Sulfate isotopes in precipitation and streamwater, Hubbard Brook Experimental Forest, 1966 - 1994

Sulfur deposition in the northeastern U.S. has been
decreasing since the 1970s and there has been a concomitant
decrease in the SO4 lost from drainage waters from forest
catchments of this region. It has been established previously
that the SO4 lost from drainage waters exceeds SO4 inputs in
bulk precipitation, but the cause for this imbalance has not
been resolved. The use of stable S isotopes and the
availability of archived bulk precipitation and stream water
samples at the Hubbard Brook Experimental Forest (HBEF) in
New Hampshire provided a unique opportunity to evaluate
potential sources and sinks of S by analyzing the long-term
patterns (1966–1994) of the d34S values of SO4. In bulk
precipitation adjacent to the Ecosystem Laboratory and near
Watershed 6 the d34S values were greater (mean: 4.5 and 4.2l,
respectively) and showed more variation (variance: 0.49 and
0.30) than stream samples from Watersheds 5 (W5) and 6 (W6)
(mean: 3.2 and 3.7; variance: 0.09 and 0.08, respectively).
These results are consistent with other studies in forest
catchments that have combined results for mass balances with
stable S isotopes. These results indicate that for those
sites, including the HBEF, where atmospheric inputs are 10 kg
S ha-1 yr-1, most of the deposited SO4 cycles through the
biomass before it is released to stream water. Results from
W5, which had a whole-tree harvest in 1983–1984 showed that
adsorption/desorption processes play an important role in
regulating net SO4 retention for this watershed-ecosystem.
Although the isotopic results suggest the importance of S
mineralization, conclusive evidence that there is net
mineralization has not yet been shown. However, S mass
balances and the isotopic result are consistent with the
mineralization of organic S being a major contributor to the
SO4 in stream waters at the HBEF. These data were gathered as part of the Hubbard Brook
Ecosystem Study (HBES). The HBES is a collaborative effort at
the Hubbard Brook Experimental Forest, which is operated and
maintained by the USDA Forest Service, Northern Research
Station.