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) - subalpin e fir (Abies lasiocarpa (Hook.) Nutt.) forest in north central Colorad o, a forest type for which little nutrient-cycling information exists. Higher concentrations of nutrient cations (Ca, Mg, Na, and K) were fo und in throughfall than in bulk precipitation, indicating canopy leach ing of nutrients and dry deposition onto the canopy followed by wash-o ff. 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 in ferred from a 5-fold increase in dissolved organic carbon and a large charge-balance deficit (23%) in throughfall relative to bulk precipita tion (8%). Canopy evaporation was estimated at 27%, accounting for a 3 7% increase in the concentration of dissolved substances in throughfal l. Dry deposition, estimated using two methods, resulted in large addi tions of most cation nutrients and N, but not SO4. Ratios of NO3 to SO 4 in wet fall (0.8) and bulk precipitation (1.1) were high at this sit e compared with other mountainous sites in the region. We suggest that the relatively high NO3/SO4 ratio results from proximity to NO(x) sou rces in the urban corridor to the east.