Wetlands as Carbon Sinks

3.3.2 Wetlands as Carbon Sinks

The results indicate that wetlands are vital carbon sinks. The majority of studies on carbon sequestration within wetlands focus on sediment, soil, and living plant communities (Krogh et al. 2003; Brevik and Homburg 2004; Bedard-Haughn et al. 2006; Euliss et al. 2006; Alongi et al. 2007).

Freeman et al. (2001a, 2004) pointed out that phenol oxidase is the main en- zyme that by remaining inactive, keeps much of the world’s terrestrial wetland carbon locked up. Actinomyces (filamentous, mostly anaerobic microorganisms of this genus), bacteria, and certain fungi are direct indicators of decomposer activity; they excrete extracellular enzymes to decompose complex high-molecular-weight compounds. The activity of phenol oxidase increases with increasing temperatures (Freeman et al. 2001b) and is reduced by low pH (Pind et al. 1994; Williams et al. 2000). Williams et al. (2000) studied phenol oxidase activity in Sphagnum spp. peat and reported that when pH was favorable, the activity of phenol oxidase de- pended more on the botanical composition of the peat and the wetland vegetation type than on the water level. Wetland carbon stores (especially peatlands) may become considerable methane sources when aerobic soil conditions activate the phenol oxidase enzyme, which triggers chain reactions breaking down lignin and humic substances, releasing methane into the atmosphere in substantial amounts (Freeman et al. 2004). Some models of small-scale constructed wetlands show that they sequester very small amounts of carbon. However, they are considered size- able carbon sinks due to the difference in energy consumption between the wetland and the equivalent wastewater treatment plant. Thus, small-scale constructed wet- lands used for the treatment of wastewater are considered carbon sinks (Ogden 2001). The main factors controlling methane emissions from wetlands are soil temperature (Christensen et al. 2003), water table depth (Moore et al. 1998), and the amount and quality of decomposable substrate (Christensen et al. 2003). The factors controlling methane oxidation are well documented by Boon and Lee (1997); they include the supply of oxygen and temperature. Methane oxidation rates can be optimized by promoting well-aerated water columns and, in turn, well- aerated sediments. Furthermore, nitrate affects oxidation, but only at relatively high nutrient concentrations, and the availability of ammonium and sulfate has little or no effect on oxidation rates (Kayranli et al. 2010). Hanson and Hanson (1996) documented that wetland soils are normally fully saturated and are often located well below the water table. These wetland conditions create mainly an-

3.3 Are Wetlands Carbon Sources or Sinks? 137

aerobic or anoxic soils, which store carbon dioxide and release methane. However, drained wetlands, which have unsaturated soils, are atmospheric methane sinks. Methane is absorbed through methanotrophs and anaerobic methane-oxidizing bacteria (Kayranli et al. 2010). Some studies identified variable methane fluxes

from wetlands in Canada. For peatlands, bogs, and fens, 2.8 ± 0.27 mg CH 4 −C/m 2 /h was calculated by Turetsky et al. (2002), for bogs and rich fens, between 1 and

10 mg CH 4 –C/m 2 /h was estimated by Bellisario et al. (1999), for fens, bogs, ponds, and palsa (i.e., low and oval rise occurring in polar climates), releases up to 11 mg CH 4 –C/m 2 /h were published by Liblik et al. (1997), and peatlands released up to

15 mg CH 4 –C/m 2 /h according to Moore and Roulet (1995). However, for freshwa- ter wetlands, a flux of only approx. 0.3 mg CH –C/m 2 4 /h was estimated by Bridgham et al. (2006). Concerning wetlands in the USA, Armentano and Menges (1986) estimated fluxes of roughly 5 mg CH 4 –C/m 2 /h for northern peatlands in northern territories and releases of roughly 26 mg CH

4 –C/m /h for Florida. Again high data variability reflects different wetland types located in various climatic regions. On the other hand, Freeman et al. (2004) pointed out that when the water table drops considerably below the peatland surface, peatlands may change from being a source to a sink for methane due to increased methane oxidation. However, drought conditions in a peatland lower methane emissions during the drawdown of the water table due to decreased methanogenesis rather than methane consumption (Kayranli et al. 2010). Flooded wetlands generally sequester carbon dioxide from and release methane into the atmosphere. The combination of these two factors determines whether these offsetting processes make a wetland system an overall contributor to the greenhouse effect. Maximizing permanent vegetation in culti- vated wetlands could provide maximum carbon sequestration, but the overall con- sequences for the gas emissions need to be carefully assessed (Bedard-Haughn et al. 2006). McCarty and Ritchie (2002) claimed that agricultural activity in- creased the rate of carbon storage within the sediment and contributed to the accu- mulation of nutrients within a wetland ecosystem. In comparison, Bedard-Haughn et al. (2006) concluded that organic carbon densities decreased from uncultivated to cultivated wetlands. McCarty and Ritchie (2002) also reported that an agricul- tural field and a riparian ecosystem in Maryland (USA) sequestered between 0.16

and 0.22 kg C/m 2 /a. Moreover, prairie wetlands in the northcentral USA are known to have sequestration values of roughly 0.3 kg C/m 2 /a (Euliss et al. 2006). Wet- lands store approx. twice the organic carbon load in comparison to cropland that is not tilled (Euliss et al. 2006). For example, northern peatlands in Scandinavia are important carbon stores. These peatlands often show large spatial and temporal variation in the atmospheric exchange of carbon dioxide and methane. The main parameters impacting carbon storage within these wetlands are erosion and soil movement (McCarty and Ritchie 2002), excessive drainage (Salm et al. 2009), water discharge, and nutrient input (Turcq et al. 2002).

Most of methane and carbon dioxide fluxes take place in the relatively thin oxic layers near the surface of peatlands. In the oxic surface layers of peatlands, the rates of litter decomposition may not generally differ from those found in the min- eral soil sites for the same litter types (Moore et al. 2002; Vavrova et al. 2009). The large amounts of carbon captured within peatlands and their low productivity

138 3 Carbon Storage and Fluxes Within Wetland Systems

highlight the potential of peatlands to significantly impact regional carbon cycling, particularly at times when climate change might lead to increased peat degradation due to increased temperatures and lower water tables (Gorham 1991; Weishampel et al. 2009). Raghoebarsing et al. (2005) have shown that methane consumption by methanotrophic bacteria living in symbiosis with some Sphagnum species leads to effective in situ methane recycling within peatlands. These findings have also helped to explain the high organic carbon burial within wetland ecosystems. In a subsequent paper, Raghoebarsing et al. (2006) demonstrated that the direct an- aerobic oxidation of methane coupled to denitrification of nitrate was possible. The reactions presented make a substantial contribution to the microbiological methane cycle. Landry et al. (2009) reported that planted wetlands may sequester between 2 and 15 times more carbon than they emit as carbon dioxide. However, respiration by stems and leaves, which was not accounted for in this study, could have reduced the reported carbon sequestration values. Moreover, they observed that methane was the most important greenhouse gas in unplanted wetland sys- tems. They also found that the presence of plants decreased methane fluxes but favored carbon dioxide production. The carbon sequestration potential of swamps is usually much higher than that of lakes. The accumulation of carbon within lake sediments depends on the water table height and on the regional climate. While low carbon storage occurs in drier climates, humid climates bring about high car- bon accumulation within most lakes (Turcq et al. 2002). Based on a wide range of assumptions, Mitra et al. (2005) calculated the net balance between methane pro- duction and carbon sequestration in the world’s wetlands and deduced that the overall impact of wetlands on climate change in the carbon cycle was minimal. High numbers of spatially distinct samples for carbon sequestration (Anderson and Mitsch 2006) and methane generation (Altor and Mitsch 2006) were collected from two created wetlands (Ohio, USA), and subsequent calculations were based on conversions proposed by Mitra et al. (2005). It was found that the created wet- lands were climate neutral or even had a cooling effect.