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.