Carbon dioxide wetland soils

Dead vegetation also afreets the global carbon cycle. Dead organic matter decomposes, releasing carbon dioxide to the atmosphere. Rates of decomposition vary with material, location, and climate. Non-woody organic matter decomposes rapidly woody organic matter slowly. Decomposition tends to occur faster at the soil surface than below. Decomposition is relatively fast in warm moist climates. In cold climates and in wetlands, decomposition is so slow that there is a net increase of stored carbon in the soil and organic soils called, "histosols, are formed. 

The substrate-indnced respiration (SIR) method was developed as a rapid means of estimating microbial activities in soils (Beare et al., 1990 Neely et al., 1991). The use of selective inhibitors such as streptomycin for bacteria and cycloheximide for fungi, in conjunction with substrate additions, has been used to quantify bacterial and fungal contributions to the total metabolism of microbial decomposers. The SIR procedure involves addition of a labile carbon source (e.g., glucose or acetate) to provide a carbon nonlimiting condition. The short-term increase in carbon dioxide production is proportional to the active microbial biomass and activity. The concept of addition of labile carbon to determine the kinetics of substrate utilization by microorganisms has been extensively studied in various ecosystems including wetlands and aquatic systems. 

Methane and carbon dioxide produced in soils are transported into the atmosphere by diffusion and mass flow via two pathways (1) the aerenchyma tissues of plant roots and stems and (2) flux from soil to the overlying water column (Figure 5.61). Gas exchange in plants is discussed in detail in Chapter 7. Carbon dioxide is highly soluble and undergoes various chemical reactions, and it may be difficult to estimate flux accurately without considering aU associated reactions. Because of the potency (on molecule-to-molecule basis, methane absorbs 25 times as much infrared radiation as carbon dioxide) of methane as greenhouse gas, we will focus our discussion on methane emissions from wetlands. 

Globally, extensive areas of peatlands and wetlands have been drained and converted into agricultural lands. Drainage of organic matter-rich soils accelerated the decomposition process and emission of carbon dioxide. Many peatlands that have accumulated organic matter for centuries... 

FIGURE 10.21 First-order Fe(III) reduction rate constants and initial cumulative carbon dioxide and methane production in a wetland soil amended with different amounts of labile organic matter. (Redrawn from Roden and Wetzel, 2002.)... 

Iron and manganese reduction in wetland soils and aqnatic sediments is linked to organic matter decomposition. In the absence of oxygen, microbial commnnities nse a wide range of electron acceptors, inclnding nitrate, Mn(IV), Fe(III), sulfate, carbon dioxide, and several simple organic componnds. 

The redox potential-pH stability diagram (Figure 12.11) indicates that between pH 7 and 8, zinc carbonate (ZnCOj) is formed when the concentration of dissolved carbon dioxide (CO2) is 10 mol L . At low redox values, zinc sulfide is the most stable combination. Zinc precipitation by the hydrous metal oxides of manganese and iron is the principal control mechanism for zinc in wetland soils and freshwater sediments. The occurrence of these oxides as coatings on clay and silt enhances their chemical activity in excess of their total concentration. The uptake and release of the metals is governed by the concentration of other heavy metals, pH, organic and inorganic compounds, clays, and carbonates. 

Soil redox potential (Eh) and the pH parameters are closely related. Production of carbon dioxide, an end product of the reduction of oxygen, has considerable influence on the soil s pH. When a reducing wetland soil system becomes oxidized, its pH may decrease drastically due to the oxidation of iron to Fe(lll) and the subsequent hydrolysis of the iron or the oxidation of sulfite to sulfate, which is accompanied by the release of protons. Lowering of the Eh of the soil due to flooding will result in a rise of pH, because many reduction reactions (such as the reduction of sulfate to sulfide, Ee to Fe, and Mn + to Mn +) involve the uptake of protons or the release of hydroxyls. 

Nutrients incorporated into herbaceous material are deposited on soil surface or exported from the wetland as detritus or dissolved nutrients released by decaying vegetation. Air-water exchange also plays an important role in biogeochemical cycling of carbon, nitrogen, and sulfur. Wetlands emit methane, carbon dioxide, nitrous oxide, and reduced sulfur gases to the atmosphere. 

Hydrology generally determines the rates of aerobic and anaerobic decomposition in wetland soils (see Chapter 5). Lowering of the water level of the highly organic wetland soils will increase decomposition rates and elevate fluxes of carbon dioxide to the atmosphere (IPCC,... 

Emission of CH4 from soils to the atmosphere is a balance between methane oxidation, production, and transport within the soil systems (Chan and Parkin, 2000 Bradford et al., 2001). Methane is released from anaerobic wetland soils to the atmosphere through diffusion of dissolved methane, through ebullition of gas bubbles, and through wetland plants that develop aerenchyma tissue (Figure 16.1). Large portions of methane formed in an anaerobic soil remain trapped in the flooded soil. Entrapped methane can be oxidized to carbon dioxide when the floodwater is drained or when the soil dries. Entrapped methane can escape to the atmosphere immediately after the floodwater is removed or recedes. 

---