Deciphering Congo Peat Chemistry; Using Plants to Understand the Peat.

The chemistry of tropical peat has been shown to vary significantly according to differences in plant litter chemistry and hence the composition of the peat-forming vegetation. In the Central Congo peatlands, peat forms under two forest types; hardwood and palm swamp forests.

The complex chemistry of the organic matter (OM) in peat involves multiple compounds including; carbohydrates, cellulose, lipids, lignin and various secondary metabolites, each with different decay rates. Thus, the chemical nature of the OM determines its recalcitrance. Likewise, due to both chemical and structural differences between plant material from different species and between different litter components, not all plant fractions are conducive to or make a similar contribution to peat production. Previous studies suggest there is a relatively greater accumulation of root material in tropical peats compared to other fractions due to its high lignin content, which renders it more resistant to decay, but linking the physiochemical properties of tropical peats and their decomposability to the botanical origins of the plant litter remains understudied. As a result, there are significant gaps in our knowledge regarding the links between plant litter inputs, peat organic geochemistry and our understanding of their role in both peat formation and GHG emissions.

Important recalcitrant moieties such as lignin can be analysed via geochemical analytical methods such as pyrolysis GC-MS, to provide insights into peat composition and vegetative origin. Lignin is an abundant and complex class of organic polymer, that forms key structural plant tissues. Categorised into three monolignols: coniferyl alcohol (Guaiacyl type; G), sinapyl alcohol (syringyl type; S) and p-coumaryl alcohol (p-hydroxyphenyl type; P). The amalgamation of these monolignols results in the creation of complex and diverse lignin structures, related to plant physiology for example between monocot and dicot angiosperms and to tissue type e.g. woody and non-woody. Consequently, key vegetation types exhibit varying concentrations of these monolignols, resulting in variations in relative proportions of G, S & P – which have identifiable pyrolysis signatures, and thus can be used to differentiate between types of lignin in the Congo peat. By examining their relative concentrations, this method allows the discrimination of plant inputs and their subsequent influence on peat organic geochemistry.

This study aims to characterise the organic geochemistry of peat from various locations across the Congo Basin; investigating the vegetative origin of the peat and OM transformations in both palm and hardwood dominated swamp forests. Initial results demonstrate distinct chemical (pyrolysis) signatures reflective of plant inputs and type, leading to discernible variations in peat chemistry over short distances and significant differences in the lignin composition corresponding to hardwood and palm dominated peat. The relevance of these findings for improved understanding of peat formation in this location is discussed.