Organic Matter–Surface Area Relationships in Acid Soils

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.