Sowers Tyler D, Stuckey Jason W, Sparks Donald L
Department of Plant and Soil Sciences and Delaware Environmental Institute, University of Delaware, 221 Academy Street, ISE Lab, Newark, DE, 19711, USA.
Biology Department, Multnomah University, 8435 NE Glisan St, Portland, OR, 97220, USA.
Geochem Trans. 2018 Feb 3;19(1):4. doi: 10.1186/s12932-018-0049-4.
Sequestration of organic carbon (OC) in environmental systems is critical to mitigating climate change. Organo-mineral associations, especially those with iron (Fe) oxides, drive the chemistry of OC sequestration and stability in soils. Short-range-ordered Fe oxides, such as ferrihydrite, demonstrate a high affinity for OC in binary systems. Calcium commonly co-associates with OC and Fe oxides in soils, though the bonding mechanism (e.g., cation bridging) and implications of the co-association for OC sequestration remain unresolved. We explored the effect of calcium (Ca) on the sorption of dissolved OC to 2-line ferrihydrite. Sorption experiments were conducted between leaf litter-extractable OC and ferrihydrite at pH 4 to 9 with different initial C/Fe molar ratios and Ca concentrations. The extent of OC sorption to ferrihydrite in the presence of Ca increased across all tested pH values, especially at pH ≥ 7. Sorbed OC concentration at pH 9 increased from 8.72 ± 0.16 to 13.3 ± 0.20 mmol OC g ferrihydrite between treatments of no added Ca and 30 mM Ca addition. Batch experiments were paired with spectroscopic studies to probe the speciation of sorbed OC and elucidate the sorption mechanism. ATR-FTIR spectroscopy analysis revealed that carboxylic functional moieties were the primary sorbed OC species that were preferentially bound to ferrihydrite and suggested an increase in Fe-carboxylate ligand exchange in the presence of Ca at pH 9. Results from batch to spectroscopic experiments provide significant evidence for the enhancement of dissolved OC sequestration to 2-line ferrihydrite and suggest the formation of Fe-Ca-OC ternary complexes. Findings of this research will inform modeling of environmental C cycling and have the potential to influence strategies for managing land to minimize OM stabilization.
环境系统中有机碳（OC）的固存对于缓解气候变化至关重要。有机-矿物结合体，尤其是那些与铁（Fe）氧化物结合的结合体，驱动着土壤中有机碳固存和稳定性的化学过程。短程有序的铁氧化物，如纤铁矿，在二元体系中对有机碳表现出高亲和力。钙通常在土壤中与有机碳和铁氧化物共同结合，但其键合机制（如阳离子桥联）以及这种共同结合对有机碳固存的影响仍未得到解决。我们探究了钙（Ca）对溶解态有机碳吸附到二线纤铁矿上的影响。在不同的初始碳/铁摩尔比和钙浓度下，于pH值为4至9时进行了叶片凋落物可提取有机碳与纤铁矿之间的吸附实验。在所有测试的pH值下，尤其是在pH≥7时，存在钙的情况下有机碳对纤铁矿的吸附程度均有所增加。在不添加钙和添加30 mM钙的处理之间，pH值为9时吸附的有机碳浓度从8.72±0.16增加到13.3±0.20 mmol有机碳/克纤铁矿。批量实验与光谱研究相结合，以探究吸附的有机碳的形态并阐明吸附机制。衰减全反射傅里叶变换红外光谱（ATR-FTIR）分析表明，羧基官能团是主要的吸附有机碳物种，优先与纤铁矿结合，并表明在pH值为9时存在钙的情况下，铁-羧酸盐配体交换增加。从批量实验到光谱实验的结果为溶解态有机碳固存到二线纤铁矿的增强提供了重要证据，并表明形成了铁-钙-有机碳三元络合物。本研究结果将为环境碳循环建模提供信息，并有可能影响土地管理策略，以尽量减少有机物质的稳定化。
