Gone or Just out of Sight? the Apparent Disappearance of Aromatic Litter Components in Soils
Tuesday, 16 December 2014: 10:50 AM
We present a literature study on controversies concerning the degradation of lignin and other aromatic litter components in soils. Lignin concentrations control rates of litter decomposition within biomes worldwide, implying that lignin is among the most resistant litter components. Consequently, lignin has been traditionally linked to the concept of soil organic matter (SOM) stability due to recalcitrance. However, studies using molecular-level analytics (e.g., CuO oxidation) suggest significant chemical alterations and losses of lignin components during litter decomposition. Lignin degradation occurs step-wise, including (i) depolymerization and (ii) further transformation of water-soluble depolymerization products. The long-term fate of depolymerization products and of other soluble aromatic litter components in mineral soils is still a mystery. Research on dissolved organic matter (DOM) indicates that they are important precursors of stable SOM attached to mineral surfaces. In laboratory experiments, aromatics were among the most resistant DOM components and prone to bind strongly to minerals. In accord, field data from various sites showed decreases of aromatic DOM during passage of mineral soil, presumably due to organic-mineral interaction. Recent research on composition of solid-phase SOM, however, contradicts DOM research. It finds that mineral-associated SOM is rather dominated by non-aromatic, microbial-derived compounds. Other studies suggest the turnover of lignin in soil to be faster than the turnover of bulk SOM. We assume that the contradictions might be due to difficulties in the analysis of aromatics. We conclude that we currently lack of understanding on long-term fate of major litter constituents and on contribution of aromatics to stable SOM. Careful data re-interpretation of available data, critical assessment of analytical limitations, and combined studies on DOM and solid-phase SOM might be ways to overcome contrasting perspectives.