Organic matter-surface area relationships in acid soils

Soil organic matter (OM) and mineral surfaces are intimately related, affec ting the dynamics of each and their reactivity with many environmentally im portant substances, We examined the coverage of mineral surfaces by OM in a cid soils of Massachusetts. Specific surface areas are controlled by a comb ination of clay and sesquioxide contents. Subsurface horizons, especially C horizons with pH 4.6 to 4.8, contained a phase with significant microporos ity (pores <2 nm) that could be eliminated by 350 degreesC muffling, Organi c C (OC) concentrations in surficial (A, O) horizons have surface area-norm alized loadings usually above the monolayer-equivalent (ME) level (approxim ate to1 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 eac h horizon type. Samples with high loadings show occlusion of the bulk of mi neral surface area by OM, as evidenced by release of significant surface ar ea after OM removal, However, a new method of assessing OM coverage of expo sed surfaces, using the energetics of gas adsorption, indicate that the bul k of surface area exposed in most untreated samples consists of mineral rat her than organic material. The data are consistent,vith a model in which th e occluding OM is present in a low-surface area configuration, such as orga noclay aggregates, rather than as dispersed coatings on mineral grains.