ATMOSPHERIC DEPOSITION TO A LOW-ELEVATION SPRUCE-FIR FOREST, MAINE, USA

A spruce-fir forest in central-Maine, USA, has been the site of dry an d wet atmospheric deposition measurements since 1988. Precipitation fl ux of hydrogen ions (H+) and sulfate (SO42-) decreased by 24 and 42%, respectively, during the study period, but no temporal trends occurred for precipitation H+ or SO(4)2- concentrations. Decreased precipitati on fluxes were likely due to a 33% decrease in precipitation amounts d uring the latter years of the study. Both H+ and SO42- concentrations and fluxes in throughfall decreased, corresponding to decreases in dry deposition of those constituents. No nitrate (NO3-) concentration or flux trends occurred in precipitation, throughfall, or dry deposition. Hydrogen, NO3-, and NH4+ had a negative net canopy exchange (NCE) att ributable to N deficiency at this site and acid neutralization in the canopy. Sulfate and magnesium (Mg2+) were conservative and had no NCE. Calcium (Ca2+) and potassium (K+) had a positive NCE due to canopy le aching. A comparison of bulk and wet-only precipitation collectors sho wed that SO42-, Ca2+, and K+ concentrations were greater in solutions from a bulk collector compared with a wet-only collector. The differen ces were partially attributable to dry deposition. Throughfall H+, Ca2 +, and dissolved organic carbon (DOC) concentrations were higher in we ekly wet-only collections composited monthly, compared with weekly bul k collections composited either weekly or monthly. Nitrate concentrati on, however, was lowest in the wet-only thronghfall collections. Diffe rences likely resulted from spatial variability. Ongoing research is a ttempting to further define trends apparent to date, and to determine the mechanisms controlling these characteristics.