Gone or just out of sight? The apparent disappearance of aromatic litter components in soils
- 1Soil Science and Soil Protection, Martin Luther University Halle-Wittenberg, von-Seckendoff-Platz 3, 06120 Halle (Saale), Germany
- 2Institute of Soil Science and Site Ecology, Technical University Dresden, Pienner Strasse 19, 01737 Tharandt, Germany
- 3Institute of Biodiversity and Ecosystem Dynamics, Earth Surface Science, University of Amsterdam, Postbus 94240, 1090 GE Amsterdam, the Netherlands
- 4Department of Land, Air, and Water Resources, University of California, One Shields Avenue, Davis, California 95616, USA
Abstract. Uncertainties concerning stabilization of organic compounds in soil limit our basic understanding on soil organic matter (SOM) formation and our ability to model and manage effects of global change on SOM stocks. One controversially debated aspect is the contribution of aromatic litter components, such as lignin and tannins, to stable SOM forms. In the present opinion paper, we summarize and discuss the inconsistencies and propose research options to clear them.
Lignin degradation takes place stepwise, starting with (i) depolymerization and followed by (ii) transformation of the water-soluble depolymerization products. The long-term fate of the depolymerization products and other soluble aromatics, e.g., tannins, in the mineral soils is still a mystery. Research on dissolved organic matter (DOM) composition and fluxes indicates dissolved aromatics are important precursors of stable SOM attached to mineral surfaces and persist in soils for centuries to millennia. Evidence comes from flux analyses in soil profiles, biodegradation assays, and sorption experiments. In contrast, studies on composition of mineral-associated SOM indicate the prevalence of non-aromatic microbial-derived compounds. Other studies suggest the turnover of lignin in soil can be faster than the turnover of bulk SOM. Mechanisms that can explain the apparent fast disappearance of lignin in mineral soils are, however, not yet identified.
The contradictions might be explained by analytical problems. Commonly used methods probably detect only a fraction of the aromatics stored in the mineral soil. Careful data interpretation, critical assessment of analytical limitations, and combined studies on DOM and solid-phase SOM could thus be ways to unveil the issues.