Extraction of aluminium and iron from organic associations in soil - developing a new selective method

• Applicant: Professor Dr. Thilo Rennert
• Subject Area: Soil Sciences
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 519407628

Metal ions such as Al3+ and Fe2+/3+ are released during soil development, and they may react with soil organic matter (SOM). Interactions between metal cations and SOM are known for decades, inducing SOM stabilization, that is, the deceleration of SOM decomposition. In acidic soil, these interactions comprise adsorption of Al and Fe ions on functional groups of SOM, and co-precipitation of the ions with SOM, resulting in organic associations of the metals. Oxalate dissolves metals in organic association, together with metals in mineral forms. Recent studies have shown that particularly the contents of oxalate-extractable Al are one of the predictors of the contents of soil organic carbon. Despite the importance of metals in organic association, there is no method for their selective extraction available. The proposed project aims to develop a scheme for the complete and selective extraction of Al and Fe in organic association and for the quantitative and qualitative characterization of the SOM associated with Al. A fluoride-based extraction (pH 6.5) will be developed and integrated into a sequential extraction scheme for this purpose. The scheme will separately extract Al and Fe from organic associations, poorly crystalline Fe oxides, and short-range ordered aluminosilicates. Selectivity and completeness of the fluoride-based extraction will be tested with models of organic associations and reference minerals before transferring the method to soils. Then, soil samples, in which enhanced contents of Al and Fe in organic association are expected, will be extracted according to the scheme. The samples will originate from horizons of reference soil groups of the WRB classification, into which metal-organic associations have been transported (Podzols, Luvisols), in which the associations have formed in situ (Andosols, Cambisols), and in which they have formed because of redox processes (Gleysols, Stagnosols). Depending on the pedogenic milieu, variable contents of metals and SOM in organic association are expected, together with variable contributions of metals in organic association to the contents of oxalate-extractable metals as a sum parameter. SOM extracted from associations with Al will be characterized with spectroscopic methods and a thermal one. These results are expected to differ as well, depending on the pedogenic milieu, for instance, larger proportions of low-molecular-mass organic acids in Podzols compared to Andosols with more aromatic SOM. The project is not only for the development and for the application of a selective extraction of Al and Fe in organic association in soil, but also for the characterization of a SOM fraction, which was not selectively accessible yet. Thus, apart from advancing soil-chemical analytics, the project will provide fundamental pedogenetic data that may be implemented into models on the controls of organic-carbon storage in soil, if larger sample sets will be studied.

• DFG Programme: Research Grants