Carbon Turnover and Removal Mechanisms

3.2 Carbon Turnover and Removal Mechanisms

3.2.1 Carbon Turnover

Various reactions utilizing carbon take place within wetlands. The key processes are respiration in the aerobic zone, fermentation, methanogenesis, and sulfate, iron, and nitrate reduction in the anaerobic zone. Organic matter typically contains between 45 and 50% carbon. Wetlands contain large amounts of dissolved organic matter, promoting microbial activity (Bano et al. 1997; Zweifel 1999). Bacterial oxidation of dissolved organic carbon subsequently results in mineralization, which is a process whereby organic substances are converted into inorganic sub- stances (Hensel et al. 1999).

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Respiration is the biological conversion of carbohydrates into carbon dioxide, and fermentation is the conversion of carbohydrates into chemical compounds such as lactic acid, or ethanol and carbon dioxide. In a wetland, organic carbon is converted into compounds including carbon dioxide and methane or stored in plants, dead plant matter, microorganisms, or peat. A significant part of the BOD may be particle-bound and, therefore, susceptible to removal by particulate set- tling (Kadlec et al. 2000; Kayranli et al. 2010).

3.2.2 Carbon Components

Wetlands contain five main carbon reservoirs: plant biomass carbon, particulate organic carbon, dissolved organic carbon, microbial biomass carbon, and gaseous end products such as carbon dioxide and methane. The latter four are present in water, detritus, and soil (Kadlec and Knight 1996). Wynn and Liehr (2001) out- lined a carbon cycle comprising the following key components: plant biomass, standing dead plants, particulate organic carbon, dissolved organic carbon, and refractory carbon (i.e., resistant carbon, which would retain its strength at high temperatures). These carbon reservoirs can be used in the description of carbon cycles (Kayranli et al. 2010).

Active biomass may comprise wetland plants and periphyton (microorganisms and detritus attached to submerged surfaces) and contributes to the transformation of inorganic carbon such as carbon dioxide into organic carbon through photosyn- thesis. The productivity of wetlands varies due to the time of year, geographic location, nutrient status, and type of vegetation. Particulate organic carbon consists of decaying plant matter, microbial cells, particulate influent, and particulate or- ganic substances found on the soil surface. Dissolved organic carbon comprises dissolved BOD and other carbon components in solution. While dissolved organic carbon typically represents <1% of the total organic carbon in soil, it represents approx. 90% of the total organic carbon in surface waters (Kadlec and Knight 1996; Wynn and Liehr 2001; Reddy and Delaune 2008). Microbial biomass car- bon occurs in heterotrophic microfloral catabolic activities, transforming organic carbon (energy reserve of the ecosystem) back into inorganic carbon and mineral- izing particulate organic carbon and dissolved organic carbon (D’Angelo and Reddy 1999; Picek et al. 2007). The turnover of active biomass happens relatively quickly, usually in the order of days, while the corresponding turnover of soil or- ganic matter takes decades. Soil microbial biomass can be regarded as a signifi- cant carbon sink (Kayranli et al. 2010).

3.2.3 Carbon Removal Mechanisms

Carbon processing in the wetland environment is complex, and the various decom- position reactions take place in different horizons; e.g., respiration and methane

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oxidation occur in the aerobic zones while methanogenesis occurs in anaerobic zones (Knight and Wallace 2008). However, the highest rates of decomposition are found closest to the wetland surface where there is an elevated input of fresh litter and recently synthesized labile organic matter (Sherry et al. 1998).

The organic matter content within wetland systems is impacted by processes such as biodegradation, photochemical oxidation, sedimentation, volatilization, and sorption. Some of these mechanisms provide natural organic matter accumula- tion via microbial or vegetative decay (Burgoon et al. 1995; Reddy and D’Angelo 1997; Stottmeister et al. 2003; Quanrud et al. 2004; Li et al. 2008). Moreover, the accumulation of organic matter is a potential energy source for microbial commu- nities (Turcq et al. 2002; Reddy and Delaune 2008).

Dissolved organic matter degradation is expected to occur via heterotrophic up- take by aerobic and anaerobic bacteria, and degradation by ultraviolet light. Sev- eral authors have reported on dissolved organic matter transformations in algae (Kragh and Søndergaard 2004), forest vegetation (Li et al. 2008), Typha spp. wet- land plant material (Pinney et al. 2000), microbial groups (Ibekwe et al. 2003; Li et al. 2008), and soils (Qualls and Haines 1992). Dissolved organic matter from plant exudates appears more dominant during warm months with active plant growth (Pinney et al. 2000).

Organic matter accumulates when primary productivity is faster than the cor- responding decomposition rate, leading to a net accumulation of organic matter (Mitsch and Gosselink 2007). Due to slow organic mater decomposition rates, strata are built up and compressed to form different soil layers. Organic matter from inflow and wetland plants is accumulated, decomposed, and subsequently buried in the system. This results in a shift from aerobic to anaerobic processes due to lack of oxygen in the wetland sediment, which drastically reduces decom- position rates (Holden 2005). It is also believed that some parameters such as temperature, organic matter quality, residence time of organic matter in the water column, vegetation pattern, wetland maturity, sedimentation rate, sediment tex- ture, and sediment reworking impact the organic matter decomposition within the water body and the organic matter compositions (Borman et al. 1995; van der Peijl and Verhoeven 1999; Barber et al. 2001; Savage and Davidson 2001; Turcq et al. 2002; Yu et al. 2002; Lafleur et al. 2005; Wolf and Wagner 2005; Shepherd et al. 2007; Yurova and Lankreijer 2007). Furthermore, organic matter compositions consist of labile and resistant fractions within the soil profile. Many of the labile compounds are accumulated on the sediment surface and decomposed within

a few months (Schlesinger 1997; Wolf and Wagner 2005).

Wetlands have aerobic and anaerobic interfaces in water, soil, and the accu- mulated organic matter (Scholz et al. 2007). Gaseous end products are formed under anaerobic and aerobic conditions. Under anaerobic conditions, carbon dioxide and methane are formed through the decomposition of organic matter. In comparison, under aerobic conditions, only carbon dioxide is formed. Previous researchers (Kadlec and Knight 1996; Scholz 2006; Mitsch and Gosselink 2007) pointed out that the aerobic respiration in wetland systems is far more effective

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with respect to organic matter degradation than anaerobic processes such as fer- mentation and methanogenesis.

The dissolved organic carbon cycle depends on the cycling and bioavailability of phosphorus and nitrogen (Craft and Richardson 1998) as well as on the bioavailability and transport of metals (Voelker and Kogut 2001; Tipping and Center 2002). Microbial death is generally assumed to only contribute to particu- late organic matter and not to dissolved organic matter because most bacteria in wetlands are associated with plant litter and soil organic matter. When the grow- ing season reaches its end, approx. 15% of the plant carbon disappears due to leaching and physical degradation in temperate climates (Kadlec and Knight 1996). The remainder degrades over approx. 1 year and becomes predominantly particulate carbon (Johnston 1991; Wynn and Liehr 2001).