Changes in Dissolved Organic Matter Structural Characteristics Induced by Exposure to Goethite and Gibbsite Surfaces: DRIFT and TG/DTA Analysis.

The release of dissolved organic matter (DOM) from forest floor material constitutes a significant flux of C to the mineral soil in temperate forest ecosystems, with estimates on the order of 120 to 500 kg C ha^-1 year^-1. Interaction of DOM with minerals and metals results in sorptive fractionation and stabilization of OM within the soil profile. Iron and aluminum oxides, in particular, have a significant effect on the quantity and quality of DOM transported through forest soils due to their high surface area and the toxic effects of dissolved aluminum. We incubated forest floor material in the presence of 1) goethite surfaces 2) gibbsite surfaces and 3) quartz sand surfaces (as a control treatment) to evaluate changes in molecular and thermal properties of dissolved organic matter as a product of interaction with oxide surfaces. Forest floor material was incubated over a period of 154 days. Dissolved organic matter was harvested on days 5, 10, 20, 30, 60, 90, and 154, and examined by Thermogravimetry/Digital Thermal Analysis (TG/DTA) and Diffuse Reflectance Fourier Transform infrared spectroscopy (DRIFT). Results indicated significant differences in DOM quality among treatments, though the effect of oxide surfaces on DOM properties did not change significantly with increasing time of incubation. Interaction with goethite produced DOM of mid-range thermal lability which was depleted in both proteins and fatty acids. Interaction with gibbsite produced DOM rich in thermally recalcitrant and carboxyl-rich compounds in comparison to the control treatment. DOM from both the goethite and gibbsite treatments had significantly lower C contents than DOM from the control treatment. This work indicates that interaction of DOM with oxide surfaces may play an important role in determining the biodegradability of DOM in forest soils.