻would be retained in lower mineral horizons. Therefore, soil contact is proposed as the most important factor in controlling terrestrial inputs of both DOC and SO₄²⁻to surface waters.