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The use of mass balances in the investigation of the biogeochemical cycle of sulfur is reviewed for three systems: 1) upland catchments, 2) wetlands, and 3) lakes. In upland catchments, the major inputs of sulfur are via wet and dry atmospheric deposition, whereas outputs or losses occur primarily through volatilization and/or runo. In addition, sulfur may be stored in vegetation and in the forest¯oor. In wetlands (particularly peatlands), a large proportion of the sulfur inputs are derived from surface and groundwater originating in the upland system. Because of the¯uctuating water table in wetlands, they can act as a source or sink for sulfate, depending on the redox conditions. Wetlands, therefore, can signi®cantly aect input-output budgets for lakes. In most lakes, only a small portion of the sulfate input is retained, (i.e. not lost from the lake via out¯ow), indicating that there is an excess of sulfate relative to biological needs. Seepage lakes are exceptions to this generalization. Although the reactivity of the sulfate input to many lakes is low, sulfate levels, especially in regions receiving substantial atmospheric sulfur deposition, are high enough that the portion reduced results in substantial in-lake alkalinity production; in fact, in many cases, alkalinity production from sulfate reduction is greater than that resulting from not only other in-lake processes but from external sources (the catchment) as well.The importance of mass balance investigations in elucidating the biogeochemical cycling of sulfur is stressed and the need for additional studies on a whole-system basis stressed.
The use of mass balances in the investigation of the biogeochemical cycle of sulfur is reviewed for three systems: 1) upland catchments, 2) wetlands, and 3) lakes. In upland catchments, the major inputs of sulfur are via wet and dry atmospheric deposition, whereas outputs or losses occur primarily through volatilization and/or runo. In addition, sulfur may be stored in vegetation and in the forest¯oor. In wetlands (particularly peatlands), a large proportion of the sulfur inputs are derived from surface and groundwater originating in the upland system. Because of the¯uctuating water table in wetlands, they can act as a source or sink for sulfate, depending on the redox conditions. Wetlands, therefore, can signi®cantly aect input-output budgets for lakes. In most lakes, only a small portion of the sulfate input is retained, (i.e. not lost from the lake via out¯ow), indicating that there is an excess of sulfate relative to biological needs. Seepage lakes are exceptions to this generalization. Although the reactivity of the sulfate input to many lakes is low, sulfate levels, especially in regions receiving substantial atmospheric sulfur deposition, are high enough that the portion reduced results in substantial in-lake alkalinity production; in fact, in many cases, alkalinity production from sulfate reduction is greater than that resulting from not only other in-lake processes but from external sources (the catchment) as well.The importance of mass balance investigations in elucidating the biogeochemical cycling of sulfur is stressed and the need for additional studies on a whole-system basis stressed.
To predict emissions of nitrous oxide (N2O) and nitric oxide (NO) from forest soils, we have developed a process-oriented model by integrating several new features with three existing models, PnET, Denitrification-Decomposition (DNDC), and a nitrification model. In the new model, two components were established to predict (1) the effects of ecological drivers (e. g., climate, soil, vegetation, and anthropogenic activity) on soil environmental factors (e. g., temperature, moisture, pH, redox potential, and substrates concentrations), and (2) effects of the soil environmental factors on the biochemical or geochemical reactions which govern NO and N2O production and consumption. The first component consists of three submodels for predicting soil climate, forest growth, and turnover of soil organic matter. The second component contains two submodels for nitrification and denitrification. A kinetic scheme, a so-called "anaerobic balloon," was developed to calculate the anaerobic status of the soil and divide the soil into aerobic and anaerobic fractions. Nitrification is only allowed to occur in the aerobic fraction, while denitrification occurs only in the anaerobic fraction. The size of the anaerobic balloon is defined by the simulated oxygen partial pressure which is calculated based on oxygen diffusion and consumption rates in the soil. As the balloon swells or shrinks, the model dynamically allocates substrates (e. g., dissolved organic carbon, ammonium, nitrate, etc.) into the aerobic and anaerobic fractions. With this approach, the model is able to predict both nitrification and denitrification in the same soil at the same time. This feature is important for soils where aerobic and anaerobic microsites often exist simultaneously. With the kinetic framework as well as its interacting functions, the PnET-N-DNDC model links ecological drivers to trace gas emissions. Tests for validating the new model are published in a companion paper
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