| 17 Jun 2026
Enrichment of lignin reveals consistent anaerobic degradation and persistent vegetation signatures in the organic matter of diverse lowland tropical peatland profiles
Abstract. Tropical peatlands store a significant amount of carbon but are also one of the most vulnerable carbon stocks due to anthropogenic pressures and climate change. The stability and accumulation of the organic carbon stored in tropical peat systems, and its sensitivity to changing temperature and/or hydrology, is intrinsically linked to the organic matter (OM) character. However, we currently lack a detailed understanding of the OM characteristics in tropical peatlands, hindering the accurate prediction of tropical peatland stability in the 21st century.
In this study, we characterise the macromolecular composition of peatland vegetation, leaf litter, and across peat depth profiles using a range of tropical (n = 7) and some temperate (n = 2) peatland ecosystems that serve as comparison. This characterisation is achieved primarily via Pyrolysis Gas Chromatography Mass Spectrometry (Py-GC-MS), complemented by Fourier-Transform Infrared Spectroscopy (FTIR). We find that silicate mineral interference in hydrologically active sites makes FTIR challenging to apply in these tropical systems. Our results also demonstrate that all sites exhibit distinct pools of putatively labile and recalcitrant (plant) OM, with both shared and distinct downcore degradation features. Most sites exhibit a downcore relative enrichment in aromatic pyrolysates, such as from lignin, vs polysaccharide pyrolysates. This relative enrichment follows a logarithmic decline, especially in the anoxic horizons. Regardless of the decomposition of the peat, however, a pyrolytic fingerprint of the original vegetation persists. This unique fingerprint is likely a driver behind the microbial community’s speciality to degrade the OM in its specific peatland, an effect known as the home advantage theory. The predicable preferential loss of polysaccharides at depth and consistent aromaticity of the leaf litter in the tropical sites can aid peatland accumulation modelling and enable more accurate predictions of peatland dynamics under future climate change.
This manuscript characterizes the macromolecular composition of organic matter (OM) in seven lowland tropical and two temperate peatlands, using pyrolysis–gas chromatography–mass spectrometry (Py-GC-MS) as the primary method. Through analysis of vegetation, leaf litter, and downcore peat profiles, the authors show a consistent enrichment of aromatic (lignin-derived) pyrolysates relative to polysaccharides with depth, following a logarithmic decline that is most pronounced in anoxic horizons of the tropical sites. They also find that a pyrolytic fingerprint of the original vegetation persists in highly degraded deep peat, and discuss what this means for peatland carbon accumulation modelling and the home-field advantage (HFA) concept. The geographic and ecological breadth of the dataset is a real strength, and Py-GC-MS provides molecular-level detail that FTIR alone cannot. That said, a number of issues need to be addressed before the manuscript is ready for publication.
1. The role of FTIR should not be dismissed
In Section 3.1, the authors show that FTIR is affected by silicate mineral interference at the 1030 cm⁻¹ band in several hydrologically active tropical sites (Pantano, Inírida, Mpologoma), producing apparent carbohydrate enrichments that the Py-GC-MS data do not support. This point is well taken. But the manuscript goes on to frame FTIR as more or less inapplicable for tropical peatland systems, which overstates things. The downcore trends revealed by the Py-GC-MS-derived PPA ratio (preferential loss of polysaccharides, relative enrichment of aromatics with depth, logarithmic decline in OM lability) are in fact very similar to what Hodgkins et al. (2018, Nature Communications) found using FTIR-based ratios on a broadly comparable set of tropical and temperate peatlands. That the two methods converge on similar conclusions, despite FTIR’s known limitations in mineral-rich settings, says something worth noting: FTIR is still a useful screening tool. It is fast, cheap, and works well where mineral input is low. A more balanced treatment would acknowledge the two methods as complementary rather than presenting FTIR as a method that ‘could be inappropriate.’
2. Section 3.2.3: The argument for aliphatics as an "early degradation" signal needs clarification
Section 3.2.3 argues that aliphatic compounds are enriched in peat relative to vegetation and that this enrichment “signals early degradation.” As written, the section is ambiguous and could easily be misread. The authors should clarify their reasoning. Specific concerns:
The text moves between several mechanisms (condensation of biological alkyl compounds during humification, selective preservation of aliphatic-rich macromolecules such as cutin and cutan, preferential degradation of polysaccharides and lignin relative to aliphatics) without specifying which diagnostic features are actually being used to infer early degradation. Is it the enrichment of aliphatics relative to the parent vegetation? A shift in chain-length distribution? The n-alkane/n-alk-1-ene ratio? It is not clear from the text.
Plant pyrolysis products themselves contain abundant aliphatics (leaf waxes, cutin, suberan, etc.). The authors note that “both peat and plants have a similar pattern among the HMW components” (lines 524–526) and that the C14/C15 doublets “probably derived from cutin, which is common in plants.” So a key question remains: how much of the aliphatic enrichment in surface peat is simply inherited from the parent vegetation (aliphatic-rich roots and leaves), and how much reflects genuine diagenetic change?
The authors should spell out the diagnostic criteria used to identify early degradation, distinguish between inherited and diagenetically enriched aliphatic pools, and revise the section title and text so that aliphatic enrichment alone is not presented as sufficient evidence of early degradation.
3. Vegetation signal preservation at depth needs refinement
The authors state repeatedly that “a pyrolytic fingerprint of the original vegetation persists” in deep peat (abstract; lines 41, 760–762, 1270–1272). This is an important and well-supported finding, but the current formulation is too general. Two points need refining:
The HCA (Section 3.2.2) and PCA (Section 3.2.1) results show that the vegetation fingerprint is not retained uniformly. Some signals, such as 4-vinylphenol (P3) as a sedge biomarker, S-lignin as a hardwood indicator, and the polysaccharide signature of Sphagnum, are detectable in shallow peat but fade with depth as degradation progresses. What persists most robustly seems to be tied to the relative stability of certain pyrolysis products, particularly lignin-derived guaiacols and syringols, and to a lesser extent high-molecular-weight n-alkanes from leaf waxes. Their resistance to microbial degradation is what allows a chemotaxonomic signature to survive even in highly decomposed peat. The authors should state explicitly which classes of pyrolysis products carry the persistent fingerprint and why (their chemical recalcitrance), rather than implying that the entire vegetation signal is preserved.
4. Section 3.3.3: The HFA discussion is overextended and should be substantially condensed
Section 3.3.3 has no microbiological data of any kind: no 16S surveys, no metagenomics, no enzyme assays, no decomposition experiments. The section is therefore speculative and should be cut back substantially. The manuscript
The home-field advantage hypothesis, as originally formulated and as the authors themselves cite (Dehaen et al., 2025; Hoyos-Santillan et al., 2017), concerns the differential decomposition of autochthonous versus allochthonous organic matter. That is, microbial communities are adapted to decompose litter from their local vegetation more efficiently than litter imported from elsewhere. The authors extend this concept to argue that microbial communities at different depths within a single peatland are specialized to degrade the OM at that depth (lines 1252–1254). These are not the same claim.
Deep peat in tropical systems can be centuries to millennia old (the authors’ own age models span “decades to millennia”; line 1143). The vegetation, climate, and hydrology under which that deep peat formed may have been very different from conditions at the surface today. Attributing the persistence of a vegetation fingerprint at depth to a specialized, depth-adapted microbial community overlooks the possibility that the deep microbial community has itself shifted over time in response to changing conditions. Without compositional or functional data on the microbial community at different depths, the authors cannot distinguish between “the microbial community is adapted to the deep OM” and “the deep OM is simply more recalcitrant, and the microbial community is whatever happens to be there.”