The SOM content is one of the most varying pedosphere properties, ranging from <0.2% in salt-affected sandy soils of arid and semi-arid climate zone (Khoshgoftarmanesh et al. 2006) up to >99% in highly organic Histosoils from uplands in humid and boreal climates of northern hemisphere (Ondrasek 2008). In cultivated topsoils, SOM represents a fairly small fraction (1-4%), but it is one of the most complex, dynamic and multifunctional soil components. Although its importance was recognised from different scientific perspectives, a consensual definition of SOM is still missing, mostly because in disparities such as the following (Ondrasek 2008 and references therein): (1) inclusion/ exclusion of living biomass, litter, fragmentation and humification layers and (2) threshold degree of decomposition. Disregarding this, SOM can be defined as soil biota and non-living biomass, i.e. a wide range of organic residues and effluents (Fig. 22.3).
Non-living SOM can be present in soil as fresh matter (litterfall) that is biologically and chemi cally altered i.e. decomposed and/or polymerised (re-synthesised) and stabilised to certain extent by soil micro-organisms (Fig. 22.3) . A typical fresh plant biomass is composed of (hemi)cellu-lose (~65%) and lignin (~20%), however, its quantity/quality vary among natural ecosystems, influencing the rate of decomposition and the balance between mineralisation/immobilisation.
As a portion of the solid SOM, there is a pool of potentially dissolved OM (DOM), which is during last few decades typically quantified as dissolved organic carbon (DOC) i.e. dissolved OM fraction able to pass through a filter 0.40.7 mm (Peichl et al. 2007; Ondrasek 2008). The concentration of DOC in the bulk soil and/or rhizosphere solution may differ considerably (Fig. 22.3) and is controlled by the following: (1) total SOM content (Ondrasek et al. 2009a), pH and ionic strength (Kalbitz et al. 2000) , soil watering status (Yallop and Clutterbuck 2009), crop species/genotype (Rengel 2002), rhizo-sphere salinity (Ondrasek et al. submitted) or trace elements (TEs) contamination (e.g. Chiang et al. 2011) . Usual DOC concentrations in mineral soil solutions may be up to one order of magnitude lower (e.g. <4 mg/L from mineral soil; Peichl et al. 2007) than in drainage solution from organic (peat) soils (20-110 mg/L; Kalbitz et al. 2000; Glatzel et al. 2003; Ondrasek et al. submitted). (Fig. 22.3).
The solid/dissolved SOM fractions may contain labile low-molecular-weight (LMW; <10%) and more stable high-molecular-weight (HMW; >90%) substances (Fig. 22.3). LMW materials refer to carbohydrates, small proteins/peptides, organic acids (OAs), amino acids, fats, sidero-phores, phenolics, vitamins, hormones, etc. (e.g. Neumann and Romheld 2000; Rengel 2002). Besides preferentially originating from decomposed biomass, LMW compounds are also released as root exudates and/or microbial metabolites (Rengel and Marschner 2005) . Recently, Chiang et al. (2011) have observed a significant increment in concentration of volatile (acetic, propionic and butyric acids) and non-volatile (lactic, maleic, succinic and malic) LMW-OAs in the millet rhizosphere contaminated by Cd. Another example is secretion of LMW siderophores by
soil microbes and phytosiderophores by plant roots that can chelate some TEs (Fe, Zn) and increase their bioavailability (Rengel 2002; Rengel and Marschner 2005).
Bulk of DOC-LMW compounds from the soil solution are rapidly decomposed/mineralised by microorganisms and are typically maintained at low concentrations (<50 mM; Van Hees et al. 2005) representing a labile (easily degradable) organic pool. Notwithstanding that the labile LMW pool has one of the shortest residence times (1-10 h), its presence in the soil (rhizosphere) solution appears to be: (1) stable, being replenished continuously by rhizosphere exudation and from fresh litterfall (Van Hees et al. 2005) , and (2) much lower in comparison to a stable HMW pool. In agricultural/forest lands, Sachse et al. (2005) found that DOC-HMW compounds
(humics+polysaccharides) dominated in all samples (55-77%), whereas DOC-LMW (e.g. acids) portion was only several %s; however, not all organic fractions were accounted for (Fig. 22.3) ; thus, contribution of DOC-LMW/-HMW pools could be higher. For example, after a 90-day incubation, Kalbitz et al. (2003) found for fresh and less humified organic material (straw, forest floor litter) that DOC was mineralised 61-93% and the labile fraction comprised 60-90% of DOC, whereas for humified peats and Oa forest layers the mineralised and labile pools represented only 4-9% and 3-6% of DOC, respectively. Therefore, for humified sources the predominant portion (>90%) of OM in dissolved phase appears to be stable DOC-HMW substances, which is similar to a solid SOM fraction (Fig. 22.3).
A negligible portion of DOC or ~1% may be lost from pedosphere by leaching (Peichl et al. 2007) , whereas the rest of SOM is stored in a complex and poorly characterised HMW pool comprising the following: (1) humics that represent the majority or 70-85% of HMW pool, (2) highly polymerised exudates from plant root (e.g. mucilage polysaccharides and secretory proteins or ectoenzymes; Neumann and Romheld 2000) or soil microbes (e.g. glomalin or Fe-containing glycoprotein from arbuscular mycorrhizal fungi) representing 10-30% of HMW and (3) an inert organic pool, i.e. black carbon (C), which is the most recalcitrant SOM fraction, contributing 5-20% of HMW (Ondrasek 2008 and references therein) (Fig. 22.3).
Even though humus is still not fully defined, there are many hypothetical chemical structures (Schnitzer 1978) from which is possible to get insight about humus complexity, heterogeneity and continuous transformation (e.g. over H-bonds or Van der Waals forces). It is known that humics are chemically highly polymerised organics containing both aromatic and aliphatic monomers. Therefore, the crucial role in humics characterisation is played by their particular constitutes i.e. functional groups; carboxyl, hydroxyl, aldehyde, ketone, ester, amino, nitro, thiol, etc. Oxygen (O) containing groups (e.g. -COOH, -OH) are much more abundant than nitrogen (N) and sulphur (S) groups (e.g. -NH), -SH) (Essington 2004), but all may act as acid/base and serve as proton (H+) donor/acceptor (i.e. TEs acceptors) under certain pHs depending on their acidity constants (p^a). Given that in the most abundant carboxyl groups of humus, the pKa values range between 4 and 6, the average pKa of humics is 4-4.5 (Tan 2003). Certain humus fractions may be extracted from soil after suspending in NaOH and filtering (e.g. XAD-8 resin column). The compounds absorbed onto the resin include humic (HA) and fulvic (FA) acids that are further fractionated by acidification. In pH < 2, precipitated fraction represents HAs, a major component of natural humus materials (Schnitzer 1978), whereas FAs remain in the solution after acidification and are soluble at all pHs. According to Shintzer's (1978) "model" HA and FA data, i.e. average values compiled from different geographical (from Artic to tropical climatic zone) and pedological (acid, neutral soils) conditions, it may be concluded that the portion of C and N is higher and that of H and O lower in HAs compared to FAs.
In the soil HMW pool there are many other organics with complex and even more recalcitrant chemical structure than those elaborated for humics. One of them is organic charred material (biochar, charcoal), i.e. black C, which is during last few decades under intensive observation, principally for its influences on properties, processes and functioning of natural ecosystems (Hockaday et al. 2006). The contribution of black C as a percentage of SOM-C may be substantial (~20-50%) in specific soil type such as terric Anthrosol (e.g. Hockaday et al. 2006 and references therein). Similarly to humics, some of biochar active radicals act as proton donors/ acceptors, resulting in coexisting areas whose properties can range from acidic to basic and from hydrophilic to hydrophobic. Due to high enrichment with polyaromatic rings, biochar is persistent even up to nano-scale fraction. Ultrafine biochar dust fraction, formed by condensed aromatic fullerene-like structures, is presumed to be the most recalcitrant OM in the nature, whose residence times is 10 to 1,000-fold longer than most of SOM (Lehmann et al. 2009 and references therein). Through natural weathering or oxidative depolymerization, soil charcoal slowly degrades to relatively less recalcitrant forms of humic substances (Hockaday et al. 2006).
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