Effects of Sorption Temperature on Soil Organic Matter and Iron Oxide Interactions

Abstract:

Soils and sediments account for the largest reservoir of actively-cycling organic carbon. Small changes in the flux of the soil carbon reservoir may affect the concentration of greenhouse gases. A natural mechanism for the protection of organic carbon from decomposition in soil is the formation of organo-mineral associations. We investigated the sorption dynamics between soil humic and fulvic acid and hematite at a five temperatures (15°C, 20°C, 25°C, 30°C, 35°C). Our research objectives were to determine the effects of sorption temperature on 1) sorption extent, 2) selectivity in the type of bound organics, and 3) the thermal stability of organo-mineral associations formed at different temperatures. The extent of adsorption of humic acid (determined by batch sorption experiments) was greater than that of fulvic acid for all five temperatures at low initial organic carbon concentrations. We also found the extent of humic acid adsorption to be significantly less with warming for two sets of temperatures (15°C-25°C, 15°C-30°C) at lower initial concentrations. The extent of fulvic acid adsorption was significantly less for three sets of temperatures (15°C-20°C, 15°C-30°C, 25°C-30°C) at higher initial concentrations. An elemental and isotopic analysis was conducted to characterize the bound organics and suggests there may be fractionation of organic components at higher temperatures. The C/N ratios may suggest enhanced sorption of amine-rich components at higher temperatures. We also conducted a thermogravimetric analysis to determine the temperatures at which organic matter decomposes as well as the energy associated with the decomposition process. Our preliminary results suggest organo-mineral associations formed at higher temperatures may possess higher thermal stability. The results of this study are not only important for understanding the response of carbon cycling in soil to climate changes but also for sustainable agriculture practices as organic matter is vital for soil fertility.