Soil leachate responses during 10 years of induced whole-watershed acidification

Soil solution was collected from zero-tension lysimeters for 10 yr on two small central Appalachian watersheds in West Virginia, U.S.A. Ammonium sulfate fertilizer was applied to one catchment 3 times per year during each year. The other watershed was used as a reference to account for ambient baseline conditions. Ca and Mg concentrations collected below the A- and B-horizons of the treated watershed increased and then decreased over time as a result of the treatment. By contrast, Ca and Mg concentrations in the C-horizon continued to increase throughout the study period. The depletion in Ca and Mg that occurred in the upper levels apparently occurred due to charge pairing and leaching of those base cations with NO₃ and SO₄. The progressively greater amounts of Ca and Mg carried through the soil with these acid anions provided their continued increasing concentrations in the C-horizon. NO₃ concentrations increased progressively with depth due to both the assimilation of NO₃ by vegetation and microorganisms in the upper soil layers and leaching of NO₃ into deeper soils by mesopore flow. NO₃ became a more important ion over time with respect to pairing and leaching with base cations because its concentrations continued to increase in all soil layers, whereas SO₄ became retained in all soil layers after several years of treatment, presumably induced by adsorption from increasing SO₄ concentrations.