Influence of source vegetation and redox conditions on lignin-based decomposition proxies in graminoid-dominated ombrotrophic peat (Penido Vello, NW Spain)

Abstract

Most knowledge about the degradation of lignocellulose in natural environments is based on woody tissue and aerobic systems; however, in peatlands the contribution of graminoids to organic matter (OM) is often significant and anaerobic conditions prevail. In order to reconstruct past environmental conditions from peatlands and predict possible feedback mechanism between peatlands and climate change, a better understanding of the decomposition of graminoid tissue and the effects of anaerobic conditions on decomposition are needed. Samples (51) from the upper 1m of the graminoid-dominated Penido Vello peatland (Xistral Mountains, Galicia, NW Spain) were analysed with pyrolysis–gas chromatography–mass spectrometry (Py–GC–MS) and 13C cross polarisation magic angle spinning nuclear magnetic resonance spectroscopy (13C CPMAS NMR). Carbon and nitrogen contents were also determined. Depth profiles of molecular groups identified using 13C CPMAS NMR were consistent with those of depth-related distributions of quantified pyrolysis products (aliphatics, polysaccharides and aromatics). Molecular proxies were selected from peat pyrolysates to reflect the state of decay of 1) lignocellulose (the summed lignin and polysaccharide pyrolysis products, Lg and Ps, respectively), 2) lignified cellulose (levoglucosan/Ps) and hemicellulose (4-hydroxy-5,6-dihydro-(2H)-pyran-2-one/Ps), 3) macromolecular lignin including syringyl (S), guaiacyl (G) and p-hydroxyphenyl (H) ratios (S/G; H/(S+G), and side chain oxidation and shortening (vanillic acid/G, syringic acid/S, 4-acetylguaiacol/G, 4-acetylsyringol/S, C3-guaicols/G and C3-syringols/S) and 4) non-lignin phenolic acids (4-vinylphenol/H, 4-vinylguaiacol/G). Factor analysis was applied to these proxies and 120 quantified pyrolysis products to examine the influence of possible underlying factors that could explain the observed variation. Botanical changes and several degradation stages including surface decay (both aerobic and anaerobic), aerobic sub-surface decay and depth related decay (long-term anaerobic) were identified with factor analysis and all affected the variance of lignin-based decomposition proxies. The net effect of these environmental factors on the lignin proxies was examined using their depth records. This revealed that some lignin decomposition proxies were not in agreement with the literature: 1) G and S moieties with a C3 alkyl side chain showed no correlation with bog hydrology at the time of peat formation; 2) G and S moieties with acetyl side chains were related to both relatively dry (secondary aerobic decay) and wet (first stage of decay) conditions; 3) vanillic acid and syringic acid were related partly to ericoids (indicating dry conditions) and partly to free phenolic acids (less depleted under wet conditions); and 4) preferential decay of G over S moieties was found during the first stage of decay (both aerobic and anaerobic) and long-term anaerobic decay. These contradictions can be explained by the dominance of non-woody lignin sources (graminoids) and the prominence of anaerobic decay in peatlands. Our findings indicate that the effect of anaerobic decay and source vegetation should be considered when using lignin proxies to deduce aerobic decay in peat records.