Organic matter accumulation

Wetlands are characterized by aerobic and anaerobic interfaces in soil and water column and accumulation of organic matter. In wetland ecosystems, the primary productivity often exceeds the rate of decomposition processes, resulting in net accumulation of organic matter. The net accumulation of organic matter is regulated by the activity of various decomposers, including benthic invertebrates, fungi, and bacteria. In a simplistic way, decomposition may be viewed as a three-step process  

Biogeochemistry of Wetlands Science and Applications Detrital plant biomass 

FIGURE 5.8 Decomposition and burial of organic matter and genesis of organic soil. 

Vertical accretion of organic matter using as an indicator in a wetlatnd soil profile (Reddy 

Long-Term Accumulation of Organic Matter in Selected Wetlands 

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Organic matter Accumulation of organic matter Bacteria Primary producers Slowed down decomposition High oxygen consumption High mineralization... 

At the boundary between uplands and wetlands there is, in some circumstances, an interaction between organic matter accumulation in sediments and the development of wetland conditions. Some level of organic matter accumulation is required to drive anaerobic metabolism. But also, because, in general, well-decomposed organic matter improves the water holding capacity of mineral soils, particularly in medium to coarse texmred sediments, and particularly if the clay mineralogy is dominated by low activity kaolinitic clays, there is a feedback between organic matter accumulation and the extent and duration of water saturation. 

Dissolntion and reduction of crystalline Fe(III) minerals is accelerated by chelation with carboxylate ligands in the presence of Fe(ll) (Zinder et al, 1986 Blesa et al, 1987 Phillips et al, 1993 Kostka and Lnther, 1994). Therefore as soil reduction proceeds and carboxylates formed in oxidation of organic matter accumulate in solution together with Fe +, dissolntion and rednction of crystalline Fe(lll) will commence. Dissolution of oxyhydroxide coatings will therefore lag behind the initial reduction of Fe(lll). 

Haslam, S. F. I., Chudek, J. A., Goldspink, C. R., and Hopkins, D. W. (1998). Organic matter accumulation following fires in a moorland soil chronosequence. Global Change Biol. 4, 305-313. 

Of the organic carbon entering the ocean, about 35% accumulated in marine sediments as organic carbon with a molar C/N/P ratio of about 250/20/1 (Mackenzie, 1981) the rest was respired and oxidized to CO2 and evaded the ocean. The nitrogen and phosphorus riverine fluxes of 1 and 0.045 x 1012 moles y1, respectively, represented the nutrient fluxes necessary to support the organic matter accumulation of 11.7 x 1012 moles C y1. 

Martens, C.S., Haddad, R.I., and Chanton, J.P. (1992) Organic matter accumulation, remineralization and burial in an anoxic marine sediment. In Productivity, Accumulation, and Preservation of Organic Matter in Recent and Ancient Sediments (Whelan, J.K., and Farrington, J.W., eds.), pp. 82-98, Columbia University Press, New York. 

Evangelou, V. P., A. D. Karathanasis, and R. L. Blevins. 1986. Effect of soil organic matter accumulation of potassium and ammonium quantity-intensity relationships. Soil Sci. Soc. Am. J. 50 378-382. 

This process is widespread in marine sediments but is most important at the continental margin sediments, where organic matter accumulation is fastest. Sulfate reduction can occur even meters below the sediment/water interface as long as seawater sulfate can diffuse, or be pumped by bioturbation induced through the actvities of sediment-dweUing organisms. The reaction yields the bisulfide anion (HS ), much of which diffuses upward and is reoxidized to SO in oxygenated seawater closer to the sediment surface. However, — 10% of the HS precipitates soluble Fe(II) to yield iron monosulfide ... 

Organic matter accumulates wherever continual or seasonal dominance of water inhibits degradation and humification of plant residues by excluding the air needed for the complete microbial oxidation of the detritus. Even when the net production of phytomass exceeds mineralization only marginally, over thousands of years this condition can result in the development of Histosols. Humic substances occur in both the solid and liquid phases of Histosols, but they have not received the same attention as the corresponding humic substances in soil. 

Sodic (black alkali) soils are a particularly difficult management problem. The water permeability of these soils to water is very slow. The pH of sodic soils is commonly greater than 9 or 9,5, and the clay and organic fractions are dispersed. Dispersed organic matter accumulates at the surface of poorly drained areas as water evaporates and imparts a black color to the surface, hence the name black alkali. Sodic soils are found in many parts of the western United States. In some locations they occur in small patches, slick spots, less than 0.5 ha in extent. Such patches occupy slight depressions, which become accentuated as surface soil particles disperse and are blown away by wind erosion. The percolation of insufficient water to satisfy plants and to control salinity is the main problem associated with sodic soils. In addition, their relatively low soluble-salt concentrations and high pH values can result in direct Na toxicities to the most sensitive plants. 

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