Is there missing S at the HBEF?

by M. J. Mitchell1, C. Alewell2, G.E. Likens3 & H.R. Krouse4

Contact Info:
1S.U.N.Y., College of Environmental Science and Forestry, Syracuse, New York 13210, USA.
email: mitchell@mailbox.syr.edu
2BITÖK, Universität Bayreuth, Dr.-Hans-Frisch-Str.1-3, 95440 Bayreuth, Germany
3Institute of Ecosystem Studies, Millbrook, New York 12545, USA
4The University of Calgary, Department of Physics and Astronomy, Calgary, Alberta, T2N 1N4, Canada

Date prepared: July 1998 (revised December 2002)

Sulfur deposition in the northeastern U.S. has been decreasing since the 1970s and there has been a concomitant decrease in the SO42-  lost from drainage waters from forest catchments of this region. It has been established previously that the SO42-  lost from drainage waters exceeds SO42-  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 SO42-.  In bulk precipitation adjacent to the Ecosystem Laboratory and near Watershed 6 the d34S values were greater (mean: 4.5 and 4.2‰, 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) (Table 1). 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 SO42- 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 SO42- 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 (Figure 1) and the isotopic results (Figure 2) are consistent with the mineralization of organic S being a major contributor to the SO42-  in stream waters at the HBEF.

Table 1a. Statistical values of the d34S values in bulk precipitation and stream water at the HBEF.
Sample Type
Location
Number of samples
d34S ‰
 
Variance
Max.-Min. (range)
Bulk Precipitation Open site adjacent to laboratory
25
4.5
0.5
6.9 - 3.5 (3.4)
Bulk Precipitation Adjacent to W6
53
4.2
0.3
6. 8 - 3.2 (3.6)
Stream W5
55
3.2
0.1
3.7 - 2.4 (1.3)
Stream W6
54
3.7
0.1
4.9 - 3.3 (1.6)
 
 


Table 1b. Statistical values of the SO42- concentrations in bulk precipitation and stream water at the HBEF. Sample values were obtained by volume weighting weekly values of SO42- concentrations.

 Sample Type
Location
Number of samples
SO42-  µmol l-1
 
variance
Max. - Min. (Range)
Bulk Precipitation Open site adjacent to laboratory
52
25
61
50 -12 (38)
Bulk Precipitation Adjacent to W6
53
24
43
40 - 10 (30)
Stream W5
52
56
50
71 - 34 (37)
Stream W6
52
58
39
73 - 48 (25)
 


Diagram of S inputs and pools         Diagram of S concentrations and fluxes
Figure 1. Summary of inputs, pools and processes that affect d34S values of SO42- for an undistubed watershed at Hubbard Brook Experimental Forest from 1968 ythrough 1993. Units are d34S ‰         Figure 2. Summary of SO42- concentrations, fluxes (kg S ha-1 for 26 years) and pools (kg S ha-1) for an undisturbed watershed at Hubbard Brook Experimental Forest from 1968 through 1993.


References:

Alewell, C., M. Mitchell, G. Likens and R. Krouse. 1999. Sources of stream sulfate at the Hubbard Brook Experimental Forest: long-term analysis using stable isotopes. Biogeochemistry 44: 281-299.

Mitchell, M.J., B. Mayer, S.W. Bailey, J.W. Hornbeck, C. Alewell, C.T. Driscoll and G.E. Likens. 2001. Use of stable isotope ratios for evaluating sulfur sources and losses at the Hubbard Brook Experimental Forest. Proceedings of Acid Rain 2000, Japan. Water, Air and Soil Pollution 130: 75-86.