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A MODEL TO SIMULATE THE RESPONSE OF A NORTHERN HARDWOOD FOREST ECOSYSTEM TO CHANGES IN S DEPOSITION
Author(s) -
Gbondo-Tugbawa Solomon S.,
Driscoll Charles T.,
Mitchell Myron J.,
Aber John D.,
Likens Gene E.
Publication year - 2002
Publication title -
ecological applications
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.864
H-Index - 213
eISSN - 1939-5582
pISSN - 1051-0761
DOI - 10.1890/1051-0761(2002)012[0008:amtstr]2.0.co;2
Subject(s) - biogeochemical cycle , environmental science , watershed , experimental forest , deposition (geology) , ecosystem , abiotic component , hydrology (agriculture) , forest ecology , leaching (pedology) , forest floor , biogeochemistry , terrestrial ecosystem , ecology , soil water , environmental chemistry , soil science , chemistry , geology , biology , paleontology , geotechnical engineering , machine learning , sediment , computer science
Watershed studies across the northeastern United States have shown that stream losses of SO 4 2− 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 SO 2 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 SO 4 2− 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 SO 4 2− 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 SO 4 2− 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 SO 4 2− concentrations and enhanced the incorporation of S in adsorbed SO 4 2− 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 NO 3 − leaching and associated acidification following this disturbance increased adsorption of SO 4 2− to soil. Compared to the reference watershed, stream‐water SO 4 2− concentrations were slightly higher, and soil adsorbed SO 4 2− 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 34 S 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 SO 4 2− 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.