Thursday, August 6, 2009: 2:50 PM
Galisteo, Albuquerque Convention Center
Background/Question/Methods Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) is a method of assessing the molecular fingerprint of a sample as it observes a diverse range of chemical constituents in fragments large enough to identify probable source molecule or structure (i.e. lignins, lipids and proteins). This is particularly useful in observing the lignin-derived isomers (guaiacyl and syringyl) and their relative degradation state. One drawback of this method is that fatty acids and acidic lignin structures are typically not observed, underestimating plant-derived lignin acids and plant- and microbial-derived large fatty-acids (derived from plant waxes and cutins). The addition of tetramethylammonium-hydroxide (TMAH) to the sample enables the detection of these structures, in particular p-coumaryl lignin-derived acids present in grasses and ferns. However, sample methylation often results in the loss of most carbohydrate-derived signal. Using both methods enables a more complete estimate of soil carbon chemistry. We explore how inherent plant tissue structural differences between a fern and angiosperm affect the chemical composition of soil organic matter and specifically whether the unique properties of fern leaf structure are inherited by the soils that form beneath this species. We used py-GC/MS and TMAH-py-GC/MS to assess the chemical composition of both plants and the soil beneath the slow-decomposing fern Dicranopteris compared to the more fast-decomposing species Cheirodendron and compare these results to those obtained by IR and 13C-NMR. Results/Conclusions Py-GC/MS shows general chemical structural differences between the two species. The fern Dicranopteris, contained more lignin-derived phenols, while the angiosperm contained more lipid-derived structures evident of the waxy protective coating, as well as a much higher carbohydrate content, corresponding to its quicker decomposition rate. Py-GC/MS was unable to distinguish between p-coumaryl and guaiacyl-derived lignin subunits, which are particularly important in grasses, ferns, or other non-woody structures. But, with the addition of TMAH, we detected p-coumaryl acidic lignin structures, and fatty acids. Fatty acids were particularly abundant in the waxy outer layer of Cheirodendron and their soil signature reaching their maximum concentrations in the A-horizon. These results suggest that the initial chemistry of the overstory vegetation influences the decomposition dynamics of organic matter found in O and to some degree A-soil horizons. Significant differences in chemistry resulting from plant-derived traits were evident in SOM under each species. These differences were detectable using py-GC/MS and TMAH-py-GC/MS, and were confirmed using NMR and IR.
