Calcium enhances adsorption and thermal stability of organic compounds on soil minerals

Increasing organic matter storage in highly weathered soils is an important strategy to improve soil fertility and to help mitigate rising atmospheric carbon dioxide levels that may cause climate change. High calcium (Ca) availability in soils resulting from lime and gypsum application may increase soil organic matter contents, and this is frequently attributed to increased adsorption of organic molecules by soil minerals. Here, we evaluated the effects of Ca on the thermal stability of organo-mineral associations in a synthetic mineral mixture of kaolinite, goethite, and aluminum (Al)-oxides mimicking the mineralogy of the clay-size fraction of weathered tropical soils. Sorption experiments were carried out with humic acid (HA) at pH 6.5 and 4.5, using Ca concentrations ranging from 0.0 to 0.4 mM. Adsorption of HA increased with decreasing pH. The presence of 0.4 mM Ca increased HA adsorption by a factor of approximately 2.4 at pH 6.5 and 1.6 at pH 4.5. The thermal stability of HA increased upon Ca addition, suggesting that Ca strengthens HA-mineral bonding. UV-Vis analyses indicated that HA is fractionated during sorption in all treatments, and that fractionation may increase in the presence of Ca, possibly by enhanced preferential adsorption of HA compounds of relatively low molecular weight. Attenuated total reflectance-Fourier transform infrared spectroscopic results indicated that HA adsorbed primarily through the carboxylate moieties under all conditions. Our combined results show that dissolved Ca not only increases the extent of HA adsorption by soil minerals, but may also enhance the chemical stability and change the physical-chemical properties of sorbed HA compounds. A stronger association of organic compounds with mineral surfaces in the presence of Ca may be a significant contributing factor to Ca-promoted OM accumulation in soils.