Associations of allophanic constituents and organic matter: Formation, properties and stabilization of organic matter
Final Report Abstract
Soil formation on volcanic parent material typically coincides with accumulation of allophanic constituents/short-range ordered aluminosilicates (SROAS) and soil organic matter (SOM). SROAS are clay-sized minerals with variable chemical composition and crystallinity. Binding of dissolved organic matter (DOM) by SROAS presumably protects SOM from microbial degradation in the long-term; however, the fundamental processes are poorly understood. Thus, we investigated interactions of DOM with SROAS by adsorption and co-precipitation. We used DOM collected in situ from a Dystric Cambisol and by water extraction from litter to clarify effects of the compositional variability of DOM. We synthesized SROAS that structurally resembled natural proto-imogolites and Si-rich SROAS. Adsorption of model organic acids (oxalic, salicylic, and octanoic acid) on SROAS was studied to resolve structural impacts on surface reactivity. Evaluation of fast adsorption kinetics (<30 s) and characterization of adsorbed species by infrared spectroscopy allowed us to detect ligand exchange with aluminol groups. Silicon-rich SROAS was less susceptible for ligand exchange than proto-imogolites due to differences in Al speciation, indicating minor stabilization of DOM by chemical bonds with Si-rich SROAS. Adsorption of soil DOM was driven by accessibility of aluminol groups on SROAS. As the poor crystallinity of proto-imogolites enabled pronounced aggregation, DOM adsorption was low after dehydration. This shows the importance of physical properties of the SROAS for SOM retention. As aromatic products of lignin oxidation preferentially adsorbed on SROAS, depth-dependent differences of aromatic DOM affected adsorption kinetics (1-168 h). Associations formed by co-precipitation contained more C than adsorption complexes. Co-precipitation of DOM increased as a function of aromatic C in DOM due to favored interactions of Al with oxidized aromatic compounds. However, in contrast to adsorption, carbohydrates were substantially sorbed by co-precipitation. Interference of DOM in SROAS formation modified the structure of the co-precipitates. Complexation of Al by ionic DOM moieties slowed crystallization of the dioctahedral sheet and enhanced abundance of ill-defined Si species. We detected formation of Si in imogolite-like configuration within 24 h; however, our results suggest that DOM impedes the assembly of precursors into tube- or sphere-shaped particles, particularly in topsoils. Degradability of co-precipitated carbohydrates was tested by addition of βglucosidase, a microbial extracellular enzyme. Depolymerisation of mineral-bound carbohydrates was lower compared to dissolved carbohydrates, evincing stabilization of easily degradable compounds by co-precipitation with SROAS. Our findings contribute to a mechanistic understanding of SOM sequestration, an important function of soils.
Publications
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Adsorption of soil-solution dissolved organic matter on allophanic constituents. International Symposium on Interactions of Soil Minerals with Organic Components and Microorganisms, Seville.
Lenhardt, K., H. Breitzke, G. Buntkowsky, I. Prater & T. Rennert
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Bildung von Assoziationen aus allophanischen Mineralen und gelöster organischer Bodensubstanz. Annual meeting of the German Soil Science Society, Göttingen.
Lenhardt, K., H. Breitzke, G. Buntkowsky, I. Prater & T. Rennert
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Synthesis of short-range ordered aluminosilicates at ambient conditions. Scientific Reports, 11(1).
Lenhardt, Katharina R.; Breitzke, Hergen; Buntkowsky, Gerd; Reimhult, Erik; Willinger, Max & Rennert, Thilo
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Characterization of poorly crystalline Al phases by FTIR and NMR spectroscopy. Recent topics in the characterization and application of minerals in soil systems, Symposium of Commission VII, German Soil Science Society, Gießen.
Lenhardt, K., H. Breitzke, G. Buntkowsky, C. Mikutta & T. Rennert
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Interactions of dissolved organic matter with short-range ordered aluminosilicates by adsorption and co-precipitation. Geoderma, 423, 115960.
Lenhardt, Katharina R.; Breitzke, Hergen; Buntkowsky, Gerd; Mikutta, Christian & Rennert, Thilo
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Interaktionen von gelöster organischer Bodensubstanz mit schwach kristallinen Aluminosilikaten bei Adsorption und Kopräzipitation. Annual meeting of the German Soil Science Society, Trier.
Lenhardt, K., H. Breitzke, G. Buntkowsky, C. Mikutta & T. Rennert
