Hydrochemical response of upland Precambrian Shield catchments to additions off H₂SO₄and NH₄NO₃during snowmelt

Two of three upland Precambrian Shield catchments received additions of H 2 SO 4 and NH 4 NO 3 prior to snowmelt in 1989 and 1990; loading levels were equivalent to 18 kg∙ha −1 ∙yr −1 as SO 4 2− and 2.62 kg∙ha −1 ∙yr −1 as N. The third catchment served as a reference. The manipulated catchments retained 96–98% of the added NH 4 + , 73–94% of the added NO 3 − , 31–48% of the added SO 4 2− , and 72–92% of the added H + . Proton neutralization was attributed to increased cation export (Ca 2+ + Mg 2+ + Na + + K + + Al n+ + Mn 2+ + Fe 2+ : 26–53%) and SO 4 2− retention (48–67%). The protonation of weak organic acids accounted for a 13–34% decline in organic anions during snowmelt. No significant change in dissolved organic carbon export was observed. In manipulated catchments, inorganic monomeric Al export increased from 30 to 80% over reference levels, and the ratio of inorganic to organic monomeric Al changed from < 1.0 to 2.7. Differences in response of the manipulated catchments were attributed to differences in soil thickness and the extent of forest cover. Interannual differences in responses were attributed to differences in snow accumulations, melt sequences, and hydrologic flushing of catchment soils.