Input-output budgets of major ions for a forested watershed in western Maryland.

Abstract

We measured concentrations and fluxesof major ions in wet deposition, throughfall andstream water in a forested watershed on the AlleghenyPlateau of western Maryland to investigatecanopy-atmosphere interactions and to calculate input–output budgets. Wet deposition was dominated byH+, SO42−, NO3− andNH4+ ions. Hydrogen and SO42−accounted for 70 and 58% of the total cation andanion equivalents, respectively. Annual wet depositionrates of SO42− (0.56 keq ha−1 yr−1), NO3− (0.31 keq ha−1 yr −1)and NH4+ (0.17 keq ha−1 yr −1)were at the high end of the range in wet depositionrates reported for other sites in the eastern UnitedStates. On an annual basis, the forest canopy consumed20% of the free acidity in incident precipitation,had no net effect on Na+ and NH4+deposition, and was a net source of K+,Ca2+, Mg2+, SO42− andNO3−; 1.5 to 22 times greater than the wetdeposition rates. On an annual basis, the watershed ofthe unnamed tributary to Herrington Creek (HCWS)retained essentially all of the throughfall H+ andNH4+ inputs, 35% of the throughfall K+input and 62% of the throughfall NO3−input. In contrast, HCWS was a net source ofSO42−, Cl−, Ca2+, Mg2+ andNa+; export rates were 2 to 5 times greater thanthroughfall inputs. Sulfate was the dominant anionassociated with cation leaching, accounting for 78%of the total anion export of 1.8 keq ha−1 yr −1 in 1996–1997.