Measurements of wet and dry deposition in a Northern Hardwood forest

Abstract

Inputs of wet and dry deposition were monitored at the Huntington Forest in the Adirondack Mountains of New York for two years in the open and beneath the canopy of a northern hardwood forest. In the open, ion flux estimates were similar using wet-only weekly (NADP protocol) and event collections, but bulk collections were higher for all ions except H⁺, which was much lower. These differences were due to the contribution of dry deposition and possible biotic alterations in bulk collectors. Dry deposition was estimated using air concentrations and ion-specific depositional velocities modeled with meteorological data, and contributed substantially to the input of all ions [H⁺ (45%), Na⁺ (24%), K⁺ (22%), NH₄ ⁺ (12%), Ca²⁺ (58%), Mg²⁺(43%), NO₃ ⁻ (55%), Cl⁻ (27%) and SO₄ ⁻² (26%)]. Dry input of base cations was dominated by coarse particles, whereas gaseous inputs were more important for S and NO₃ ⁻. Atmospheric concentrations of SO₂ and inputs of SO₄ ²⁻ and H⁺ were lower at this site than sites closer to point sources of S gas emission. The importance of estimating atmospheric inputs was examined using examples of elemental budgets. For example, different estimates of the contribution of dry deposition of SO₄ ²⁻ (9–21 meq m⁻² y⁻¹) resulted in conclusions ranging from no net retention to a net loss of this element. Such differences have important implications in assessing the current and future role of atmospheric inputs in affecting elemental cycling.