Application of 13C-TMAH thermochemolysis to the study of forest floor decay dynamics

Background/Question/Methods The macromolecular components of plant tissue are often used as proxies for source input to litter, soils, sediments, and aquatic fractions, as predictors to decay rate in litter, and turn over time in soil fraction.  Decay state or degree of decomposition of the plant material is also frequently inferred through molecular break down products of lignin through the ratios of oxidized and reduced released compounds.  For plant components such as lignin, cutin, and suberin thermolytic and chemolytic techniques are the dominant analytical methods.   For non hydrolysable components, such as lignin, it is analytically challenging to extract a representative fraction of the material from any matrix and reconstruct source and process given extraction biases and chemical byproducts.  Any competent interpretation must address these biases.   One such technique which combines both chemolytic and thermolytic qualities is the thermally assisted hydrolysis in the presence of 13C-labeled -tetramethyl ammonium hydroxide.  This technique, like it non-¹³C labeled analog, primarily cleaves the b-O-4 linkage in lignin and all hydrolyzable ester linkages and then methylates phenolate, carboxylate and alkoxide moieties.  Additionally,  the methyl derivative is 100%¹³C permitting spectral quantitation of the degree of methylation.  This ¹³C-methyaltion has been used to quantify the degree of demethylation in wood and litter by soil and insect-hosted fungi, relative input and decay of hydrolysable tannins in soils, and selective decomposition of petiole and leaf blades in forest litter.

Results/Conclusions We discuss applications of this technique to a variety of leaf litter, coarse woody debris, and microbial inoculation studies where the paths of decay and source are inferred from the molecular ratios and level of ¹³C-labeleing on the lignin ring.  For example, wood decay by brown rot fungi exhibits only minor alteration to lignin, primarily a small degree of demethylation.  Selective analysis, however,  of water soluble fractions liberated during decay (up to 10% by weight) indicates that a mobile pool of highly demethylated (~20%) and oxidized lignin is generated progressively with time.  Soluble sugars exhibit a pulse during early decay stages.  Analysis of frass from a variety of  invertebrates shows a wide degree of alteration.  Some organisms exhibit demethylation and oxidation while others facilitate hydroxylation of the lignin ring.  The different trajectories in degraded biopolymer chemistry inferred with this technique can have large impacts on long term stabilitzation of soil organic matter, binding of forest floor cations, and the nature of dissolved organic matter.