Element fluxes and landscape position in a northern hardwood forest watershed ecosystem

Chemical changes along headwater streams at the Hubbard Brook Experimental Forest in New Hampshire suggest that important differences exist in biogeoc hemical cycles along an altitudinal gradient within small watershed ecosyst ems. Using data collected during the period 1982-92, we have constructed el ement budgets [Ca, Mg, It, Na, Si, Al, dissolved organic carbon (DOC), S, a nd N] for three subcatchments within watershed 6, a forested watershed last logged around 1917-20. The biogeochemistry of the high-elevation spruce-fi r-white birch subcatchment was dominated by processes involving naturally o ccuring organic compounds. Stream water and soil solutions in this zone had elevated concentrations of organic acidity, DOG, and organically bound mon omeric aluminum (Al,), relative to lower-elevation sites. The middle-elevat ion subcatchment, dominated by hardwood vegetation, had the greatest net pr oduction of inorganic-monomeric aluminum (Al,), and exhibited net immobiliz ation of DOC and Al,. The low-elevation subcatchment, also characterized by deciduous vegetation, had the highest rates of net production of base cati ons (Ca2+, Mg2+, K+, Na+) among the subcatchments. Living biomass of trees declined slightly in the spruce-fir-white birch subcatchment during the stu dy period, remained constant in the middle-elevation zone, and increased by 5% in the low-elevation subcatchment. Coupling the correspending changes i n biomass nutrient pools with the geochemical patterns, we observed up to 1 5-fold differences in the net production of Ca, Mg, K, Na, and Si in soils of the three subcatchments within this 13.2-ha watershed. Release of Ca, Na , and dissolved Si in the highest-elevation subcatchment could be explained by the congruent dissolution of 185 mol ha(-1) y(-1) of plagioclase feldsp ar. The rate of plagioclase weathering, based on the net output of Na, incr eased downslope to 189 and 435 mol ha(-1) y(-1) in the middle-elevation and low-elevation subcatchments, respectively. However, the dissolution of fel dspar in the hardwood subcatchments could account for only 26%-37% of the o bserved net Ca output. The loss of Ca from soil exchange sites and organic matter is the most likely source of the unexplained net export. Furthermore , this depletion appears to be occurring most rapidly in the lower half of watershed 6. The small watersheds at the Hubbard Brook Experimental Forest occupy a soil catena in which soil depth and soil-water contact time increa se downslope. By influencing hydrologic flowpaths and acid neutralization p rocesses, these factors exert an important influence on biogeochemical flux es within small watersheds, but their influence on forest vigor is less cle ar. Our results illustrate the sensitivity of watershed-level studies to sp atial scale. However, it appears that much of the variation in element flux es occurs in the first 10-20 ha of drainage area.