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Review article2015Peer reviewed

Organic acid induced release of nutrients from metal-stabilized soil organic matter - The unbutton model

Clarholm, Marianne; Skyllberg, Ulf; Rosling, Anna

Abstract

Processes of soil organic matter (SOM) stabilization and the reverse, destabilization of SOM resulting in subsequent release and mobilization of nutrients from SOM, remain largely unresolved. The perception of SOM as supramolecular aggregates built of low molecular mass biomolecules is currently emerging. Polyvalent metal cations contribute to SOM tertiary structure by bridging functional groups of such molecules (Simpson et al., 2002). The strong bond to metals protects high quality organic material from being immediately accessed and decomposed. Here we propose a three-step process by which low molecular mass organic acids (LMMOAs) and hydrolytic enzymes act in series to destabilize SOM supramolecules to release organic nitrogen (N) and phosphorus (P) for local hyphal and root uptake. Complexation of the stabilizing metals by fungal-released LMMOA gives fungal-root consortia direct access to organic substrates of good quality. Because of their small sizes and carboxyl group configuration, citric and oxalic acids are the most effective LMMOAs forming stable complexes with the main SOM bridging metals Ca and Al in SOM. Citrate, forming particularly strong complexes with the trivalent cations Al and Fe, is dominant in soil solutions of low-productive highly acidic boreal forest soils where mycorrhizal associations with roots are formed predominantly by fungi with hydrophobic hyphal surfaces. In these systems mycelia participate in the formation of N-containing SOM with a significant contribution from strong Al bridges. In less acidic soils of temperate forests, including calcareous influenced soils, SOM is stabilized predominantly by Ca bridges. In such systems mycorrhizal fungi with more hydrophilic surfaces dominate, and oxalic acid, forming strong bidentate complexes with Ca, is the most common LMMOA exuded. A plant-fungus driven biotic mechanism at the supramolecular aggregate level (10(3)-10(5) Da) resolves micro-spatial priming of SOM, where the destabilization step is prerequisite for subsequent release of nutrients. (C) 2015 Elsevier Ltd. All rights reserved.

Keywords

Citrate; Hydrophobic mycorrhizae; Organo-metal complexes; Oxalate; Rhizosphere priming; Soil organic matter

Published in

Soil Biology and Biochemistry
2015, Volume: 84, pages: 168-176
Publisher: Elsevier

      SLU Authors

        Sustainable Development Goals

        Protect, restore and promote sustainable use of terrestrial ecosystems, sustainably manage forests, combat desertification, and halt and reverse land degradation and halt biodiversity loss

        UKÄ Subject classification

        Forest Science

        Publication identifier

        DOI: https://doi.org/10.1016/j.soilbio.2015.02.019

        Permanent link to this page (URI)

        https://res.slu.se/id/publ/66116