Molecular Trickery in Soil Organic Matter: Hidden Lignin

Abstract

Binding to minerals is one mechanism crucial toward the accumulation and stabilization of organic matter (OM) in soils. Of the various biochemicals produced by plants, lignin-derived phenols are among the most surface-reactive compounds. However, it is not known to what extent mineral-bound lignin-derived phenols can be analytically assessed by alkaline CuO oxidation. We tested the potential irreversible binding of lignin from three litters (blue oak, foothill pine, annual grasses) to five minerals (ferrihydrite, goethite, kaolinite, illite, montmorillonite) using the CuO-oxidation technique, along with bulk organic carbon (OC) sorption. Up to 56% of sorbed lignin could not be extracted from the minerals with the CuO-oxidation technique. The composition of the irreversibly bound lignin component differed markedly between minerals and from that of the parent litter leachates, indicating different bonding strengths related to individual monomers and conformations. The difference in extractability of individual phenols suggests that abiotic processes, such as sorption/desorption, should be taken into account when using CuO oxidation data for assessing lignin turnover in mineral matrixes. However, given the apparent relationship between aromaticity as indicated by carbon-specific UV absorbance (SUVA) and bulk OC sorption, it is likely that irreversible sorption is a concern for any technique that addresses the broad class of aromatic/phenolic compounds in soils and sediments.

The Genesis and Exodus of Vascular Plant DOM from an Oak Woodland Landscape

“…Average weighted carbon-normalized yields in the other six sample types ranged from 1.72 to 2.56 mg 100 mg OC −1 . These values are consistent with previous studies involving plant leachates (Hernes et al, , 2013aPellerin et al, 2010). Carbon-normalized yields in the litters and duffs prior to leaching ranged from 0.72 mg 100 mg OC −1 in the annual grass duff to 6.05 mg 100 mg OC −1 in the annual grass litter, and averaged 3.82 mg 100 mg OC −1 , while post-leaching values were 1.03-6.87 mg 100 mg OC −1 with an average of 4.17 mg 100 mg OC −1 , indicating preferential leaching/degradation of non-lignin plant constituents ( Table 1).…”

“…Characterizing fractionation effects in lignin biomarkers has significantly improved interpretation of DOM and lignin parameters in various environments, explaining apparent degradation trends that previously were conundrums (Hernes et al, , 2013aSpencer et al, 2008Spencer et al, , 2016. Lignin fractionation studies to date have primarily consisted of snapshot measurements, in which dissolved and particulate lignin is allowed to reach equilibrium, then a single measurement of each is compared to assess the magnitude of fractionation (Hernes et al, , 2013a.…”

Iron addition to soil specifically stabilized lignin

“…This is consistent with preferential interactions between Fe oxide minerals and aromatic lignin constituents (Gu et al, 1995;Kaiser, 2003;Riedel et al, 2013). Given that short-range-order Fe oxides are present in most soils (Reyes and Torrent, 1997), Fe-lignin interactions deserve attention as contributors to "hidden lignin" pools that have potentially been obscured due to methodological artifacts associated with common lignin measurements (Hernes et al, 2013). Lignins synthesized with labeled isotopes provide one method to surmount these analytical challenges.…”

“…In support of the new paradigm, several studies reported that lignin might decompose as fast or faster than bulk SOM (Dignac et al, 2005;Gleixner et al, 1999;Guggenberger et al, 1994;Heim and Schmidt, 2007;Kiem and K€ ogel-Knabner, 2003). Yet, these findings are difficult to reconcile with other studies demonstrating preferential association of aromatic lignin constituents with minerals, especially Fe oxides (Chorover and Amistadi, 2001;Gu et al, 1995;Hernes et al, 2013;Huang et al, 1977;Kaiser, 2003;Riedel et al, 2013). These associations would presumably decrease lignin mineralization, as has been observed for coprecipitates of lignin and ferrihydrite (Eusterhues et al, 2014).…”

Dynamics of Organic Matter in Leaf Litter and Topsoil within an Italian Alder (Alnus cordata (Loisel.) Desf.) Ecosystem

“…An important contribution to the dissolved organic matter leaching from the litter to the mineral soil is provided by oxidised lignin fragments [63,64]. The nature and role of lignin, especially in mineral soil, is still poorly understood [65,66]. Lignin was a large contributing variable in forest floor leaf litter within our two-block PLS.…”