Assessment of lignin and (poly-)phenol transformations in oak (Quercus robur) dominated soils by 13C-TMAH thermochemolysis

Two sandy soil profiles under oak forest were examined for their phenolic composition using 13C labelled tetramethylammonium hydroxide (TMAH) thermochemolysis, the first application of this method to soils. Lignin, demethylated lignin, hydrolysable tannin, and other phenolic acids could be distinguished, and as such indications of the relative distribution of these sources among the soil horizons was determined. Some compounds extracted from soils using TMAH thermochemolysis, all of which are permethylated phenols, can have multiple structural and botanical sources, such as 3,4-dimethoxybenzoic acid methyl ester from vanillic acid or demethylated vanillic acid derived from lignin, or protocatechuic acid as the initial compound. Correcting the common lignin proxies for contributions of non-lignin phenols and altered lignin, it was found that Λ (yield of monomers upon thermochemolysis), F/P (ferulic acid/p-coumaric acid), Ac/Al (acid/aldehyde) ratios, and Γ (indicator of side chain shortening) were severely influenced by non-lignin phenols. By contrast, the S/G (syringyl/guaiacyl) and C/G (cinnamyl/guaiacyl) ratios remained rather similar. The proxies that changed the most were those that affect interpretations of lignin patterns in terms of source, alteration and preservation. From L to Ah horizons, a progressive lignin alteration was found for both soils, but the differences in litter input among plant materials (leaves, bark, branches, roots) also potentially affected the lignin patterns within and between both profiles. Therefore, important insights into the rapid changes in lignin chemistry between organic and mineral horizons were provided as this method permitted enhanced distinction between fresh source input and decomposition. Even with the corrections to lignin proxies, the soil lignin and polyphenol chemistry remained complex and highlights the limitations of using a few or only one lignin proxy in assessing soil organic matter dynamics.