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⁺, SO ²⁻₄ , NO ⁻₃ andNH ⁺₄ ions. Hydrogen and SO ²⁻₄ accounted for 70 and 58% of the total cation andanion equivalents, respectively. Annual wet depositionrates of SO ²⁻₄ (0.56 keq ha⁻¹ yr⁻¹), NO ⁻₃ (0.31 keq ha⁻¹ yr ⁻¹)and NH ⁺₄ (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 NH ⁺₄ deposition, and was a net source of K⁺,Ca²⁺, Mg²⁺, SO ²⁻₄ andNO ⁻₃ ; 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⁺ andNH ⁺₄ inputs, 35% of the throughfall K⁺input and 62% of the throughfall NO ⁻₃ input. In contrast, HCWS was a net source ofSO ²⁻₄ , 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.