Gone or just out of sight? The apparent disappearance of aromatic litter components in soils

Abstract

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.

Molecular Signals of Heterogeneous Terrestrial Environments Identified in Dissolved Organic Matter: A Comparative Analysis of Orbitrap and Ion Cyclotron Resonance Mass Spectrometers

“…During potential passage through deeper soil, aquifers, and downstream aquatic systems, these signal become less dominant and NOSC, AI MOD , and DBE of DOM decrease simultaneously with a shift from bi-or multi-modal ion abundance patterns to unimodal ones. Similar losses of aromatic constituents have been described for soils and rivers by other techniques (Creed et al, 2015;Klotzbücher et al, 2016). This implies that freshly decomposing materials can be traced by their contribution of aromatic-and phenolic-type formulae (Roth et al, 2013(Roth et al, , 2014(Roth et al, , 2016Nowak et al, 2017;Schwab et al, 2017).…”

Microbial and abiotic controls on mineral-associated organic matter in soil profiles along an ecosystem gradient

“…Several studies showed that lignin-derived aromatic compounds sorb preferentially to soil minerals due to their high content of carboxyl groups (e.g., summarized in Klotzbücher et al . 37 ). Sanderman et al .…”

Lignin Dimers as Potential Markers for 14C-young Terrestrial Dissolved Organic Matter in the Critical Zone

“…High abundances of this new marker in ground water could indicate fast transport of water and DOM through the subsurface. As aromatic organic matter is known to interact with mineral surfaces, the potential applicability of the proposed molecular markers in groundwater will depend on the mineral structure of the respective subsurface environment (Klotzbücher et al, 2016). Recent investigations show that a considerable proportion of lignin-derived phenols might be irreversibly sorbed to minerals (Kaiser and Guggenberger, 2000;Hernes et al, 2013), thereby limiting the potential traceability of lignin-derived markers into groundwater.…”