Adsorption of humic acid to interlamellar surfaces of montmorillonite

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The world's soils are the largest repository of terrestrial organic carbon, nearly 3 times larger than the terrestrial biotic C pool, and more than twice the size of the atmospheric C pool. Despite the magnitude of the soil organic carbon (SOC) reservoir, there is considerable potential for further SOC additions, especially in managed ecosystems, with rates of SOC sequestration estimated to be as high as 1.2 Gigatons per year. These SOC additions have been proposed as a means to offset greenhouse gas emissions, thereby helping to achieve targets established within the Kyoto Protocol. Among the various fractions to which SOC may be sequestered, only the passive pool, with mean residence times in excess of 1,000 years, has sufficient stability to serve as a suitable long-term store of organic C. Placement of organic C in this refractory pool therefore remains the aim of all comprehensive C sequestration strategies. Among the most recalcitrant organic molecules are the humic substances, such as humic acid and fulvic acid. These materials are particularly resistant to degradation when they are strongly adsorbed to mineral surfaces. Thus, mineral-complexed humic materials are an important and desirable component of the stable SOC reservoir. The aim of this investigation is to determine how humic acid becomes adsorbed to the internal surfaces of montmorillonite, a common soil mineral. We will test the hypothesis that a certain group of natural organic molecules, possessing long saturated hydrocarbon chains and bound to montmorillonite edges, serve as attachment points for the humic acid, thus positioning this organic molecule near the gallery entrance and so facilitating interlamellar adsorption. Our data will reveal the mechanisms through which humic materials become intercalated by expanding layer silicates, contributing to the stable SOC pool. This research will also inform global soil C management strategies, guiding C sequestration efforts to those soils in which long-term organic C storage is most favoured.