7.4: Organic matter storage in wetlands
Organic Matter Decomposition
The organic matter generated by plants can be mineralized or accumulate in the soil matrix as sequestered organic carbon and other elements (Elsey-Quirk & Cornwell, 2022). Slow cycling of organic resources may indicate inherent resistance to enzymatic attack but may also be due to environmental factors, e.g., moisture, pH, temperature, particle size, and nutrient and oxygen availability (Li et al., 2022). Natural polymers (e.g., lignin, cellulose, hemicellulose, pectin) can bind to inorganic ions, clay, or other organic residues that protect these materials from degradation. Plant detritus can also be naturally enclosed to other molecules, e.g., cellulose is usually tightly associated with lignin, which limits cellulase access.
As oxygen-dependent peroxidases and dioxygenases primarily degrade lignin, lignified cellulose is not readily decomposed in anaerobic environments. Other oxidants, such as nitrate, metal oxides or sulfate (e.g., Equations 1 and 2, 4, 5, and 8, respectively) do not seem to promote anaerobic degradation of these resources, which leads to lignocellulose accumulation in water-saturated soils and sediments, as bogs (Kirk, 2004).
The storage of organic matter in soils and sediments constitutes an essential reservoir of carbon in the biosphere (Fenchel, et al., 2012). Storage is derived from balancing detritus input and mineralization (Six & Jastrow, 2002; Kirk, 2004). Element turnover is often quantified as mean residence time (MRT) or half-life (t½). The MRT of an element is defined as: (i) the average time the element resides in the medium at a steady state or (ii) the average time required to replenish the contents of the element at a steady state completely. The half-life of soil organic matter (Soil OM) is the time required for decomposing 50% of the currently existing stock.
The typical model used to describe the dynamic behavior or turnover of the Soil OM is the first order model, which assumes a constant (zero-order) input with a proportional (constant) mass loss per unit of time. For Soil OM, the MRT = 1⁄k; where k = coefficient of mass loss of organic matter (t-1); Six & Jastrow (2002). Depending on latitude, humidity, management, soil type and composition and method used for evaluation, the MRT of Soil OM ranges from tens to thousands of years (Stevenson, 1994; Six & Jastrow, 2002; Torn et al., 2009).
Evidence suggests that the persistence of organic carbon oxidation in the environment (soil and sediment) is determined by the interaction between substrates, microbial communities, and abiotic conditions. Therefore, organic matter turnover must be seen as dependent on microbial ecology and the state of a specific environment. Varying degrees of structural organization, microbial ability, and resource constraints within a given environment (soil aggregate, soil horizon) make it likely that identical organic compounds can be recycled at different rates due to variations in driving force (i.e., controlling environmental variables). Soil organic matter can be classified as a reservoir of reduced carbon in different states (Kleber, 2010). This categorization leads to observing the structural heterogeneity of the Soil OM and organic detritus. On a carbon basis, it is possible to distinguish that decomposing resources are formed by labile organic carbon (LC), soluble organic carbon (which will be released in dissolved form; DOC) and refractory particulate organic carbon (POCR; comprising basically of structural compounds); Bianchini Júnior & Cunha-Santino (2011)
Inputs and Exports of Organic Matter
Organic matter enters wetlands from various sources (i.e., point and diffuse) and in different ways (e.g., functions: impulse, step, linear, exponential, and periodic). The atmospheric sources (precipitation and dry deposition) that enter the air-water interface are partially dependent on the surface size of the aquatic environment and the amount of rainfall. In wetlands, detritus additions are naturally (and periodically) linked to: (i) the seasonality of the climate; (ii) GHG emissions (Bloom et al., 2012) and (iii) hydrological dynamics (Baker et al., 2009; Gilvear & Bradley, 2009; Grootjans & Van Diggelen, 2009).
Seasonal additions of detritus tend to be more evident in temperate than in tropical environments, where seasonal temperature variations are less intense (Gonçalves Junior et al., 2014). Hydrological dynamics (e.g., variations in aquifer height, fluviometric variations, runoff ), which define the frequency and duration of floods, are an important source of allochthonous detritus (Barker & Maltby, 2009; Dise, 2009). Detritus enters a wetland from various sources and in several different ways. Sources of adduction related to the hydrological cycle, such as mass transported by flow or precipitation can often be characterized by periodic functions. The general pattern of high spring/summer runoff, with the relatively low and constant flow for other seasons, is repeated in a very predictable way.
The origin of the detritus (autochthonous or allochthonous) can also interfere with cycling rates, with allochthonous resources being normally more inaccessible since potentially labile fractions have already been consumed, remaining detritus refractory fractions (Thurman, 1985; Gimenes et al., 2010). In wetlands, hydrology alters many variables related to detritus cycling, e.g., humidity depends on the flood regime, running water carries oxygen and nutrients, while in stagnant water, oxygen is quickly depleted, and nutrients are transformed into less available forms. Hydrological changes induced by climatic and anthropogenic disturbances can also define the rates and predominant composition of GHG emissions; for example, drainage lowers the water table and raises the oxygen content of the soil, increasing \(CO_2\) emissions. In temperate wetlands, the highest emissions were found where the water level remained close to the soil surface, suggesting that mainly litter and not burial organic matter contributes to \(CH_4\) emission (Wang et al., 2021). \(CH_4\) emissions from drained wetland soils are generally negligible because soil carbon is preferentially oxidized to \(CO_2\) (Hiraishi et al., 2014).
Three different reactions generate \(CH_4\) under strictly reductive conditions (Boon, 2006). The first uses \(CO_2\), acetate (HCOO-), or carbon monoxide (CO) to produce \(CH_4\) (Equation \(\PageIndex{1}\)). In the second reaction, \(CH_4\) can be produced by the reduction of the methyl group of methyl compounds, such as methanol (Equation \(\PageIndex{2}\)). In the third reaction, \(CH_4\) is produced by the breakdown of acetate into methane and carbon dioxide (Equation \(\PageIndex{3}\)).
\[\begin{array}{c}\mathrm{CO}_2+4 \mathrm{H}_2 \rightarrow \mathrm{CH}_4+2 \mathrm{H}_2 \mathrm{O} \\ +\operatorname{Energy}\left(\Delta \mathrm{G}_0{ }_0=-31.3 \mathrm{kcal} \mathrm{mol}^{-1}\right) \end{array}\]
\[\begin{array}{c}4 \mathrm{CH}_3 \mathrm{OH} \rightarrow 3 \mathrm{CH}_4+\mathrm{CO}_2+2 \mathrm{H}_2 \mathrm{O} \\ + \text { Energy }\left(\Delta \mathrm{G}_0{ }_0=-76.2 \mathrm{kcal} \mathrm{mol}^{-1}\right) \end{array}\]
\[\begin{array}{c} 4 \mathrm{CH}_3 \mathrm{COO}^{-}+\mathrm{H}_2 \mathrm{O} \rightarrow 3 \mathrm{CH}_4+\mathrm{HCO}_3^{-} \\ + \text {Energy }\left(\Delta \mathrm{G}_0{ }_0=-7.4 \mathrm{kcal} \mathrm{mol}^{-1}\right)\end{array}\]
Anaerobic mineralization bioassays indicated that \(CO_2\) is the main product, instead of \(CH_4\) (Romeiro & Bianchini Júnior, 2006; Cunha-Santino & Bianchini Júnior, 2013; Bianchini Júnior & Cunha-Santino, 2016). The controlling factors of \(CH_4\) production are: (i) availability of electron acceptors (Segers, 1998); (ii) quantity and quality of the organic matter supply (Bianchini Júnior et al., 2010); (iii) temperature (Romeiro & Bianchini Júnior, 2008); (iv) pH (Kiene, 1991; Cunha-Santino et al., 2006; Bloom et al., 2012); (v) coenzymes and prosthetic groups (Schlegel, 1997) and (vi) micronutrients (Banik et al., 1996; Basiliko & Yavitt, 2001). Interactions between these abiotic factors that influence metabolic pathways in generating of specific intermediate products can influence \(CH_4\) production (Bergman et al., 1999). The intermediate compounds (e.g., methanol, propanol, formic acid, butyric acid) and acetate (Equation 34) is the main substrate for methanogenesis (Boone, 1991; Conrad, 1999).
The physical and biotic structure and resulting metabolism of a wetland ecosystem are tightly coupled to hydrological and chemical loads from the watershed (Wetzel, 2006). The importance of hydrology for the export of organic carbon is evident; in general, higher export rates are expected from wetlands that are open to water flow. Riparian wetlands provide large amounts of organic detritus to streams, including coarse detritus. It is evidence that watersheds that drain wetlands export more organic material but maintain more nutrients than watersheds that have no wetlands (Mitsch & Gosselink, 2015). In wetlands, the export of organic matter is predominantly associated with dissolved organic matter derived from relatively recalcitrant chemical compounds, often associated with the origin of lignin and cellulose structural tissues of higher plants and various products of bacterial degradation (Wetzel, 2006).
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Excerpted from
Cunha-Santino, M. B. D., & Bianchini Júnior, I. (2023). Reviewing the organic matter processing by wetlands. Acta Limnologica Brasiliensia , 35 , e19. Accessed December 2023 from https://www.scielo.br/j/alb/a/ypwb635W6PGrxXSZWPtgt4S CC-BY