THROUGHFALL CHEMISTRY IN AN ENGELMANN SPRUCE - SUB-ALPINE FIR FOREST IN NORTH CENTRAL COLORADO

Bulk precipitation and throughfall quantity and quality were measured in an old-growth Engelmann spruce (Picea engelmannii Parry) - subalpine fir (Abies lasiocarpa (Hook.) Nutt.) forest in north central Colorado, a forest type for which little nutrient-cycling information exists. 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, estimated using two methods, 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 at this site compared with other mountainous sites in the region. We suggest that the relatively high NO3/SO4 ratio results from proximity to NO(x) sources in the urban corridor to the east.