Dissolved organic matter in soil: challenging the paradigm of sorptive preservation

doi:10.1016/S0016-7061(02)00366-X

Geoderma

Volume 113, Issues 3–4, May 2003, Pages 293-310

https://doi.org/10.1016/S0016-7061(02)00366-X Get rights and content

Abstract

During the last decade, research in sedimentary systems led to the paradigm of sorptive stabilization of organic matter (OM). Studies on soils also show that sorptive interactions between dissolved organic matter (DOM) and mineral phases contribute to the preservation of soil OM. In the first part of the paper, we summarize evidence for sorptive stabilization of OM in forest soils including (a) pronounced retention of DOM in most subsoils, (b) strong chemisorptive binding exhibiting strong hysteresis, and (c) similarity in the composition of DOM and OM in illuvial soil horizons and clay-sized separates. However, the capacity of soils for sorption of DOM is not infinite. In the second part of the paper, we present a case study where we relate the yearly retention of dissolved organic carbon (DOC) in the mineral soil to the available sorption capacity of seven forest soils. We estimate that the saturation of the sorption complex would occur within 4–30 years. Assuming these soils are in steady-state equilibrium with respect to carbon cycling, this suggests a mean residence time of the sorbed organic carbon (OC) of about the same time, therefore providing little evidence for a long-term stabilization of sorbed OM.

One explanation for this discrepancy may be because in forest soils most surfaces are not characterized by juvenile minerals but are covered with OM and colonized by microorganisms. This is the case mainly in topsoil horizons but occurs also along preferential flow paths and on aggregate surfaces. Biofilms develop particularly at sites receiving high input of nutrients and organic substrates, i.e., DOM, such as preferential flow paths. The OM input enhances the heterotrophic activity in the biofilm, converting the DOM into either organic compounds by microbial resynthesis or inorganic mineralization products. Recent studies suggest that Fe hydrous oxides embedded within the biofilms may serve as a sorbent and shuttle for dissolved organic compounds from the surrounding aqueous media. We assume that sorption of DOM to the biofilm does not lead to a stabilization of OM but is a prerequisite for its rapid turnover. Only when DOM is transported by mass flow or diffusion to fresh, juvenile mineral surfaces, may sorption effectively stabilize OM. This stabilization would involve complexation of functional groups, changed conformation, and incalation in small pores.

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

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Dissolved organic matter in soil: challenging the paradigm of sorptive preservation

Abstract

Introduction

Section snippets

Processes of dissolved organic matter retention in forest soils

Capacity of soils to sorb organic matter

Sorption of organic matter to surfaces differing in the microbial activity

Conclusion

Soil Biol. Biochem.

Soil Biol. Biochem.

Water Res.

Geoderma

Org. Geochem.

Mar. Chem.

Org. Geochem.

Geoderma

Geoderma

Soil Biol. Biochem.

Geochim. Cosmochim. Acta

Chem. Geol.

Geochim. Cosmochim. Acta

Water Res.

Soil Biol. Biochem.

Mar. Geol.

Geochim. Cosmochim. Acta

Org. Geochem.

Geoderma

Geoderma

Geochim. Cosmochim. Acta

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