Throughfall chemistry in Engelmann spruce - subalpine fir forest in north central Colorado

Higher concentrations of nutrient cations (Ca, Mg, Na, and K) were found in throughfall than in bulk precipitation, indicating canopy leaching of nutrients and dry deposition onto the canopy followed by wash-off. Ammonium was retained by the canopy. Concentrations of anions (NO₃, SO₄, and ortho-P) were similar in throughfall and bulk precipitation. Large additions of organic acids to throughfall from foliage were inferred from a 5-fold increase in dissolved organic carbon and a large charge-balance deficit (23%) in throughfall relative to bulk precipitation (8%). Canopy evaporation was estimated at 27%, accounting for a 37% increase in the concentration of dissolved substances in throughfall. Dry deposition resulted in large additions of most cation nutrients and N, but not SO₄. Ratios of NO₃ to SO₄ in wet fall (0.8) and bulk precipitation (1.1) were high and result from proximity to NO_x sources in the urban corridor to the east. -from Authors