pH Effects on DOM Dynamics via Physico-chemical Processes
The effect of pH on the release of DOM is sometimes contradictory insofar as pH can affect different controls on DOM release. Since DOM probably consists mainly of high molecular, polyfunctional electrolytes, its solubility should depend on its charge density (Tipping and Hurley, 1988; Tipping and Woof, 1990), which in turn depends on the pKa value and the pH of the soil solution. Tipping and Woof (1990) calculated that an increase in soil pH of 0.5 units would lead to increases of about 50% in the amount of mobilized organic matter. Therefore, the solubility of DOM is diminished by the high degree of protonation resulting from low pH (Tipping and Hurley, 1988).
Protonation of functional groups might, therefore, reduce the solubility of DOM by altering the steric conformation when intramolecular bonds are cleaved and van der Waals forces and proton bridging become more effective. On the other hand, the dissolution of organic complexes containing polyvalent cations, and the advancing saturation of exchange sites, should result in enhanced mobility of DOM. The interaction of these and other mechanisms make the net effect of pH variation in soil layers difficult to estimate.
Almost all laboratory studies show that DOC release from organic soil horizons is positively correlated to pH (Whitehead et al. 1981; Hay et al. 1985; Tipping and Hurley 1988, Gödde et al. 1996; Jozefaciuk et al. 1996; Kennedy et al. 1996; Hajnos et al. 1999; You et al. 1999).
In contrast to organic soil horizons, both batch and column experiments in the laboratory have shown enhanced mobilization of DOC from the solid phase in spodosol soil mineral horizons with decreasing pH (David et al. 1989; Jardine et al. 1989; Vance and David 1989; Guggenberger et al. 1994b). These findings have been attributed to enhanced solubilization of metal-organic complexes by proton competition. In opposite to these mobilizing studies (use of DOC-free extracting solutions), most adsorption experiments (use of DOC solutions) showed the opposite effect (increasing adsorption with decreasing pH; see section Controls on DOM Adsorption).
In addition to the quantity of DOM mobilized, changes in solution acidity can affect its quality. Cronan (1985), David et al. (1989), Vance and David (1989), and Guggenberger et al. (1994b) reported a decrease in the relative amount of hydrophobic acids and a corresponding increase in hydrophilic acids under acid irrigation of spodosol soil columns.
pH Effects on DOM Dynamics via Biological Processes
Guggenberger and Zech (1993), Guggenberger et al. (1994a), and Dai et al. (1996) all characterized DOM as plant-derived organic substances, transformed and modified by microbial metabolism. Therefore, the effect of acidity on microbial activity should be regarded as a crucial factor in controlling DOM in natural ecosystems. Unfortunately, information about the effects of pH on microbial DOM production is only fragmentary.
Chang and Alexander (1984) and Guggenberger et al. (1994b) showed that decreasing DOC mobilization with decreasing pH from incubated spodosol soil columns was, in most cases, accompanied by reduced carbon mineralization. However, Cronan (1985) observed neither reduced DOC mobilization nor lowered carbon mineralization with decreasing irrigation water pH (at pH >= 3.5) for intact soil cores from coniferous, hardwood, and mixed stands. Curtin et al. (1998) observed no significant correlation between soil pH (ranging from 5.1 to 7.9) and the N mineralization rates of 61 agricultural soil samples from throughout Canada. Experimentally raising the pH from 5.7 to 7.3 (by addition of Ca(OH)2) in these soils, however, increased DOC and DON concentrations of aqueous soil extracts and resulted in higher mineralization rates for C and N (see liming effects).
Cronan (1985) also reviewed the effects of acid deposition on leaching and organic matter decomposition. He postulated that increased H+ deposition increases the short-term leaching of cations and soluble organic constituents from forest litter. However, the studies he reviewed generally concur that the biologically mediated mineralization of organic matter is relatively unaffected by acid precipitation at pH > 3.0. Vance and David (1991a) showed that soil respiration is affected little by acid inputs. They also showed that DOC release from columns filled with either forest floor or a reconstructed profile (forest floor and B horizon) was not affected by pH 3.7 solution compared with pH 4.8. Even lowering the extractant pH to 3.0 resulted in only a minimal DOC decrease.
Blagodatskya and Anderson (1998) showed that the fungal-to-bacterial respiratory ratio was significantly higher at pH 3.0 compared with pH 6.0. Taking into account the importance of fungi for DOM release (Guggenberger et al., 1994a; Møller et al., 1999), these results suggest that decreasing pH could cause elevated DOM release.
pH Effects as a Combination of Physico-chemical and Biological Processes as
Revealed in Field Studies
The controls on DOM mobilization identified in the laboratory are often not corroborated in the field because of the spatial variability of soil properties and the much longer timescale (months to years vs. hours to days for lab studies). Information about pH-DOM relationships is seldom available at the field scale. Studies investigating the effects of acid deposition on hardwood forests (Liechty et al., 1995), Scots pine forests (Schaaf et al., 1995), or soil solution pH in peaty podzols (Chapman et al., 1995) or Norway spruce (Michalzik and Matzner, 1999) found no effect of acidity on DOC concentrations or fluxes. Michalzik and Matzner (1999) also showed this for DON. Nitrogen fertilization experiments also show no effect of elevated proton fluxes on DOC and DON fluxes in both forest floor leachate and soil solution (Fernandez and Rustad, 1990; Emmett et al., 1998; Stuanes and Kjønass, 1998). Five years of artificial catchment acidification also failed to induce any significant changes in DOC concentrations in a lake in western Norway (Hessen et al., 1997).
One reason for the lack of sufficient correlation between DOC and pH in soil solutions from the field might be that the soil solution in a particular soil horizon has already equilibrated with the bulk soil. This may be especially true with tension lysimeters, which collect water that is held against the gravitational gradient and thus, probably, have a high residence time. Michalzik et al. (1998) accounted for this by correlating the DOC concentrations of leachates from different O layers of spruce and hardwood forest floors with the solution pH of the overlying horizon. Remarkably, these correlations were also not statistically significant.
In contrast, Schindler et al. (1992, 1997) demonstrated a rapid decline in DOC concentration in bog pools of an experimentally acidified wetland, and Cronan and Aiken (1985) found a strong negative correlation between DOC concentrations and the pH between 4.8 and 3.5 of O/A horizon leachates for three forested watersheds. The same was shown by Ross and Bartlett (1996) for spodosol soil solutions from another site. However, Guggenberger and Zech (1993) and Zech et al. (1994) detected significantly elevated DOC concentrations and fluxes at coniferous sites with high acidic inputs and lower soil pH in relation to other sites. They suggested that higher acidic inputs intensified the leaching of bridging polyvalent cations such as Al3+, Ca2+, and Mg2+ and, therefore, enhanced the solubilization of organic matter.
Whether the mobilization of DON and DOP follows the same patterns as DOC is still unknown as information about these components is scarce. Raubuch and Beese (1997) found that increasing soil acidification led to reduced N mineralization and higher carbon respiration. The decline of DOC concentrations induced by artificial acidification in a northwestern Ontario lake was accompanied by rising DON concentrations, suggesting that DOC and DON may respond differently to acid inputs, at least in aquatic systems (Schindler et al. 1992).
In conclusion, mobilization of DOM from forest floors and soil organic matter seems to be favored by increasing pH values due to physicochemical properties (Fig. 4). The influence of pH on the microbial production of DOM is not clear. DOM release from subsurface horizons is caused primarily by desorption of formerly adsorbed organic matter. At high pH values, adsorption capacity is diminished and, therefore, DOM mobilization is enhanced. At low pH values, dissolution of organo-metal complexes can contribute to leaching of DOM. The most important conclusion is that despite the reported pH effects on the dynamics of DOM in the laboratory, in the field these effects seem to be small, i.e., within the normal pH range of the soils (Fig. 4).
Fig. 4. pH effects on the release and adsorption of DOM in batch and field studies.
The effect of ionic strength on DOM mobilization is ambiguous and may involve several processes that differ in significance for each soil compartment. Solubilization of organic matter might be impeded by high ionic strength solution reducing the charge density of organic substances (Tipping and Hurley, 1988) leading to coagulation. On the other hand, competition between DOM and other anions could displace DOM from adsorption sites, thus enhancing leaching.
Evans et al. (1988) found DOC mobilization from soil columns of A, E (eluvial) and Bs (illuvial accumulation of sesquioxides) horizon material leached with acid solutions to be negatively correlated to ionic strength, a result supported by Kaiser (1992) with A and B horizon samples from acid forest soils, by Tipping and Hurley (1988) with organic soils, by Vance and David (1989) with mineral B horizons, and by Davis (1982) with Al-oxides. Skyllberg and Magnusson (1995) reported the same relationships for Oa and A horizon material of Humic Gleysols but found DOC mobilization from Oe and Oa horizon material of Haplic Podzols from the same site to be insensitive to changes in ionic strength. They hypothesized that the differences were caused by different stages of humification of organic matter in both soils. In a column-leaching experiment with soil from a mixed forest stand, Berry et al. (1990) observed a decrease in DOC mobilization from Oa horizon material with increasing ionic strength of percolation solutions but increased DOC leaching from mineral horizon material and from a reconstructed profile (Oa/EB). The authors gave no explanation for this different behavior of forest floor and mineral soil horizons. Gu et al. (1994) found no difference in the amount of DOC adsorbed onto hematite between NaCl solutions of 0.01 and 0.1M. Jardine et al. (1989) also found no significant correlation between DOC adsorption to B-horizon soil samples and the ionic strength of the solution used.
Fotovat and Naidu (1998) investigated DOC adsorption onto acidic and alkaline soil samples in relation to ionic strength. They found DOC adsorption on acid soils diminished with increasing ionic strength up to 0.05 M (up to 0.03 M for alkaline soils) but remained constant at higher electrolyte levels up to 0.2 M.
Although information about ionic strength effects on DOC release from litter and soils is not always consistent, increasing ionic strength seems to reduce mobilization of DOC, at least in organic horizons. In addition it may be difficult to tease effects of ionic strength apart from pH effects as the former strongly influences the latter in acid forest soils (Wiklander, 1975; Richter et al., 1988; Kaiser and Kaupenjohann, 1998).