Iron oxides as major available interface component in loamy arable topsoils

To characterize biogeochemical interface properties in soil with respect to oxide surfaces, the contribution of weakly crystalline and crystalline oxides to soil specific surface area (SSA) of particle size fractions was determined. Th ree arable topsoils with intermediate to high clay content were subjected to ultrasonic dispersion and particle size fractionation. The obtained silt and clay fractions were treated with hydrogen peroxide, extracted with oxalate and dithionite and the SSA of all fractions was determined using BET-N ₂. Results show that stable microaggregates were present in the coarse and medium silt fractions of all soils that could not be dispersed physically even at the highest ultrasonic dispersion energy and were probably stabilized by organic matter and iron oxides. Iron oxides were a major contributor to the SSA of all particle size fractions and the losses of carbon after oxalate and dithionite extraction showed that a major part of the organic matter in all particle size fractions was stabilized by iron oxides, even in these clay-rich soils. Weakly crystalline oxide surface area did not increase with decreasing particle size and calculated negative surface areas for some of the fine fractions indicated that weakly crystalline oxides were present as coatings on other minerals. The results demonstrate the importance of (iron) oxides for microaggregation and stabilization of organic matter in soil. However, the actual interface provided by these oxides depends on particle size and crystallinity due to the possible occlusion of mineral surfaces by organic matter and weakly crystalline oxides.