Organic matter decomposition reactions

The elements deposited within the sediment matrix show that mobilization processes may be occurring in the upper layers. At Station SIN 3, figure 4d for example, the element deposited (pg-cm-2) in the topmost layers decreases, often much more than in the concentration (Mg g 1). This may be due to organic matter decomposition and/or to environmental chemical reactions of solubility and precipitation of the given element. The metal must have been removed rapidly from the water column since the sediment concentration is shown to decrease rapidly with distance from the shipyard (Stations SIN 3 and SIN 2). Lead may not be mobilized significantly after deposition since any diffusion in the pore water would tend to "smooth" the concentration profile with time. 

It has been shown experimentally that organic matter decomposition in sediments can be explained as a set of quasi-first-order reactions of the sort... 

Many studies of the impact of chemical diagenesis on the carbonate chemistry of anoxic sediments have focused primarily on the fact that sulfate reduction results in the production of alkalinity, which can cause precipitation of carbonate minerals (see previous discussion). However, during the early stages of sulfate reduction (—2-35%), this reaction may not cause precipitation, but dissolution of carbonate minerals, because the impact of a lower pH is greater than that of increased alkalinity (Figure 4). Carbonate ion activity decreases rapidly as it is titrated by CO2 from organic matter decomposition leading to a decrease in pore-water saturation state. This process is evident in data for the Fe-poor, shallow-water carbonate sediments of Morse et al. (1985) from the Bahamas and has been confirmed in studies by Walter and Burton (1990), Walter et al. (1993), and Ku et al. (1999) for Florida Bay, Tribble (1990) in Checker Reef, Oahu, and Wollast and Mackenzie (unpublished data) for Bermuda sediments. 

There are two species in this grouping— ammonia (NH3) and ammonium (NH4) with one valence state (—III) (Table 1). The primary species emitted to the atmosphere is NH3 produced during organic matter decomposition and emitted when the partial pressure in the soil, water, or plant is greater than the partial pressure in the atmosphere. It is the most common atmospheric gaseous base and, once in the atmosphere, can be converted to an aerosol in an acid-base reaction with a gas (e.g., HNO3) or aerosol (e.g., H2SO4) ... 

The study has been broken into two parts the first concentrates on describing diagenetic processes involving organic-matter decomposition and production or consumption of the nutrients SO ", NH4, alkalinity, and HP04. The second emphasizes the associated chemical behavior of Fe and Mn (Part II). Several types of measurements were made (1) seasonal pore water and solid-phase analyses, (2) direct measurement of solute fluxes out of the sediment, (3) rates of reaction as a function of depth and temperature, and (4) the abundance and composition of the fauna at each station. Taken together, these measurements provide one of the most detailed descriptions of controls on diagenesis near the sediment-water interface that is presently available. 

Oxidation-reduction reactions of iron and manganese are also involved in nutrient release in flooded soil and sediments. Fe(III) and Mn(IV) serve as electron acceptors for organic matter decomposition or turnover. Organic nitrogen mineralization results in the release of nutrients such as ammonium nitrogen. Iron reduction is also coupled to phosphorous release in soils dominated by iron redox couples. 

It can be concluded from the vertical distribution of nitrogen forms that the contents of OC, ON, BP, and BSi decrease remarkably within a depth of 0 10 cm (Song et ah, 2000b), indicating that organic matter decomposition mainly occurs at the surface and sub-surface. The decomposition rate constant of OC can be evaluated by its decrease. Taking M9-5 as an example, the decomposition rate constant of OC, ON, BP, and BSi can be estimated. It is supposed that, OC decomposition is a first degree reaction, Z is the sediment depth (cm), Co and Zq are OC contents at a depth of 0 and Z cm respectively, K is the decomposition rate constant (yr ), and S is the sedimentation rate (cm/yr). The formula is as follows ... 

An excess of a standard solution of iron(II) must therefore be added and the excess back-titrated with standard cerium(IV) sulphate solution. Erratic results are obtained, depending upon the exact experimental conditions, because of induced reactions leading to oxidation by air of iron(II) ion or to decomposition of the persulphate these induced reactions are inhibited by bromide ion in concentrations not exceeding 1M and, under these conditions, the determination may be carried out in the presence of organic matter. 

Soil solution is the aqueous phase of soil. It is in the pore space of soils and includes soil water and soluble constituents, such as dissolved inorganic ions and dissolved organic solutes. Soil solution accommodates and nourishes many surface and solution reactions and soil processes, such as soil formation and decomposition of organic matter. Soil solution provides the source and a channel for movement and transport of nutrients and trace elements and regulates their bioavailability in soils to plants. Trace element uptake by organisms and transport in natural systems typically occurs through the solution phase (Traina and Laperche, 1999). 

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