Abstract 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.

中文翻译：

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钙增强有机化合物对土壤矿物质的吸附和热稳定性

摘要 在高度风化的土壤中增加有机质储存是提高土壤肥力和帮助缓解可能导致气候变化的大气二氧化碳水平上升的重要策略。施用石灰和石膏可提高土壤中钙 (Ca) 的有效性，这可能会增加土壤有机质含量，这通常归因于土壤矿物质对有机分子的吸附增加。在这里，我们评估了 Ca 对模拟风化热带土壤粘土大小部分矿物学的高岭石、针铁矿和铝 (Al) 氧化物的合成矿物混合物中有机矿物组合热稳定性的影响。吸附实验是用腐殖酸 (HA) 在 pH 6.5 和 4.5 下进行的，使用钙浓度范围为 0.0 到 0.4 mM。HA 的吸附随着 pH 值的降低而增加。0.4 mM Ca 的存在使 HA 吸附在 pH 6.5 时增加了约 2.4，在 pH 4.5 时增加了 1.6。添加 Ca 后 HA 的热稳定性增加，表明 Ca 增强了 HA 与矿物的结合。UV-Vis 分析表明，在所有处理中，HA 在吸附过程中被分馏，并且在 Ca 存在下分馏可能增加，这可能是由于相对低分子量的 HA 化合物的优先吸附增强。衰减全反射-傅立叶变换红外光谱结果表明，在所有条件下，HA 主要通过羧酸盐部分吸附。我们的综合结果表明，溶解的 Ca 不仅增加了土壤矿物质对 HA 的吸附程度，但也可能提高化学稳定性并改变吸附的 HA 化合物的物理化学性质。在 Ca 存在下，有机化合物与矿物表面的更强关联可能是 Ca 促进土壤中 OM 积累的重要因素。