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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

View larger version (15K): [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 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

View larger version (18K): [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 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 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)

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

View larger version (19K): [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 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