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.