Wetland Carbon and Environmental Management. Группа авторов

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Wetland Carbon and Environmental Management - Группа авторов


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phenolics (C). Lower inhibition causes higher hydrolase activity (D) and organic matter decomposition (E) releasing CO2 and nutrients (F), which can feed back on microbial activity (G) through pH effects related to CO2 production and nutrition effects due to release of nitrogen, phosphorus, and other nutrients.

      Source: Modified from Fenner & Freeman (2011).

       Physical protection.

      Organic matter can be physically protected from decomposition through chemical associations with mineral surfaces, by being physically inaccessible in soil pores, or as a result of encapsulation by humic materials. The importance of these mechanisms has been well‐illustrated by studies in terrestrial soils and marine sediments, where organic matter associated with mineral particles can be preserved for thousands of years yet is rapidly mineralized once desorbed (Keil et al., 1994; Nelson et al., 1994). For wetlands, more than half of the soil carbon pool could be protected by minerals at low soil carbon concentrations, but the mineral‐protected fraction necessarily drops as soil carbon concentrations increase (Needelman, Emmer, Emmett‐Mattox et al., 2018).

      Organic matter can be chemically stabilized through sorption and coprecipitation with Fe(III) (oxyhydr)oxides (Kaiser & Guggenberger, 2000; Lalonde et al., 2012) or by forming a non‐crystalline floc with Fe2+ (Henneberry et al., 2012). For example, the aeration of porewater from a fen removed 27% of the dissolved organic carbon (DOC) due to coagulation with newly formed Fe(III) hydroxides (Riedel et al., 2013) and salt marsh soils had up to 50% of their soil organic carbon stabilized due to associations with Fe(III) (oxyhydr)oxides (Cui et al., 2014). Protection by iron helps explain why lignin is preferentially preserved in wetland soils. Iron minerals strongly sorb phenolic molecules (Riedel et al., 2013) and inhibit the mineralization of lignin but not that of bulk soil organic matter (Hall et al., 2016). At redox interfaces like the wetland plant rhizosphere, there is dynamic redox cycling of Fe (e.g., Weiss et al., 2004) where the microbial and chemical dissolution of Fe(III) can release sorbed carbon into solution (Chin et al., 1998; Knorr, 2013). However, many wetlands contain solid‐phase Fe(III) as a coating on vascular plant roots, in shallow soils where atmospheric O2 penetration occurs, and as Fe‐rich concretions (R. M. Chambers & Odum, 1990; Duan et al., 1996; Emerson et al., 1999; Mendelssohn et al., 1995). While there is an overall decline in Fe(III) with increasing soil depth (Cutter & Velinsky, 1988; Griffin et al., 1989), oxidized iron can persist under anaerobic conditions over geologically relevant timescales (Haese et al., 1997). We have focused here on the preservation of organic carbon, but wetlands can contain measurable amounts of inorganic carbon in the form of siderite (FeCO3) (Duan et al., 1996; Hansel et al., 2001; T. Wang & Peverly, 1999).

      Lastly, proteins and amino acids can become encapsulated in humic acids and protected from hydrolysis. In soils and sediments, humic acid fractions can be hundreds or thousands of years old yet have high concentrations of amide and amino nitrogen, forms of organic matter which are often highly reactive (e.g., Hedges & Keil, 1995; Knicker et al., 1996; Mahieu et al., 2002; Zang et al., 2000). The humic acids may be forming a micelle‐like structure that traps reactive organic molecules within the hydrophobic interior of the structure (Zang et al., 2000), which is consistent with observations that hydrophobic organic contaminants also have a high affinity for humic acids (De Paolis & Kukkonen, 1997). This protective mechanism may be most important at low pH where humic acids form structures with a lower surface–volume ratio (versus a chainlike structure at higher pH), which enhances the ability of the humic materials to physically trap organic matter (Myneni et al., 1999). Given the low pH of many peatlands and their general paucity of mineral matter, the encapsulation of organic matter by humic acids in peat is likely to be more important than interactions with aluminosilicate clays or iron minerals.

       pH.

       Temperature.

      Temperature affects the efficiency of carbon preservation through several related mechanisms. Firstly, biological processes such as decomposition generally slow down at cooler temperatures, as demonstrated for multiple indices of decomposition including litter decay, soil enzyme activities, biological oxygen demand, CO2 and CH4 production and emissions to the atmosphere, and the hydrologic export


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