A model to simulate the response of a northern hardwood forest ecosystem to changes in S deposition

Solomon S. Gbondo-Tugbawa, Charles T. Driscoll, Myron J. Mitchell, John D. Aber, Gene E. Likens

Research output: Contribution to journalArticlepeer-review

28 Scopus citations


Watershed studies across the northeastern United States have shown that stream losses of SO42- exceed atmospheric sulfur (S) deposition. Understanding the processes responsible for this additional source of S is critical to quantifying ecosystem response to ongoing and potential future controls on SO2 emission. An integrated biogeochemical model, PnET-BGC, was used to investigate inputs and dynamics of S in a northern hardwood forest at the Hubbard Brook Experimental Forest (HBEF), New Hampshire, USA. The changes in soil S pools and stream-water SO42- were simulated to assess the response to both atmospheric S deposition and forest clear-cutting disturbances. Model simulation using the measured dry-to-bulk deposition ratio of 0.21 resulted in an underprediction of soil S pools and stream-water SO42- concentrations in the biogeochemical reference watershed (Watershed 6). However, the depiction of biotic processes (e.g., plant uptake, mineralization) in the model reduced the discrepancy in stream SO42- concentration between measured and model predicted value by -50% compared to a previous modeling effort that only considered abiotic processes. Long-term simulations (-150 yr) indicated that elevated anthropogenic S deposition has increased stream SO42- concentrations and enhanced the incorporation of S in adsorbed SO42- and organic S soil pools. Following the implementation of the 1970 and 1990 Amendments to the Clean Air Act, decreases in S deposition resulted in the net release of S from soil pools, including soil organic S. Model simulation of forest clear-cutting of Watershed 5 at the HBEF showed that NO3- leaching and associated acidification following this disturbance increased adsorption of SO42- to soil. Compared to the reference watershed, stream-water SO42- concentrations were slightly higher, and soil adsorbed SO42- pools were substantially higher in the clear-cut watershed 4-5 yr after disturbance. Simulation of stable S isotopes showed that fractionation associated with the mineralization of soil organic S might explain the depletion in 34S observed between throughfall and stream water in the reference watershed. There is a need for further research on: (1) the rates of dry S deposition and (2) the rate of weathering of various minerals and the isotopic composition of these minerals in order to fully assess the discrepancy (i.e., greater export of S than can be accounted for by atmospheric deposition) in watershed S mass balances. The results of forecasts of the future response to anticipated decreases in S deposition are highly dependent on the nature of this additional source of S to forest watersheds. However, the large size and relatively long turnover time of soil organic S pools compared to adsorbed SO442- pools suggest that a model depicting only abiotic processes will not be suitable for predicting the long-term recovery of stream water from acidification by atmospheric deposition in northern forests.

Original languageEnglish (US)
Pages (from-to)8-23
Number of pages16
JournalEcological Applications
Issue number1
StatePublished - 2002


  • Acidification
  • Biogeochemistry
  • Dry sulfur deposition
  • Hubbard Brook Experimental Forest (New Hampshire, USA)
  • Model, simulation of sulfur-deposition effects
  • Organic sulfur mineralization
  • SO emissions
  • Stable sulfur isotopes
  • Sulfate adsorption
  • Sulfur deposition
  • Sulfur weathering
  • Surface-water sulfate

ASJC Scopus subject areas

  • Ecology


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