Organic matter operationally defined fractions

Table 22.1 Operationally Defined Fractions of Organic Matter. ...

Fractionation of Soil Organic Matter. There are numerous approaches to separating SOM pools for analysis, with a corresponding number of underlying conceptual frameworks. The objective of fractionation is usually to reduce the chemical, physical, and/or C-cycling time variation in the fractions compared to the bulk soil. Nonetheless, most isolated SOM fractions are operationally defined and... 

In operationally defined speciation the physical or chemical fractionation procedure applied to the sample defines the fraction isolated for measurement. For example, selective sequential extraction procedures are used to isolate metals associated with the water/acid soluble , exchangeable , reducible , oxidisable and residual fractions in a sediment. The reducible, oxidisable and residual fractions, for example, are often equated with the metals associated, bound or adsorbed in the iron/manganese oxyhydroxide, organic matter/sulfide and silicate phases, respectively. While this is often a convenient concept it must be emphasised that these associations are nominal and can be misleading. It is, therefore, sounder to regard the isolated fractions as defined by the operational procedure. Physical procedures such as the division of a solid sample into particle-size fractions or the isolation of a soil solution by filtration, centrifugation or dialysis are also examples of operational speciation. Indeed even the distinction between soluble and insoluble species in aquatic systems can be considered as operational speciation as it is based on the somewhat arbitrary definition of soluble as the ability to pass a 0.45/Am filter. 

A high level of residual complexation capacity, large pseudo stability constants, and metal molecular size distributions that closely parallel trends in organic carbon suggest that metal speciation is dominated by organometallic complexes with either soluble, colloidal, or microparticulate organic matter in the upper Mississippi River. This conclusion is consistent with the observations that trace metals are associated with an operationally defined organic fraction in water-borne particulates ( > 0.4 jLt) in the upper Mississippi River. 

Humin s insolubility obviates its presence in a dissolved organic matter aquatic sample. In aquatic systems, the operational definitions of humic and fulvic acids are stated somewhat differently [4]. Humic acid and fulvic acid are defined by the aqueous solution pH necessary to desorb them from XAD-8 resin fulvic acid can be desorbed with water humic acid is desorbed with an alkaline aqueous solution. However, the defining operational characteristic, solubility in an aqueous solution as a function of pH, is consistent in both operations. Thus, the use of the term fulvic acid in this paper refers to the humic substance fraction soluble in aqueous solution at any pH value, and the term humic acid refers to the humic substance fraction soluble in alkaline aqueous solution but insoluble under acidic conditions. 

They constitute 75% w/w of the organic matter in most soils and 50% of the organic carbon in surface waters. Humic substances are composed of complex heterogeneous mixtures of organic compounds and are characterized as being yellow to brown in color, of high relative molecular mass, and refractory. Unlike many other natural organic products, they cannot be described in terms of unique chemical structures and are operationally defined by the technique used for their extraction and fractionation. 

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