Wetlands (2010) 30:111–124 DOI 10.1007/s13157-009-0003-4
ORIGINAL PAPER
Carbon Storage and Fluxes within Freshwater Wetlands: a Critical Review Birol Kayranli & Miklas Scholz & Atif Mustafa & Åsa Hedmark
Received: 10 February 2009 / Accepted: 4 September 2009 / Published online: 9 December 2009 # Society of Wetland Scientists 2009
Abstract We critically review recent literature on carbon storage and fluxes within natural and constructed freshwater wetlands, and specifically address concerns of readers working in applied science and engineering. Our purpose is to review and assess the distribution and conversion of carbon in the water environment, particularly within wetland systems. A key aim is to assess if wetlands are carbon sinks or sources. Carbon sequestration and fluxes in natural and constructed wetlands located around the world has been assessed. All facets of carbon (solid and gaseous forms) have been covered. We draw conclusions based on these studies. Findings indicate that wetlands can be both sources and sinks of carbon, depending on their age, operation, and the environmental boundary conditions such as location and climate. Suggestions for further research needs in the area of carbon storage in wetland sediments are outlined to facilitate the understanding of the processes of carbon storage and removal and also the factors that influence them. B. Kayranli : M. Scholz (*) : A. Mustafa Institute for Infrastructure and Environment, School of Engineering, The University of Edinburgh, William Rankine Building, The King’s Buildings, Edinburgh EH9 3JL Scotland, UK e-mail:
[email protected] Å. Hedmark Swedish University of Agricultural Sciences, Department of Forest Products, Box 7008, 750 07 Uppsala, Sweden Å. Hedmark Air Policy Unit, Corporate Office, Scottish Environment Protection Agency, Erskine Court, Castle Business Park, Stirling FK9 4TR Scotland, UK
Keywords Carbon dioxide . Constructed wetland . Global warming . Greenhouse gases . Methane . Peatland
Introduction Wetlands and Processes Wetlands are areas of water saturated soil, and include small lakes, floodplains, and marshes. Wetlands only cover a small proportion of the earth’s land surface (approximately between 2% and 6%, depending on definitions), but contain a large proportion of the world’s carbon (approximately 15×1014 kg) stored in terrestrial soil reservoirs (Schlesinger 1991; Amthor et al. 1998; Whitting and Chanton 2001). Wetlands play an important role in carbon cycling because they represent 15% of the terrestrial organic matter losses to the oceans (Hedges et al. 1997; Stern et al. 2007). Among all terrestrial ecosystems, they have the highest carbon density. Furthermore, wetlands are a diffuse source of humic substances for some receiving freshwater systems (Stern et al. 2007). Decomposition within wetlands is a complicated process as it involves aerobic and anaerobic processes. Organic matter decomposition is often incomplete under anaerobic conditions. The lack of oxygen is therefore the main factor determining plant detritus turnover. Consequently, plant remains coming from the inflow, the wetland biomass, and/ or from the vegetation growing along the wetland margins accumulate within the wetland system, and different decomposition stages can be identified (Gorham et al. 1998; Collins and Kuehl 2001; Holden 2005). A net retention of organic matter and plant detritus can be observed in most wetlands (Mitsch and Gosselink 2007). Organic matter accumulation in wetland sediments depends
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on the ratio between inputs (organic matter produced in situ and ex situ) and outputs. The latter may be due to the decomposition under waterlogged conditions, erosion due to high precipitation, and soil disturbance in general (Gorham et al. 1998). Since 1980, treatment wetland systems have gained popularity, and have been applied successfully for the treatment of numerous waste streams (Kadlec et al. 2000; Haberl et al. 2003; Zhang et al. 2005; Vymazal 2007) and runoff from urban areas (Scholz 2006), farm yards (Carty et al. 2008), and log yards (Hedmark and Scholz 2008). The concept of constructed wetlands applied for the purification of wastewaters has received growing interest because most of these systems are easy to use, require only little maintenance and have low construction costs (Machate et al. 1997). Dissolved organic matter is a very important water quality parameter associated with the performance of treatment wetland systems. Some microorganisms including bacteria use dissolved organic matter as an energy source for processes such as denitrification. However, too high levels of dissolved organic matter can prevent light penetration within the water column (Pinney et al. 2000; Li et al. 2008). The treatment efficiencies of wetlands vary depending on climate, vegetation, microorganism communities, and type of wetland system (Waddington et al. 1996; Schlesinger 1997; Joabsson et al. 1999; Trettin and Jurgensen 2003; Whalen 2005; Picek et al. 2007; Ström and Christensen 2007; Weishampel et al. 2009). Scientists have carried out detailed investigations concerning wetland biochemistry and hydrology. Nevertheless, there is no commonly accepted agreement if wetlands are actually carbon sources or sinks. There is disagreement in the interpretation of variables, reactions and the impact of environmental conditions on carbon storage and release. Therefore, recommendations on how to adapt policies and planning processes to enhance carbon storage vary considerably. Comparisons of carbon storage and flux data vary greatly as a function of region and climate.
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On the other hand, wetlands are considered to be greenhouse gas sources, particularly with respect to the emission of methane gas to the atmosphere. Methane has a much higher global warming potential than carbon dioxide, and contributes to the atmospheric sorption of infrared radiation and subsequent warming (Carroll and Crill 1997; Whitting and Chanton 2001; Zhang et al. 2005). Minimizing methane fluxes from created and restored wetlands should therefore be a vital aim in combating climate change. Improved design, construction, and operation of wetlands used for treatment and conservation purposes should therefore help to mitigate global warming by reducing the release of greenhouse gases and enhancing carbon storage at the same time.
Purpose and Review Method Our critical review focuses on the assessment of the key processes determining carbon removal, sequestration, and fluxes within wetlands. It has specifically been written for applied scientists and engineers working world-wide, and complements other more ecology-based papers such as Bridgham et al. (2006) focusing on Northern America. The aims of the key sections are: & & &
To discuss carbon turnover and removal processes within wetlands. To highlight processes where wetlands can be described as carbon sources or carbon sinks. To discuss the effect of global warming on wetlands.
The authors undertook a comprehensive literature search predominantly with the help of the literature data base ISI Web of Knowledge, version 4.6 (www.isiknowledge.com). Grey literature and reports were also assessed, but an attempt was made to find the same information in more easily accessible journal papers.
Global Warming
Carbon Turnover and Removal Mechanisms
Global warming mitigation is becoming increasingly important as the effects of climate change are becoming apparent around the world. Depending predominantly on the meteorological and hydrological conditions, wetlands can absorb carbon dioxide from the atmosphere and capture it within the sediment, and may therefore be greenhouse gas sinks. The high productivity, high water table, and low decomposition rate associated with wetlands lead to carbon storage within the soil, sediment, and detritus (Whitting and Chanton 2001). The process of locking carbon dioxide away from the atmosphere is called carbon sequestration.
Carbon Turnover The major components of the carbon cycle within a wetland are illustrated in Fig. 1. 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,
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Fig. 1 Schematic diagram showing the major components of the carbon cycle
Methane and carbon dioxide fluxes
Air
Particulate and dissolved carbon input
Carbon dioxide uptake
Carbon dioxide Methane and carbon dioxide fluxes
Surface water Decomposition (aerobic) Oxidized soil layer
Photosynthesis and respiration
Carbon output
Decomposition (anaerobic)
Reduced soil layer Sulfate and nitrate reduction Fermentation of dissolved organic carbon (lactic acid and ethanol) and methanogenesis
which is a process where organic matter is converted to inorganic substances (Hensel et al. 1999). Respiration is the biological conversion of carbohydrates to carbon dioxide, and fermentation is the conversion of carbohydrates to 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 and/or stored in plants, dead plant matter, microorganisms, or peat. A significant part of the biochemical oxygen demand may be particle-bound and, therefore, susceptible to removal by particulate settling (Kadlec et al. 2000). 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 (Fig. 1). The latter four are present in water, detritus, and soil (Kadlec and Knight 1996). Wynn and Liehr (2001) outlined 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. 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 to organic carbon through photosynthesis. 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 organic substances found on the
soil surface. Dissolved organic carbon comprises dissolved biochemical oxygen demand and other carbon components in solution. While dissolved organic carbon typically represents
