Controls on DOM Release from Soil Horizons of High Organic Carbon Content

Amount of Litterfall and Soil Organic Matter
Litterfall represents the most important source of C inputs to the forest floor (Gosz et al., 1976; Nordén, 1994). Yearly terrestrial total litter production is almost equivalent to net terrestrial primary production of about 60 Pg C yr-1 (Post, 1993). However, there has been little research on the quantitative effect of litterfall on DOM release. Considering that the largest ecosystem internal flux of DOC occurs as percolate from the forest floor (Zech et al., 1996; Michalzik and Matzner, 1999), the rate of litter incorporation in the forest floor and its rate of degradation into products with varying degrees of humification may ultimately determine the rate of DOM production.

A few studies have found that seasonal changes in the concentration of DOC and DON in forest floor leachate are related to litterfall inputs (Lundström, 1993; Casals et al., 1995; Currie et al., 1996). However, Casals et al. (1995) did not rule out the possibility that this was related to cycles of drying and rewetting and/or to the high temperature that was observed in the same period as high leaching. Although the periods of maximum litterfall and high DOM leaching do not generally coincide (Cortina et al., 1995), it is probable that DOM dynamics in soils are related significantly to the amount of recent litter and organic matter in the soils. Gundersen et al. (1998) observed that DOC flux was correlated to litterfall amount, not to the N status of the sites. They suggested that in forest floor material of several forest sites across Europe, C supply and turnover rate determine DOC leaching from the forest floor. In a modeling study, Currie and Aber (1997) found that relatively high fluxes of DOC and CO2 were caused by either high relative rates of decay or by high litterfall fluxes. In field studies in experimental forests, the same researchers found that DOC leaching and CO2 mineralization were correlated positively with the organic matter mass of the forest floors. This is consistent with the results of Tipping et al. (1999), who observed in a field manipulation experiment on three different soils that the soil with the highest organic matter content exported more DOC than the other two soils.

In conclusion, increasing litter production and humus content presumably results in increasing DOM concentration and fluxes. However, this effect cannot be quantified.

Substrate Quality
The quality of litter is determined largely by the dominant vegetation, which thus plays an essential part in controlling DOM concentrations in soil solutions (Kuiters, 1993). Soil solutions from mixed and coniferous stands often contain significantly more DOC and DON than those from hardwood stands (David and Driscoll, 1984; Cronan and Aiken, 1985; Currie et al., 1996). Cronan (1990) pointed out that DOC export from coniferous forests was 50% higher than from hardwood stands. Modeling work by Currie and Aber (1997) suggest that DOC and DON fluxes are higher in coniferous forests despite slower rates of litter decomposition. Until now, the reason for this difference has been poorly understood. Moreover, Kuiters (1993) showed in a laboratory leaching experiment that much more DOC was released from deciduous leaves (10-25 mg C g-1 dry matter) than from coniferous needles (<5 mg C g-1 dry matter), possibly as a result of differences in the water permeability of the leaves.

Studies examining the interactions between substrate quality and decomposition rates have focused on the concentrations of nitrogen, lignin, polyphenols, or some combination of these chemical constituents (e.g., Melillo et al., 1982; Melillo et al., 1989). However, few attempts have been made to relate DOM production rates in the soil to the substrate quality of litter and humus. Northup et al. (1995) showed that in a Pinus muricata forest ecosystem characterized by extremely acidic and infertile soil, the polyphenol concentration of needle litter controls the release rate of DON. The DON released is assumed to be directly utilized by mycorrhizae minimizing nitrogen loss to competing organisms.

The potential decomposition rate of organic soil is conventionally characterized by its C/N ratio. In the laboratory, however, Michel and Matzner (1999) observed no correlation between DOC and DON release rates and the C/N ratios of forest floor materials sampled from 12 different Norway spruce stands. In contrast to these results, Kalbitz and Knappe (1997) showed in a column leaching experiment that a wide C/N ratio of soil organic matter, a high content of hot-water soluble organic C, and a high proportion of hot-water soluble organic C to the total organic C content determine the amounts of DOC released from the topsoil if the soils have a low capacity to adsorb and precipitate DOC. This is supported by Gödde et al. (1996), who found in a column leaching study that soils with higher C/N ratios showed higher rates of respiration and DOC mobilization. They attributed this result to higher fractions of the more labile Oe (partially decomposed litter) versus Oa (well decomposed organic matter) material in the high C/N soils, making them easier to degrade.

Little information exists about the effect of substrate quality on in situ DOM dynamics in soils. Parameters such as the content of carbon or nutrients (N, P) in soil, or ratios between carbon and nutrients (e.g. C/N, C/P) are often not significantly related to DOM release rates (Cortina et al., 1995). Williams and Edwards (1993) also suggested that leaching of DOM is not necessarily constrained by the ratios of C to Na or C to P because during the early stages of decomposition (up to two years), decaying litter can release organic leachates independent of microbial demand for inorganic nutrients (as indicated by the C/N and C/P ratios of the litters). Other possibilities for controls on DOM dynamics in the field are the substrate accessibility for microbes (Bosatta and Ågren, 1991) and the content of soluble organic C in the substrate (Hu et al., 1972; Reinertsen et al., 1984; Cogle et al., 1989).

Decomposer Community
The decomposer community in soil consists of a wide range of bacteria, fungi, protista, and invertebrates (Swift et al., 1979). Microorganisms have received considerable attention because of their "dual roles as a pool of labile nutrients and the agent of decomposition of organic materials" (Lundquist et al., 1999a). Guggenberger et al. (1994a) and Møller et al. (1999) suggested that fungi are the most important agents in the process of DOM production, probably because of incomplete degradation of organic matter by fungi. As described in the previous section, microbial metabolites constitute a significant portion of DOM released from the forest floor. In addition, microbial biomass itself provides an important pool of potential DOM. Yavitt and Fahey (1984), for example, observed a 10-fold increase in DON concentrations in leachate from biocide-treated microcosms, which was considered to be caused mainly by cell death and lysis.

Some evidence indicates that the soil fauna also influences the rate of DOM release by enhancing the rate of turnover of microbial biomass (Williams and Edwards, 1993) and through the release of organic compounds at death (Whalen et al., 1999). In leaching experiments using litter and humus with and without soil fauna, addition of enchytraeid worms (Williams and Griffiths, 1989) and a diverse soil fauna (Huhta et al., 1988; Setälä et al., 1990) increased the rates of inorganic and organic N leaching, presumably as a consequence of reduced microbial immobilization caused by faunal grazing. According to Whalen et al. (1999), 13 to 18% of the N from 15N-labeled decomposing earthworms was transformed into DON in the incubation period of 2 to 16 days, indicating the significance of direct N flux from faunal tissues through mortality.

Controls on DOM Adsorption in Mineral Soil Horizons of Low Organic Carbon Content
Several field studies have shown that concentrations and fluxes of DOM in soil solutions decrease significantly with soil depth (Fig. 2). It is generally assumed that adsorption of DOM to mineral surfaces is far more important than decomposition of DOM in reducing DOM concentrations in mineral soil. A variety of mechanisms has been postulated, including anion exchange, ligand exchange-surface complexation, cation bridging, hydrogen bonding, van der Waals forces, and physical adsorption. Jardine et al. (1989) concluded that physical adsorption (hydrophobic interactions) driven by favorable entropy changes is the dominating process. Kaiser (1996), however, argues that adsorption of hydrophobic DOC components is diminished with increasing soil organic matter content and that ligand exchange should be the most important process (Gu et al., 1994; Edwards et al., 1996). The steric position of bound ligands would play an important role in the strength of the bond (Kaiser, 1996), and the formation of strong covalent bindings and multiple site bindings causes a pronounced hysteresis of the adsorption. Thus, DOC sorption is largely irreversible under natural soil conditions. Gu et al. (1994) report that 72 to 92% of DOC sorbed to Fe oxides was irreversibly bound. Inasmuch as Fe and Al oxides and hydroxides are the most important sources of variable charge in soils (Jardine et al., 1989; Moore et al., 1992; Kaiser and Zech, 1998a), DOC adsorption can be related quantitatively to the Fe and Al oxide/hydroxide content of soil samples (Moore et al., 1992). In addition to oxides and hydroxides, clay minerals are also important adsorbents for DOC in soils. DOC adsorption to kaolinite is more effective than its adsorption to illite (Jardine et al., 1989), and DOC concentrations in catchment runoff are negatively correlated to the clay content of soils in the catchment (Nelson et al., 1993). The surface area of minerals is a key factor influencing the adsorption capacity (Gu et al., 1994; Mayer 1994a and b). The adsorption process is relatively rapid: in batch experiments ad- and desorption was completed almost within 2 to 12 hours (Dahlgren and Marrett, 1991; Kaiser and Zech, 1998b).

Fig. 2. DOC concentrations (a) and DOC fluxes (b) in different soil horizons (Michalzik et al., 1999; data from 23 references).

Organic matter already adsorbed to mineral surfaces reduces the potential adsorption capacity of the adsorbent (Vance and David, 1992; Moore et al., 1992). The maximum adsorption of unfractionated DOC onto Al and Fe oxides/hydroxides ranges from 0.2 to 2.2 mg C m-2 surface area (Tipping, 1981; Day et al., 1994; Gu et al., 1994; Kaiser and Zech 1997a). That DOC sorption reaches a maximum suggests that only a limited number of adsorption sites is available and a monolayer of adsorbed DOC is formed on mineral surfaces (Mayer 1994a and b).

The adsorption of DOM leads to fractionation of DOM: hydrophobic compounds are removed selectively from the soil (Jardine et al., 1989; Kaiser and Zech, 1998a), and hydrophilic substances are released into the soil solution. The observation that hydrophilic DOM compounds have higher N contents than hydrophobic DOM compounds (Qualls and Haines, 1991; Gu et al., 1995; Kaiser et al., 1997) suggests that DON would be less strongly adsorbed than DOC, but this has not been investigated to date. One field observation showing that the DOC/DON ratio of soil solutions decreases with soil depth (Qualls and Haines, 1991) supports this hypothesis, although the changing ratio could be attributable to other factors (e.g., preferential release of hydrophilics) (Kaiser and Zech, 1998b). High molecular weight fractions are preferentially adsorbed compared with low molecular weight components (Gu et al., 1995). The presence of aromatic rings, carboxylic acids, N- and S-containing groups, and amino acid residues in organic molecules increases the adsorption capacity (McKnight et al., 1992).

Anions in soil solutions such as sulfate and phosphate compete with DOC for adsorption sites in forest soils (Tipping, 1981; Jardine et al., 1989; Vance and David, 1992; Gu et al., 1994), but DOC shows a greater affinity for soil than sulfate (Kaiser and Zech, 1998a). Furthermore, DOC adsorption is decreased under anaerobic conditions (Kaiser and Zech, 1997b).

Studies on the effects of pH on DOC sorption have shown widely varying results. The adsorption of organic matter on iron oxides (Tipping 1981; Gu et al., 1994), aluminum hydroxides (Parfitt et al., 1977; Davis 1982), and bulk soil (Jardine et al., 1989; Kennedy et al., 1996) has been shown to increase with decreasing pH as a result of increasing positive charge on the hydroxides. Tipping (1981) showed that maximal adsorption of humic substances onto geothite occurs at pH 5. However, David and Zech (1990) found decreasing DOC adsorption in the B horizon of an acid forest soil with decreasing pH, and, in a batch adsorption experiment, Vance and David (1992) saw no effect of pH (between pH 3 and 6) on DOC adsorption in mineral soil samples. The main reason for these observed differences in the laboratory is that soil minerals have their maximum adsorption capacities at different pH values. Kaiser (1996) concluded that the pH of most natural soils (normally ranging between 3.5 and 6) does not significantly affect DOM adsorption because the adsorption capacity of sesquioxides is diminished substantially only at a pH greater than 6.0 or less than 4.5 to 3.5.

Laboratory studies show that disturbed soil samples can adsorb DOC rapidly and have large adsorption capacities, suggesting low rates of DOC transport to deeper horizons. However, under field conditions, the transport of DOC in soil profiles is often dominated by the flow regime and macropore transport (Fig. 3). Jardine et al. (1990) found significant transport of DOC during storm events and postulated reduced contact time between the solid and solution phases as the cause for this unexpected finding (see Precipitation and Water Fluxes). In contrast, Li and Shuman (1997) concluded that adsorption properties derived from batch experiments could be used to predict the transport of DOC in sandy soils. The limited value of laboratory findings for the prediction of DOC transport in a sandy aquifer under field conditions was pointed out by McCarthy et al. (1996) in an experimental field study. They report the unexpected transport of large, strongly-binding DOC compounds from sandy soils under field conditions due to saturation of sorption sites.
Fig. 3. Effects of soil aggregation on DOM sorption.

In summary, DOM is strongly adsorbed in the mineral soil to Al and Fe oxides/hydroxides and clay minerals, especially if the surface of the adsorbants has low pre-existing levels of adsorbed C. This results in low DOM output from these types of soils. The individual behavior of DON in relation to DOC has not been investigated. Predictions of DOM mobility in soils based on laboratory studies are potentially misleading if macropore fluxes dominate the field situation.

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