Physical and Chemical Stabilization of Organic Matter by Iron Oxides: Reconciling Observations at the Nano and Landscape scales

Concentrations of iron oxides are sometimes well correlated with concentration of organic carbon in the soil. But, this is highly dependent on the type of oxides, and their concentrations. On a northern California toposequence we found that high concentrations of crystalline Fe oxides (mainly goethite in subsoils of flat or convergent slopes) is accompanied with low concentrations of organic carbon, that is old (very negative δ14C values). It has been hypothesized that this could be because the concentrations of Fe oxides in some Mediterranean, and temperate soils may be too small to have significant effect on carbon accumulation and stabilization. In this study we employ different field and lab measurements to determine if there exists a threshold oxide concentration for soil organic carbon storage and stabilization; and determine whether the organic matter is primarily stabilized physically (by aggregation) or chemically (organo- mineral complexation). Here we are using (a) selective dissolutions of Fe oxides to determine concentration of crystalline Fe oxides; (b) density fractionations to determine amount of organic carbon chemically bound to the mineral fraction; (c) hydrogen fluoride demineralization to determine the stock of recalcitrant organic carbon; (d) batch sorption/desorption experiments with synthetic goethite and natural organic matter to determine reversibility of the oxide-organic carbon interactions at different concentrations of sorbent and sorbate, along with ultrasonic disruption of aggregates; and (e) concentrations 13C NMR and FTIR to determine aromaticity of the organic constituents. In this study we show that, as the concentration of oxides increases beyond a threshold level (that depends on type of oxide and chemistry of sorbing organic matter) there is no proportional increase in amount of carbon that is stored by the oxides, or in storage and stabilization of aromatic functional groups. But increasing oxide concentrations were well correlated with fraction of stabilized organic carbon.