Dissolved organic matter in soil: challenging the paradigm of sorptive preservation
Introduction
Three types of pathways are commonly considered in the formation of stable organic matter (OM) in soils Christensen, 1996, Sollins et al., 1996. Selective enrichment of organic compounds refers to the inherent recalcitrance of specific organic molecules against degradation by microorganisms and enzymes. Chemical stabilization involves all intermolecular interactions between organic substances and inorganic substances leading to a decrease in availability of the organic substrate due to surface condensation and changes in conformation, i.e., sorption to soil minerals and precipitation. Physical stabilization is related to the decrease in the accessibility of the organic substrates to microorganisms caused by occlusion within aggregates.
Recently, increasing evidence from studies in soils and sedimentary systems indicates that sorptive protection of OM may be of particular importance, although chemisorption of OM to clay-sized particles and physical protection of OM within organo-clay aggregates often cannot be clearly distinguished. A first indication for the importance of sorptive protection in soils is the frequently reported positive relationship between the organic carbon (OC) content and the clay content (e.g., Burke et al., 1989, Hassink, 1997). Additional evidence comes from close relations between OC and BET surface areas in coastal sediments Mayer, 1994a, Keil et al., 1994 and subsoil horizons (Mayer, 1994b), giving calculated surface loadings of 0.6–1.5 mg OC m−2. These loadings were considered to represent the “monolayer equivalent” (ME) range for OM associated with mineral particles (Mayer, 1994a). Hedges and Keil (1995) assumed that this finding is indicative of dissolved organic matter (DOM) sorption to mineral grains. Keil et al. (1994) showed that simple desorption of OM from marine sediments with water increased mineralization rates by up to five orders of magnitude. Similar results were observed for soils by Nelson et al. (1994).
A reappraisal of the BET surface data using calculations of the reaction enthalpies (Mayer, 1999; Mayer and Xing, 2001) and TEM studies Ransom et al., 1997, Salmon et al., 2000 showed that only a fraction of the mineral surface was covered with OM, which occurred as patches and formed microaggregates with clay-sized particles. These OM patches seem to be related to mineralogy. Ransom et al. (1998) reported that OM appears to be preferentially sequestered in sediments, being rich in smectite but also in metal oxyhydroxides, suggesting an influence of mineralogy on sorptive protection. Likewise, Kaiser and Guggenberger (2000) calculated close correlations between measures for Al and Fe hydrous oxides and OC concentrations in soils. In a chronosequence study, Torn et al. (1997) identified the concentrations of short-range ordered and noncrystalline minerals as the primary control for soil OC concentrations and turnover.
Together, these results suggest that stabilization of OM by interactions with distinct mineral matrices may be the single largest factor controlling OM preservation on the Earth's surface today (Keil et al., 1994). Because sorption is defined as the transfer of a solute (the sorbate) from solution to an existing solid phase (the sorbent) (Sposito, 1984), a prerequisite for the sorptive stabilization of OM is that it must occur in a dissolved state prior to sorption. Likewise, if precipitation (the accumulation of a solute as a new solid phase) is considered as a process of OM stabilization (Boudot et al., 1989), the precursor substance must be dissolved. Hence, the paradigm of sorptive stabilization includes a significant fraction (if not all of the OM) must have passed through the dissolved phase before undergoing sorption or precipitation (Hedges and Keil, 1995). This gives rise to the investigation of DOM sorption to minerals not only in rivers and sedimentary systems but also in soils.
In this discussion paper, we will first summarize results from DOM research in forest soils to show that DOM sorption to soil minerals contributes to the formation of stable soil OM. Then we will challenge on the paradigm of sorptive stabilization. Using a combination of field data on dissolved organic carbon (DOC) fluxes in forest soils and laboratory data on the soils' sorption capacity for DOC, we will provide evidence that the turnover of sorbed OC may be quite rapid. We will present a line of evidence showing that, depending on the type of surface, DOM sorption can be a stabilizing process but it may be also a prerequisite for OM mineralization.
Section snippets
Processes of dissolved organic matter retention in forest soils
In the last two decades, many studies have dealt with the dynamics of DOM in forest ecosystems. It has been reported that about 10–40 g DOC m−2 year−1 is translocated from the organic surface layer into the mineral soil horizons (summarized in Michalzik et al., 2001; Fig. 1). This means that about 10–25% of total C input to the forest floor with litter fall is leached from the organic surface layers McDowell and Likens, 1988, Guggenberger, 1992. In deeper mineral soil horizons, the DOC fluxes
Capacity of soils to sorb organic matter
In the previous section it was shown that relatively large quantities of DOM enter the mineral horizons. Biodegradation of organic matter in the dissolved phase is too slow to remove a large portion of the DOM percolating through the soil Qualls and Haines, 1992, Kalbitz et al., 2003, whereas sorption to mineral phases is an efficient sink for DOM in subsoil horizons Kaiser and Zech, 1998, Kaiser and Guggenberger, 2000. However, sorption of DOM to mineral phases and mineral soil is not infinite
Sorption of organic matter to surfaces differing in the microbial activity
In podzols, sorption of DOM has been considered an important process in the formation of OM in subsoil horizons. The Bh horizon is generally thought to result from DOM sorption. Often, Bh horizons have large OC concentrations, which is a prerequisite for the high density of microorganisms found in these horizons (McKeague et al., 1986; Fig. 5). Concurrently, the 14C age of OM shows a minimum in the Bh horizon and is only about 100 years (Rumpel et al., 2002; Fig. 5). This is about the same
Conclusion
Based on these findings, we propose a model shown in Fig. 7 where the location of the OM is decisive for its fate. In forest soils, organic matter in the soil solution is mineralized at rates that are much slower than the mean residence time of DOM in the mineral soil (e.g., Qualls and Haines, 1992). This suggests that mineralization cannot be responsible for the DOM retention in the mineral soil. Kalbitz et al. (2003) identified that the stable DOM component, which dominates OM in the solution
References (68)
- et al.
Biodegradation of synthetic organo-metallic complexes of iron and aluminium with selected metal to carbon ratios
Soil Biol. Biochem.
(1989) - et al.
Preferential flow paths: biological ‘hot spots’ in soils
Soil Biol. Biochem.
(2001) - et al.
Effect of molecular size and charge on biofilm sorption of organic matter
Water Res.
(1998) - et al.
Dissolved organic carbon control in acid forest soils of the Fichtelgebirge (Germany) as revealed by distribution patterns and structural composition analyses
Geoderma
(1993) - et al.
Formation and mobilization pathways of dissolved organic carbon: evidence from chemical structural studies of organic carbon fractions in acid forest floor solutions
Org. Geochem.
(1994) - et al.
Sedimentary organic matter preservation: an assessment and speculative synthesis
Mar. Chem.
(1995) - et al.
The role of DOM sorption to mineral surfaces in the preservation of organic matter in soils
Org. Geochem.
(2000) - et al.
Sorption of DOM and DOM fractions to forest soils
Geoderma
(1996) - et al.
Biodegradation of soil-derived dissolved organic matter as related to its properties
Geoderma
(2003) - et al.
Effect of substrate location in soil and soil pre-water regime on carbon turnover
Soil Biol. Biochem.
(1993)
Surface area control of organic carbon accumulation in continental shelf sediments
Geochim. Cosmochim. Acta
Relationships between mineral surfaces and organic carbon concentrations in soils and sediments
Chem. Geol.
Extent of coverage of mineral surfaces by organic matter in marine sediments
Geochim. Cosmochim. Acta
A comparative study of the nature of biopolymers extracted from anaerobic and activated sludges
Water Res.
Availability of organic carbon in soluble and particle-size fractions from a soil profile
Soil Biol. Biochem.
TEM study of in situ organic matter continental margins: occurrence and the “monolayer” hypothesis
Mar. Geol.
Organic matter preservation on continental slopes: importance of mineralogy and surface area
Geochim. Cosmochim. Acta
Protection of organic matter by mineral matrix in a Cenomanian black shale
Org. Geochem.
Stabilization and destabilization of soil organic matter: mechanisms and controls
Geoderma
Interactions between montmorillonite and fulvic acid
Geoderma
The adsorption of aquatic humic substances by iron oxides
Geochim. Cosmochim. Acta
Chemical characteristics and acidity of soluble organic substances from northern hardwood forest floor, central Maine, U.S.A.
Geochim. Cosmochim. Acta
Adsorption of dissolved organic carbon extracted from sewage sludge on montmorillonite and kaolinite in the presence of metal ions
J. Environ. Qual.
Impact of preferential flow on radionuclide distribution in soil
Environ. Sci. Technol.
Sorption and transport of metals in preferential flow paths and soil matrix after the addition of wood ash
Eur. J. Soil Sci.
Texture, climatic and cultivation effects on soil organic matter content in U.S. grassland soils
Soil Sci. Soc. Am. J.
Carbon/sesquioxide ratios in organic complexes and the transition of albic-spodic horizon
J. Soil Sci.
Carbon in primary and secondary organomineral complexes
Organic carbon sorption in arctic and subalpine Spodosol B horizons
Soil Sci. Soc. Am. J.
Microenvironments of soil microorganisms
Biol. Fertil. Soils
Adsorption and desorption of natural organic matter on iron oxide: mechanisms and models
Environ. Sci. Technol.
Eigenschaften und Dynamik gelöster organischer Substanzen (DOM) unterschiedlich immissionsbelasteten Fichtenstandorten
Bayreuth. Bodenkdl. Ber.
Retention of dissolved organic carbon and sulfate in aggregated forest soils
J. Environ. Qual.
The capacity of soils to preserve organic C and N by their association with clay and silt particles
Plant Soil
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