The effects of acidic deposition on cation leaching from three deciduous forest canopies

Abstract

Canopy leaching or retention of all major ions was determined for three deciduous forests in eastern Tennessee [USA] that are currently receiving wet and dry acidic deposition. For all three canopies, the retention of atmospherically deposited cations (H+ and NH4+) exceeded retention of deposited anions (NO3-) on a charge-equivalent basis. Charge balance was maintained by ion exchange between the deposited and K+, Ca2+, and Mg2+ on the canopy surfaces. It was found that 40-60% of the leaching of K+, Ca2+, and Mg2+ from these canopies resultes from this exchange process, with the remainder attributable to coleaching with plant-derived anions. Because of the cation exchange can be largely attributed to retention of deposition H+ we interpret it as a measure of the acidic deposition effect on foliar cation leaching. Dry deposition was found to be an important input for most of the major cations and anions. The data indicate that organic anions may be important counter ions to cation leaching, especially in the absence of acidic deosition, and that acid-base reactions in the canopy can obscure the true H+-exchange reactions between the deosition and the canopy.