The contribution of polyhydroxyl aromatic compounds to tetramethylammonium hydroxide lignin-based proxies

A problem inherent in analytical procedures where thermochemolyis or pyrolysis is coupled with in situ methylation by tetramethylammonium hydroxide is the inability to determine if an observed aromatic methoxyl group was originally present as a hydroxyl or a methoxyl functionality. This analytical insensitivity makes it impossible to ascertain relative inputs of fresh lignin, demethylated lignin and polyphenols (hydrolysable) into natural organic matter pools. 13C-TMAH thermochemolysis, however, methylates using a 13C-labeled methyl group permitting the differentiation of original and analytically-added methyl groups and therefore the potential source of many 'lignin' phenols. Common oak root, senescent leaf, wood, and bark as well as brown rot decayed oak wood are compared herein to test if the 13C-TMAH modification can be used to differentiate sources of phenols. Oak was chosen as it exhibits extremely high contents of hydrolysable tannins which interfere with the study of wood decomposition but also confound any comparative studies of the fate of lignin and tannin in soil profiles, dissolved organic matter, and sediments when using the unlabeled TMAH thermochemolysis technique. The average % monohydroxyl (i.e. 3,5-dimethoxyl, 4-hydroxyl in source) content of syringyl compounds for each sample varies as leaves (70%) < root (75%) < bark (77%) < brown rot wood (85.9%) < wood (89%). Some compounds such as permethylated syringic acid were almost 90% 3,4,5-trihydroxyl benzoic acid in origin as was determined for oak leaf, or, put another way, S6 was derived almost exclusively from gallic acid. Similarly, what was assumed to be permethylated ferulic acid in roots turned out to be predominantly caffeic acid with a 3,4-dihydroxyl substitution rather than the 3-methoxy, 4-hydroxy substitution. When individual compound concentrations are corrected for the polyhydroxyl content to better reflect 'true' lignin, or at least lignin predominantly bound via a ß-O-4 configuration as is released by this chemolysis procedure, the values for lignin proxies changed substantially. As such, it must be considered that many unlabeled TMAH methylated compounds released from natural systems and ascribed to a 'lignin' source may in fact be from tannins or other fresh or degraded phenolics

A comparative study of the molecular composition of a grassland soil with adjacent unforested and afforested moorland ecosystems

On-line thermally assisted hydrolysis and methylation (THM) in the presence of both unlabelled and 13C-labelled tetramethylammonium hydroxide (TMAH) was used to assess the relative contributions of phenolics (lignin, demethylated lignin and non-lignin phenolics) in a peaty gley soil profile beneath an unimproved grassland (LL), from a study site located at Harwood (Northumberland, northeast England, UK). This site also includes an unforested moorland (ML) and a second rotation Sitka spruce stand (SS). The common lignin proxies have been corrected for contributions of non-lignin phenols and demethylated lignin in the LL ecosystem and then compared with those from the ML and SS ecosystems. The phenolic compositions from the contributory vegetation inputs (i.e. grasses, heather and Sitka spruce) to all three soils (LL, ML and SS) were also analysed. By using 13C-labelled TMAH it was possible to show that the chemical composition of soil organic matter (SOM) reflected the different vegetation inputs in each of the L/F layers but these characteristics were lost from the deeper organic and mineral layers. Similar changes in the yield of lignin monomers (Λ) with increasing soil depth were displayed in the LL soil profile as reported previously in the ML soil in that no maxima were observed in these amount-depth profiles. The tannin input to the LL soil is low and as a consequence, unlike the ML and SS soils, there is no progressive decrease in the amounts of these non-lignin phenolics with increasing depth. Finally the methylated carbohydrate derivatives (MC) become more abundant relative to the phenolics with increasing soil depth in all three ecosystems (LL, ML and SS)