Extent and nature of organic coverage of soil mineral surfaces assessed by a gas sorption approach

Organic matter (OM) in soils often associates intimately with the surfaces of fine-grained minerals. We used two measures of OM–mineral associations, based on either the energetics of N2 gas sorption (C-constant in BET equation) or changes in mineral specific surface area (SSA) upon removal of OM, that provide complementary information on organic coverage of mineral surfaces. Undisturbed surface mineral soils along an altitudinal transect in Borneo provided a gradient of OM levels (30 to 140 mg-OC g− 1 soil) with which to address modes of mineral surface coverage by OM, and were compared with a variety of soils throughout the USA. Increasing OM levels, either as OM content or as a ratio to SSA (i.e. loading), led to coverage of the mineral surfaces proportional to the OM content. In all surface horizon samples from Borneo and the USA, cross-plots of the two measures show that these surfaces were covered by OM having low SSA (presumably in globular forms) rather than thin coatings of adsorbed OM. The latter mode was found only in some podzolic B-horizon samples. At loadings of > 3–4 mg-OC m− 2, virtually all mineral surfaces in soils derived from aluminosilicate parent materials were covered by OM. Iron-rich soils on ultrabasic parent material did not achieve this full coverage. At loadings < 2–3 mg-OC m− 2, exposed surfaces of the soils were dominantly mineral even though 10–70% of total mineral surface was inaccessible to N2 gas due most likely to organic occlusion of the mineral phases that have high SSA. These modes of OM-surface coverage may have important implications for aqueous–solid reactions in soil.