Cation and anion fluxes in northern hardwood throughfall along an acidic deposition 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+, SO4 2-, 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. -from Author