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Lignin is an aromatic plant compound that decomposes more slowly than other organic matter compounds; however, it was recently shown that lignin could decompose as fast as litter bulk carbon in minerals soils. In alpine Histosols, where organic matter dynamics is largely unaffected by mineral constituents, lignin may be an important part of soil organic matter (SOM). These soils are expected to experience alterations in temperature and/or physicochemical parameters as a result of global climate change. The effect of these changes on lignin dynamics remains to be examined and the importance of lignin as SOM compound in these soils evaluated. Here, we investigated the decomposition of individual lignin phenols of maize litter incubated for 2 years in-situ in Histosols on an Alpine elevation gradient (900, 1300, and 1900 m above sea level); to this end, we used the cupric oxide oxidation method and determined the phenols' (13) C signature. Maize lignin decomposed faster than bulk maize carbon in the first year (86 vs. 78% decomposed); however, after the second year, lignin and bulk C decomposition did not differ significantly. Lignin mass loss did not correlate with soil temperature after the first year, and even correlated negatively at the end of the second year. Lignin mass loss also correlated negatively with the remaining maize N at the end of the second year, and we interpreted this result as a possible negative influence of nitrogen on lignin degradation, although other factors (notably the depletion of easily degradable carbon sources) may also have played a role at this stage of decomposition. Microbial community composition did not correlate with lignin mass loss, but it did so with the lignin degradation indicators (Ac/Al)s and S/V after 2 years of decomposition. Progressing substrate decomposition toward the final stages thus appears to be linked with microbial community differentiation., (© 2013 John Wiley & Sons Ltd.)

Abstract: CuO oxidation is traditionally used for soil lignin study, although, like other degradation methods it might give access only to a part of the lignin polymers. For structural characterization, lignins are conventionally isolated from plant material by combining ball-milling, cellulolysis and solvent extraction to recover a milled wall enzymatic lignin (MWEL) fraction. This method might isolate condensed lignins, which are not accounted for CuO oxidation. MWELs are still associated with polysaccharides. This study aimed to evaluate if the MWEL method can be used complementary to CuO oxidation for soil lignin studies. We assessed the purity of isolated lignins by pyrolytic characterization of MWEL as well as the efficiency of the isolation method by characterization of the MWEL-free residues. MWELs isolated from maize stems and leaves and soil were primarily composed of lignin units, with few associated polysaccharides. MWELs isolated from soils were more degraded and contained less polysaccharides than MWEL from maize tissues. Fewer lignin-derived pyrolysis products were detected in the soil residue compared to the residues obtained from plant tissues. The MWEL method appeared very efficient in isolating relatively pure lignins from soil, most probably because the lignin–polysaccharide complex is less intact than in plants. [Copyright &y& Elsevier]

Abstract: Lignin transformation and decomposition products are generally considered a major source of stable soil organic matter (SOM). Nevertheless this process remains poorly understood in part because lignin is a heterogeneous biopolymer composed of several types of phenol monomers, which potentially display specific and contrasting decomposition kinetics in soils. Here, we compared the specific in situ turnover kinetics of individual lignin monomers in a Paris-basin loamy soil through natural 13C labeling of SOM generated in a series of 1–9-year chronosequences of maize monoculture (C4, δ 13C −12‰) replacing wheat (C3, −27‰). Lignin monomers were released by CuO oxidation, quantified by gas chromatography (GC)/flame ionization detection (FID) and their 13C content was determined by GC coupled via a combustion interface to isotope ratio mass spectrometry (GC/C-IRMS). We calculated the proportion of C4-derived OC in lignin monomer pools by applying the isotopic mass balance equation to each lignin monomer. Individual C4-derived phenols displayed contrasting accumulation rates in soils over time, confirming the monomer-specific nature of their transformation kinetics. In proportion to total lignin phenols in soils, syringyl (S) and cinnamyl (C) phenols from maize accumulated faster than their vanillyl (V) counterparts. Consequently, the turnover kinetics of lignin-derived V-moieties may be slower than those of S and C ones. Incorporation of maize-derived carbon was faster in the aldehyde than in the acid pool for V-type units, while the opposite was observed for S-units. These in situ trends for phenol monomers and V-, S- and C-moieties were remarkably similar to the trends described in the literature with laboratory incubation or litterbag studies. None of the observed kinetics had a linear shape. Using only the extreme points of the chronosequence to calculate the kinetic parameters would result, for all the lignin monomers, in underestimating the turnover kinetics at the beginning of the kinetics and overestimating it for longer times. This observation underlines the importance of comprehensive 13C time-sequence labelling experiments such as provided at the Closeaux site, to accurately compute the kinetic parameters of SOM for the 1st years after the vegetation change. [Copyright &y& Elsevier]