Long-Term Ionic Increases from a Central Appalachian Forested Watershed

The electrical conductivity of stream water draining from an unmanaged and undisturbed control watershed has been increasing rather steadily, about 0.03 mS m-1 yr-1, since 1971. During this period, NO~ and Ca 2÷ concentrations increased and were shown to mathematically account for the ionic contribution to conductivity; therefore, they are believed to be primarily responsible for the increase. However, the percentage of conductivity explained by the two ions was different over time. The percentage of conductivity attributable to NO~ increased in a pattern very similar to concentration. In contrast, the percentage of conductivity attributable to Ca 2+ decreased slightly over time. The Ca :÷ is believed to be pairing with the NO] as the NOg ions leach through the soil. While nitrification in mature stands can be strongly inhibited, limited nitrification, especially in forest gaps, and high anthropogenic inputs of NO~ probably were primary sources of the leached NO~. Preferential adsorption of SOl-, rather than NO~, on soil colloids is given as an explanation for the lack of retention of NO~ in the soil system and subsequent leaching to the stream. S THE INITIATION of experimentation on the Fernow Experimental Forest, Watershed 4 has been maintained as a control for use in paired watershed studies. As such, it has not been treated or managed. However, the mean annual electrical conductivity of the stream water from this catchment has increased rather steadily since 1971 (Helvey et al., 1982), indicating that the export of dissolved solids is increasing (Kunkle and Wilson, 1984). This article attempts to identify the ionic constituents that were responsible for causing the long-term conductivity increase, quantify the percentages of conductivity composed of these ions, examine changes in these percentages over time, and discuss possible mechanisms that might control these ionic responses