Wetlands, organic matter

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. 

Denier van der Gon HAC, Neue HU. Influence of organic matter incorporation on the methane emission from a wetland rice field. Global Biogeochem. Cycl. 1995 9 11— 22. 

Biomass can provide substitutes for fossil fuels as well as electricity and heat. Its resource base is varied. Arid land, wetlands, forest, and agricultural lands can provide a variety of plants and organic matter for biomass feedstock. 

The principal distinguishing feature of wetland soils is that they develop under predominantly anoxic conditions. Although anoxia is also sometimes found in other ecosystems, it prevails in wetlands and dominates soil properties. Because of the very large organic matter content of some wetland soils, a rough separation into organic and mineral types based on organic matter content is a useful delineation. 

The USDA (1999) defines organic wetland soils as having an organic carbon content of at least 12 % if the mineral fraction has no clay, 18 % if > 60 % clay, or 12-18 % if < 60 % clay. Further differentiation is based on the botanical origin of the organic matter-whether mosses, herbaceous plants, or woody plants-and its state of decomposition fibrists contain predominantly recognizable, little-decomposed plant debris, saprists predominantly well-decomposed plant debris. 

At the boundary between uplands and wetlands there is, in some circumstances, an interaction between organic matter accumulation in sediments and the development of wetland conditions. Some level of organic matter accumulation is required to drive anaerobic metabolism. But also, because, in general, well-decomposed organic matter improves the water holding capacity of mineral soils, particularly in medium to coarse texmred sediments, and particularly if the clay mineralogy is dominated by low activity kaolinitic clays, there is a feedback between organic matter accumulation and the extent and duration of water saturation. 

AG° = -17.7kJmor at pH 7. Consequently the microbes mediating the decomposition derive less energy and produce fewer cells per unit of carbon metabolized. The accnmnlation of organic matter in marshes and peat bogs illns-trates this point. (Bnt note the rarity of tropical wetland soils with large organic matter contents, discnssed in Section 3.7.)... 

Wetland remediation involves a combination of interactions including microbial adsorption of metals, metal bioaccumulation, bacterial oxidation of metals, and sulfate reduction (Fennessy Mitsch, 1989 Kleinmann Hedin, 1989). Sulfate reduction produces sulfides which in turn precipitate metals and reduce aqueous metal concentrations. The high organic matter content in wetland sediments provides the ideal environment for sulfate-reducing populations and for the precipitation of metal complexes. Some metal precipitation may also occur in response to the formation of carbonate minerals (Kleinmann Hedin, 1989). In addition to the aforementioned microbial activities, plants, including cattails, grasses, and mosses, serve as biofilters for metals (Brierley, Brierley Davidson, 1989). 

Detrital iron (oxy)(hydr)oxides, organic matter, and other arsenic-bearing materials in sediments may be transported by water or wind into wetlands and contribute arsenic to peats. Once buried, reductive dissolution releases sorbed arsenic from iron (oxy)(hydr)oxides. Under sulfate-reducing conditions, the arsenic coprecipitates in sulfide minerals or organic matter. During diagenesis, additional arsenic may be released from the organic matter and coprecipitate in sulfide minerals (Eskenazy, 1995), 253. 

Kalbitz, K. and Wennrich, R. (1998) Mobilization of heavy metals and arsenic in polluted wetland soils and its dependence on dissolved organic matter. Science of the Total Environment, 209(1), 27-39. 

At whole lake scales, littoral zones are a major component of autochthonous DOM production and important sources of labile organic matter for aquatic bacteria. Of the approximately one billion lakes in the world, the littoral zone accounts for more than 95% of lake surface area in nearly 99.8% of all lakes (Wetzel, 1983 Fig. 3a). The importance of shallow waters is even more marked when the bounds of lakes are expanded to include wetlands. With such an expanded view, the littoral zone and wetlands comprise more than 95% of the area in 99.999% of all lakes (Fig. 3a). Clearly, shallow waters are a dominant feature of lentic ecosystems. 

FIGURE 3 Proportion of lake area accounted for by littoral zones for the world s lakes (a), and the proportion of extracellular (ER) dissolved organic matter inputs derived from littoral zones (b see text for description of the model). The solid lines illustrate relationships in which lake boundaries are restricted to littoral and pelagic zones and the dotted lines illustrate patterns in which lake boundaries are expanded to include adjacent wetlands. In (b), the two sets of lines illustrate the range in the contribution of littoral zones to total lake ER with variation in rates of primary production for phytoplankton (0.1-2.0 kg organic matter m-2 yr 1) and macrophytes (0.6-3.8 kg organic matter nT2 yr 1). The relationship between littoral zone area and number of lakes is from Wetzel (1983). 

McLatchey, G. P., and K. R. Reddy. 1998. Regulation of organic matter decomposition and nutrient release in a wetland soil. Journal of Environmental Quality 27 1268-1274. 

Chin, Y. P., C. R. Swank, S. J. Traina, and D. Backhus. 1998. Abundance and properties of natural organic matter in the pore waters of a freshwater wetland. Limnology and Oceanography 43 1287. 

Bano, N., M. A. Moran, and R. E. Hodson. 1998. Photochemical formation of labile organic matter from two components of dissolved organic carbon in a freshwater wetland. Aquatic Microbial Ecology 16 95-102. 

Jackson, C., C. Foreman, and R. L. Sinsabaugh. 1995. Microbial enzyme activities as indicators of organic matter processing rates in a Lake Erie coastal wetland. Freshwater Biology 34 329-342. 

Direction 2. A large portion, usually >90%, of the organic matter imported from allochthonous and littoral/wetland sources to these aquatic ecosystems is predominantly in dissolved or colloidal form. Although a portion of the dissolved organic compounds may aggregate and shift to a particulate and hence gravitoidal form that may sediment out of the water, most of the imported dissolved organic matter is dispersed within the water... 

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