Distinct recalcitrant and labile pools of organic matter in tropical peatlands: The effects of vegetation input and downcore degradation in the peat profile

Tropical peatlands contain 65-105 Gt of carbon and are one of the most vulnerable carbon stocks, currently under threat from anthropogenic exploitation 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. Large surveys of OM composition suggest that it is more recalcitrant in tropical peatlands than temperate ones, because of the former’s relatively high aromatic content. However, the precursor aromatic-bearing macromolecules, such as lignin, are produced in different proportions by diverse plant communities, and are often not equally recalcitrant (i.e. syringyl vs guaiacyl-rich lignin); nor are they the only recalcitrant compounds. To predict tropical peat organic matter stability in the 21^st century, both on a global scale and in individual peatlands, we need to properly characterise the complexity of the ‘recalcitrant’ carbon pool and how it varies across diverse peatland types.

In this study, we characterise bulk organic matter in plants, leaf litter, and peat depth profiles from a range of tropical (n = 7) and temperate (n = 1) peatland ecosystems. This characterisation is achieved primarily via Pyrolysis Gas Chromatography Mass Spectrometry (Py-GC-MS), complemented by Fourier-Transform Infrared Spectroscopy (FTIR). Our results show that each site exhibits distinct pools of putatively labile and recalcitrant (plant) organic matter, with both shared and distinct downcore degradation features. For example, all tropical sites exhibit a downcore enrichment in lignin, but that is not evident in the temperate site which exhibits little downcore change in composition – a difference that likely is driven by the intense degradation in the tropics at high temperatures. Some sites show a shift with depth from the preferential degradation of hemicellulose and cellulose to primarily cellulose degradation. This regime may promote carbon stability at depth, although notably a putatively “labile” pool of carbohydrate-rich OM persists at depth in all sites. The diversity in the chemical properties of these carbon pools between peatland ecosystems could explain the variability in peat accumulation and stability.