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DIVISION S-9-SOIL MINERALOGY

Organic Matter–Surface Area Relationships in Acid Soils

^a Darling Marine Center, Univ. of Maine, Walpole, ME 04573
^b Plant and Soil Sciences Dep., Univ. of Massachusetts-Amherst, Amherst, MA 01003

Corresponding author (lmayer{at}maine.edu)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Soil organic matter (OM) and mineral surfaces are intimately related, affecting the dynamics of each and their reactivity with many environmentally important substances. We examined the coverage of mineral surfaces by OM in acid soils of Massachusetts. Specific surface areas are controlled by a combination of clay and sesquioxide contents. Subsurface horizons, especially C horizons with pH 4.6 to 4.8, contained a phase with significant microporosity (pores <2 nm) that could be eliminated by 350°C muffling. Organic C (OC) concentrations in surficial (A, O) horizons have surface area-normalized loadings usually above the monolayer-equivalent (ME) level (1 mg OC m^-2), while B and C horizons usually have loadings at this level. Surface area-normalized loadings are inversely related to pH for each horizon type. Samples with high loadings show occlusion of the bulk of mineral surface area by OM, as evidenced by release of significant surface area after OM removal. However, a new method of assessing OM coverage of exposed surfaces, using the energetics of gas adsorption, indicate that the bulk of surface area exposed in most untreated samples consists of mineral rather than organic material. The data are consistent with a model in which the occluding OM is present in a low-surface area configuration, such as organoclay aggregates, rather than as dispersed coatings on mineral grains.

Abbreviations: BET, Brunauer–Emmett–Teller • EG, ethylene glycol • ME, monolayer-equivalent • OC, organic C • OM, organic matter

	INTRODUCTION

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ORGANIC MATTER concentrations and fine-grained mineral contents often covary with one another in soils (e.g., Nichols, 1984). This correlation has suggested sorptive control of OM contents that is limited by surface area (Sollins et al., 1996), though protection of OM within clay–OM aggregates provides an alternative hypothesis. Organic matter concentrations, fine-grained mineral contents, and surface area can be important correlates for other important soil properties or processes, such as contaminant accumulation, nutrient dynamics, or physical characteristics. Distinguishing causal relationships among the latter processes and these three properties requires a deeper understanding of the relationships among the three properties.

Several studies have shown that OC concentrations in continental shelf marine sediments, normalized to sediment specific surface area, are 1 mg OC m^-2 (Mayer, 1994a, 1994b; Keil et al., 1994). This loading has been previously termed the monolayer-equivalent (ME) level, because its value is equivalent to that to be expected for a monolayer of moderately sized organic molecules coating all surfaces. Organic matter loadings relative to surface area have received only minor attention for terrestrial soils. We reported previously that about one-half of soil samples from geographically diverse areas showed OM loading in A horizons, normalized to mineral surface area, similar to the ME level (Mayer, 1994b). However, these loadings have not been examined for other soil horizons.

This ME loading of OM may or may not represent a dispersed coating over all mineral surfaces. Such a question is difficult to answer by most spectroscopic methods for the types of surfaces found in soils. However, to address this question we have developed a method based on the energetics of N gas adsorption (Mayer, 1999). Nitrogen shows higher enthalpy of adsorption onto mineral than onto organic surfaces, so that the difference in enthalpy between an untreated sample and one with its OM removed can be used to determine the extent to which the untreated surfaces are organic or mineral in nature. For marine sediments, we found that OM coats only a small fraction of sedimentary minerals, making the ME phrase somewhat misleading.

Little attention has been given to the degree of coating of soil mineral grains by OM, though there has been active consideration of the relative contributions of minerals and OM to total surface area (e.g., Chiou et al., 1990; Pennell et al., 1995). Does OM form continuous coatings on soil mineral surfaces, or does it coat only a minor fraction of mineral surfaces as found for sediments? This question is relevant to a variety of environmental processes such as mineral formation and dissolution or contaminant sorption. Here we address OM/surface area ratios and the question of mineral coverage by OM for a series of acidic soils from the northeastern USA.

	MATERIALS AND METHODS

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Soil Sampling
We sampled various soil series in Massachusetts (Table 1). The soil types examined here are mostly sandy, well-drained, and acidic. Soil pits (1 m²) were dug to a depth of at least 1 m or to the bedrock surface. Standard soil profile descriptions were made at each site (Bartos, 1994). Soil samples were collected from identifiable soil horizons. Soil samples were transported back to the laboratory, air-dried, sieved (<2 mm), ground mechanically with a Model 8510 Spex Pulverizer (Spex Industries, Edison, NJ) for 5 min, and stored for subsequent analyses.

View this table: [in this window] [in a new window]   Table 1. Soil description, horizon, depth of sampling, surface area of unmuffled samples (SFAu), surface area of muffled samples (SFAm), excess enthalpy of N2 adsorption on unmuffled samples (Hxsu), excess enthalpy of N2- adsorption on muffled samples (Hxsm), organic C concentration (OC), total N concentration (TN), percentage clay, dithionite-extractable Fe and Al (Dith-Fe and Dith-Al), and pH of samples in H2O.

To assess the coverage of mineral surfaces by OM, we measured gas adsorption energetics using the approach of Mayer (1999). Briefly, we measured N₂ adsorption across the partial pressure range 0 to 0.3, and the C constant of the BET transform (Brunauer et al., 1938) was obtained from the most linear portion of the isotherm. This C constant was converted to its enthalpy equivalent by the expression

(1)

where M is a preexponential term assumed to equal 1, H_ads is the adsorption enthalpy of the gas directly on the surface; H_cond is the enthalpy of gas condensation, normally considered equivalent to adsorption of the second and higher layers of gas (Steele, 1974); H_xs (excess enthalpy) is their difference; R is the universal gas constant; and T is degrees Kelvin. This H_xs term was determined on samples before and after 350°C muffling. To assess the fraction of exposed surface that consists of OM, these two values are processed by an algorithm developed from model systems of various organic molecules adsorbed onto various mineral adsorbants (Mayer, 1999).

Other Measurements
Organic C and N were analyzed by Perkin-Elmer 2400 CHN analyzer (Perkin-Elmer, Norwalk, CT), according to Mayer (1994a). Particle-size analysis (clay, silt, and sand) was determined by the pipette procedure after pretreatment with H₂O₂ to remove OM. The pH of each soil was measured in both distilled water and CaCl₂ solutions (Thomas, 1996). Citrate-dithionite-extractable Fe and Al were measured according to Holmgren (1967).

	RESULTS

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The surface area values of these soils ranged from <1 to 49 m² g^-1 (Table 1). There were significant increases in surface area for many samples upon 350°C muffling, indicating occlusion of mineral surface area by OM. These increases were strongest and most consistent for horizons rich in OM (O, A; Fig. 1) and are similar to increases found in other soil studies (Burford et al., 1964; Feller et al., 1992; Pennell et al., 1995) but significantly greater than are typically found for marine sediments (Mayer, 1994a). For the organic-poor C horizon samples, surface area values showed small differences between the two treatments, indicating little impact of muffling on surface area values of the minerals themselves.

View larger version (15K): [in this window] [in a new window]   Fig. 1. Ratio of specific surface area values for soils subjected to only degassing to those subjected to 350°C muffling to remove organic matter, plotted against the organic C content of the unmuffled samples. Samples with ratio of 1 (i.e., most C horizons) thus had little change in surface area upon muffling. Abscissa expanded to log scale only to show data more clearly

View larger version (15K): [in this window] [in a new window]   Fig. 2. Specific surface area (m2 g-1) of 350°C-treated samples vs. the percentage clay, which are positively correlated . Diagonal line represents the relationship: Surface Area (m2 g-1) = 0.9(Percentage Clay), and roughly bounds the lower limit of the data

The numerical relationship

(2)

nicely delineates the minimum surface area values found for any given clay content (Fig. 2), but most samples clearly had higher surface area values than are predicted by this line. The y-axis residuals from this clay–surface area relationship correlated positively and strongly (P < 0.001) with both the Fe and Al extracted by dithionite-citrate solution, implying that sesquioxides also contributed significantly to the surface area values in samples. The slopes of these correlations were consistent with surface area values of several hundred square meters per gram for the sesquioxide phases, in agreement with previous measurements (e.g., Borggaard, 1982; Crosby et al., 1983). This finding is compatible with a previous report of the importance of Fe oxides in contributing to the surface area of soils (Borggaard, 1982). Because the clay content and dithionite-extractable Fe and Al all correlated with one another, it is impossible from our data to specify their relative influence on the surface area. Stepwise regression showed that each variable could explain surface area variance left over from regression on another variable, so it is likely that each of these phases plays a role in the overall surface area values. Further, analytical artifacts such as 350°C muffling or 150°C outgassing prior to surface area analysis may reduce or enhance the surface areas of sesquioxides (e.g., Aldcroft et al., 1968), making it more difficult to determine the relative contributions of these three variables.

Organic C concentrations ranged from 0.9 to 417 mg g^-1—that is, from essentially organic-free C horizons to essentially pure organic litter in an O horizon (Table 1). Total N content ranged from 0.02 to 21.7 mg g^-1, with the consequent C/N ratios ranging from 8.7 to 45.5. Lower C/N values are typical of OM stabilized in organoclay aggregates (Stevenson, 1994), and here we found that higher clay contents generally led to lower C/N ratios, although low C/N values were also found in some samples with low clay content.

Plotted against specific surface area, the OC concentrations of most of the samples from B and C horizons studied here fell into the ME zone as defined by the marine sediments studied previously (Fig. 3) . All O, all E, and most A horizon samples had OC loadings higher than the ME level. However, OC concentrations did not show overall significant correlations with either clay content or surface area, although the C horizons alone did show significant correlations. The loadings, parameterized as a ratio of OC to surface area, instead showed a strong inverse relationship with soil pH (Fig. 4) , whether measured in H₂O or CaCl₂. This inverse relationship was driven by trends seen with the A and B horizons, with the A horizons having generally higher ratios at any given pH. Considered alone, the C horizon loadings showed no dependence on pH.

View larger version (18K): [in this window] [in a new window]   Fig. 3. Organic C concentrations vs. specific surface area. The diagonal lines bound the 95% confidence interval of the monolayer-equivalent zone from Mayer (1994a)

View larger version (13K): [in this window] [in a new window]   Fig. 4. Surface area-normalized organic C (OC) concentrations (mg OC m-2) vs. soil pH (measured in H2O). Inverse correlation significant at

View larger version (17K): [in this window] [in a new window]   Fig. 5. Excess enthalpy (Hxs) of N2 gas adsorption onto untreated soil samples, vs. (A) the percentage of Brunauer–Emmett–Teller surface area contained in micropores of <2 nm; (B) pH of the soil, measured in H2O; (C) the ratio of organic C (OC) concentration to the mineral surface area (SFA) (i.e., of muffled samples) (OC/SFA: mg OC m-2)

The highest H_xs values were found in soil horizons with pH (H₂O) values of 4.6 to 4.8 (Fig. 5b). C horizons with low OC/surface area ratio exhibited these anomalous H_xs values most strongly. Loss of these H_xs values upon muffling (see below) implies a loss of microporosity. A possible reason for this loss is that the microporosity in the untreated samples originates from a phase such as Fe or Al oxyhydroxides, which can exhibit significant microporosity (Torrent et al., 1992) and might recrystallize upon muffling to new phases with loss of surface area and microporosity. However, there was no correlation of these high H_xs values with high levels of dithionite-citrate-extractable Fe or Al. This lack of correlation does not necessarily exclude Fe or Al oxyhydroxides as the source of the microporosity, but the responsible phases remain unidentified. The high H_xs values are consistent with the common presence of sesquioxides in the lower horizons of podsols (e.g., Gustafsson et al., 1999). The maximal H_xs values in the C rather than the B horizons, in which sesquioxide concentrations generally peak, may be due to occlusion of micropores by adsorption of OM in B horizon sesquioxides (Farmer, 1982). An alternative explanation for this microporosity is the possibility that OM causes clay platelets to orient in a fashion that creates interparticle micropores, which are lost upon destruction of the OM via muffling. Microporosity is known to arise from clay mineral interparticle contacts (Murray and Quirk, 1990), but this explanation seems less likely than one involving sesquioxides.

The H_xs values of the untreated samples (i.e., those with their OM still present) were inversely correlated with the ratio of OC to the surface area of the mineral fraction (i.e., the surface area of the muffled samples) (Fig. 5c). Most C and B horizon samples had H_xs values similar to those of mineral surfaces, while A horizons showed a gradation from mineral to organic surface values and O horizons showed values characteristic of largely organic surfaces.

Upon muffling of the samples, the range of H_xs values narrowed to 10.2 to 13 kJ mol^-1, so that samples with previously low values yielded higher H_xs values and the converse for samples with previously high values (Table 1). This narrower range is consistent with bulk aluminosilicate mineral surfaces (Mayer, 1999). Although the unusually high H_xs values disappeared upon muffling, there were no accompanying decreases in surface area values, consistent with the relatively minor (<30%) contributions to surface area from microporosity calculated by the t-plot approach (Fig. 5a). Moreover, none of the relationships between H_xs values and percentage microporosity, pH, or OC/surface area ratio (Fig. 5) persisted after muffling. Thus, phases affected by muffling were implicated for the low and high H_xs values found with the untreated samples—that is, phases such as OM for samples with low H_xs values and metal oxyhydroxides for samples with high H_xs values. As noted above, micropores in inorganic phases may have been present in samples with higher organic loading, but filled with OM. Such samples would exhibit less microporosity in the unmuffled treatments but would lose both microporosity and OM upon muffling.

	DISCUSSION

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Organic Matter Loadings
It is clear that OC levels in these soils were often higher than is consistent with ME loadings found in marine sediments and soils from other regions (Mayer, 1994a, 1994b). The A horizons studied here had higher loadings than found for many A horizons, but in keeping with those of acidic soils in other parts of the USA (compare Fig. 3 with Fig. 5 in Mayer, 1994b). It is also clear that the higher loadings occurred in association with lower pH values (Fig. 4). There was also an inverse correlation between C/N ratio and pH (Fig. 6) so that these excess loadings in lower pH soils may represent largely undecomposed vascular plant detritus, which was visible in many of the samples. Samples from soils with higher pH, or from lower soil horizons, had OC/surface area ratios and C/N ratios similar to organomineral aggregates (Mayer et al., 1993; Bock and Mayer, 2000; Stevenson, 1994). This pH trend is in accord with the preferential preservation of lignin biomarkers in acid soils and relative abundance of nitrogenous compounds in higher pH soils found by van Bergen et al. (1998).

View larger version (13K): [in this window] [in a new window]   Fig. 6. Carbon/N ratio (w/w) vs. soil pH (measured in H2O)

View larger version (19K): [in this window] [in a new window]   Fig. 7. Schematic diagram of clay minerals in soil, with organic matter binding some fraction of them into aggregates in untreated soil (above, with organic matter represented as solid ovals), such that a major fraction of the exposed surface area is due to uncoated minerals. Upon removal of the organic matter by muffling (below) the clays previously contained within aggregates are exposed, vastly increasing the surface area

View larger version (19K): [in this window] [in a new window]   Fig. 8. Fraction of surfaces in soils that is organic, plotted vs. soil pH: (A) fraction of exposed surfaces (in the untreated samples) that is organic, as calculated according to the excess enthalpy method of Mayer (1999); (B) fractional coverage by organic matter of all mineral surface area, and is a minimum estimate based on the extent of occlusion of total mineral surface area that is exposed by organic matter removal. O horizon samples are not plotted because of lack of pH data; their organic coverage values are 1 according to either calculation method

(3)

where SFA_m is the surface area of the muffled sample (i.e., total mineral surface area) and SFA_u is the surface area of the untreated sample. This estimate is conservative because it assumes that all surface area measured on the untreated sample had no OM coating. This assumption is generally supported by the enthalpy-based calculations (Fig. 8a), which show that exposed surfaces in most samples were largely inorganic mineral in nature. To the small extent that this assumption is incorrect, fractional cover of minerals by OM will be even greater than the estimates derived from Eq. [3]. Nevertheless, using Eq. [3] with this assumption, only samples with higher pH values have exposed surfaces dominated by uncoated minerals (Fig. 8b). Samples with lower pH, especially from upper soil horizons, have most mineral surfaces covered by OM.

The differing fractional coverages in Fig. 8a and 8b are not different answers to the same question, but rather differ in the question they address. Figure 8a shows that most exposed surface area (accessible to N₂ at 77°K) is inorganic mineral, while Fig. 8b implies that most mineral surface is covered by OM. These relationships imply that coverage of soil mineral surfaces occurs via an occlusion rather than a simple adsorption mechanism.

The pH dependence of surface coverage is consistent with simple adsorption of OM as predicted by laboratory experiments (e.g., Gu et al., 1995). However, adsorption by relatively small molecules in a thin coating probably would not reduce surface area of the minerals (Mayer, 1999).

Implications for Soil Reactivity
The high degree of mineral surface occlusion in these lower pH soil horizons differed considerably from the greater mineral exposure found in marine sediments (Mayer, 1999) and perhaps higher pH soils. One consequence of this occlusion may be greater protection of minerals from equilibration with bulk soil solutions. This protection may lead to either inhibited reactivity (e.g., via simple armoring of the mineral surface) or enhanced reactivity (e.g., via formation of microenvironments enriched in organic acids).

The high occlusion also implies that organomineral aggregates in the lower pH soils have reduced diffusional access for contaminant sorption, relative to their organic content. Organic matter types vary in their abilities to sorb contaminants (Xing et al., 1994; Xing and Pignatello, 1997; Huang and Weber, 1997). It seems likely that the type of OM responsible for ME loadings, with its minimal occlusion of mineral surface area, will have different sorbing characteristics than the OM in low-pH soils that strongly occludes the minerals. Again, it is difficult to predict even the direction of the net effect; for example, the sorption potential of occluded aggregates might be greater because of geometric considerations but lesser because of compositional ones. The different sorbant characteristics may be exhibited as different sorption capacities, kinetics, or degree of adsorption–desorption hysteresis; these implications merit further attention.

	ACKNOWLEDGMENTS

 
This work was supported by NSF-Marine Chemistry. We thank L. Schick for laboratory assistance and Dr. Peter Veneman for his help with soil sampling. This work represents Contribution no. 357 of the Darling Marine Center.

Received for publication August 4, 1999.

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