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.