Soil organic carbon (SOC) is the percentage measure of carbon (C) derived from living organisms in soil. Stability of soil organic matter (SOM) can be defined in terms of how easily C and nitrogen in the SOM can be decomposed. Due to the implications in the permanence of SOC during sequestration there is scientific interest in fractionation of SOM into different fractions. A large number of SOM fractionation procedures have been developed to distinguish between SOM to study whether it is liable or recalcitrant to activities of soil microbes. There are physical and chemical fractionation techniques. The former is based on particle size and density of soil samples or combination of the two, and the latter on the reaction of chemical on SOM for the separation of stable SOC. Each fraction of SOC in the laboratory can be commonly determined using wet oxidation by Walkley-Black method and dry combustion by LECO CN Analyzer. With the advancement in chemometric statistical techniques; faster, robust, cheaper and non-destructive methods are emerging. The chemometric statistical techniques do not require any reagents for analysis compared with the wet oxidation or dry combustion methods. Thus, these emerging techniques are highly attractive for studies where a large number of analyses are required. For in situ measurement of SOC, spectral reflectance technology is developed to facilitate instant measurement in the field using the sensors or by remote sensing.
Soil organic matter (SOM) is the vast array of carbon (C) compounds in soil and the soil organic carbon (SOC) is the C component of the SOM. Therefore, the SOC is the percentage measure of C deriving from living organisms and it is commonly represented as SOM = SOC ´ 1.72, with 1.72 as the most commonly used conversion factor [
Stability of SOM can be defined in terms of how easily C and nitrogen in the SOM can be decomposed. The identification, isolation and characterisation of SOM fractions have received a great deal of scientific interest because of their implications in the permanence of SOC during sequestration [4,5]. The estimates of different pools are used in mechanistic models (e.g. Roth-C and Century) that predict changes in SOM storage [6,7]. A large number of SOM fractionation procedures have been developed that seek to distinguish between SOM that is more easily decomposed (low stability) and less easily decomposed (high stability) by the soil microbes. The procedures have been recently reviewed and include physical fractionation by size or density, and various chemical fractionation methods that separate SOM by solubility, hydrolysability, or resistance to oxidation [3,8].
Physical fractionation procedures by size is based on the idea that the association of soil particles and their spatial arrangement play a key role in SOM dynamics as bioaccessibility is a prerequisite for decomposition [9,10]. Physical fractionation involves the application of various degrees of disaggregating treatments by dry and wet sieving [11-13], dispersion [
The general wet sieving process uses sieves of 250 μm and 53 μm to isolate fractions into three aggregate groups. The >250 μm is macroaggregate, 250 - 53 μm is microaggragate and <53 μm is silt + clay size fraction [12,13]. Density fractionation is applied to isolate SOM that is not firmly associated with soil minerals from organo-mineral complexes [11,15]. Associations of SOM to mineral surfaces are most often characterised by a density >1.6 - 2 g cm−3. The fraction of SOM that floats in the >1.6 - 2 g cm−3 medium is the light fraction and that settles down is the heavy fraction. Lighter fraction or POM with a density of <1.6 - 2 g cm−3 consists mostly of pieces of plant residues and heavier fraction consists of mineral-associated organic C [11,15]. POM also has been separated by a combination of size and density fractionation [17-19] in several steps for each particle size group.
Of the five SOC pools in the Roth-C model, the inert pool [
The most commonly used methods for determination of SOC are wet oxidation by Walkley-Black method [
With the development and emergence of reflectance spectroscopy techniques, near infrared (NIR) and mid-infrared (MIR) are also used to determine SOC in the laboratory. Using chemometric statistical methods, both NIR and MIR spectroscopy techniques have been used in the past two decades to determine soil properties, including soil organic C and total nitrogen [33-35]. Compared with wet and dry combustion methods, the NIR and MIR spectroscopy techniques are faster, robust, cheaper and non-destructive, and they do not require any reagents for analysis [36,37]. These emerging techniques are, therefore, likely to replace the tedious job of wet and dry combustions in future, and are highly attractive for studies where a large number of analyses are required [
The techniques used for in situ measurement of SOC in the field are by sampling, or by on-the-go detection with sensors mounted on a tractor [38,39] that facilitate instant measurement of SOC. These methods are also non-destructive and relatively cheaper compared to the destructive wet laboratory [
It is advantageous to report SOC rather SOM to consistently and reliably compare between studies. Due to the implications in the permanence of SOC during sequestration, there is a need to fractionate of SOM into different fractions. Further, mechanistic models that predict changes in SOM storage require the estimates of different SOC pools. Thus, a large number of SOM fractionation procedures have been developed to distinguish between SOM to study whether it is liable or recalcitrant. Once fractionated, the most common methods for determination of SOC in laboratory are wet oxidation and dry combustion. More advanced; chemometric statistical techniques are faster, robust, cheaper and non-destructive than wet oxidation and dry combustion methods. With the advancement in spectral reflectance technology, the in situ measurement of SOC is done using with sensors or by remote sensing to facilitate instant measurement of SOC in the field.
The first author, a Bhutanese national, was funded by the Endeavour Postgraduate Award of Australia Awards at the University of New England, Armidale, Australia for his PhD study.