Cation and anion fluxes in northern hardwood throughfall along an acidic deposition gradient

Abstract

Ionic concentrations and fluxes were measured for 2 years in five northern hardwood stands along an acidic deposition gradient that extends from northern Minnesota (lowest deposition) to southeastern Michigan (highest deposition). Precipitation fluxes of H+, SO42−, and NO3− were, respectively, 340, 69, and 83% greater at the site with the highest deposition than at the site with the lowest deposition. No significant differences among sites were evident for precipitation fluxes of cations along the gradient. Fluxes of H+, SO42−, NO3−, Ca2+, and Mg2+ in throughfall increased along the gradient and were positively correlated with increased atmospheric inputs of H+, SO42−, and NO3− measured at the sites. Fluxes of SO42− and NO3− in throughfall were greater than precipitation fluxes, indicating dry deposition in excess of any assimilation of these anions from precipitation. Dry deposition inputs of SO42− increased from the northwestern to southeastern sites and were estimated to range from 23 to 49% of precipitation inputs. Precipitation acidity was neutralized by the canopy in all stands, but the amount of H+ retained by the canopy was significantly greater at sites with the greatest precipitation acidity. Throughfall fluxes of Ca2+ and Mg2+ in excess of precipitation fluxes were positively correlated with the canopy retention and deposition of H+ along the gradient. Increased throughfall fluxes of these cations were consistent with hypothesized increases in canopy leaching of cations with increased acidic deposition. Increased canopy leaching of Ca2+ and Mg2+, resulting from elevated acidic deposition, was estimated to represent as much as 6.2 and 12.9% of foliar contents of these cations, respectively. Although HCO3− and organic anions were found to play an important role in maintaining electroneutrality in throughfall along the gradient, strong acid anions increasingly dominated anionic composition of throughfall with increasing acidic deposition.