Exploring the biogeochemical composition of a coastal tropical peatland

Peatlands are wetland ecosystems composed of Histosols, which may act as source or sink of carbon upon climate warming. The role and functioning of tropical peatlands in this context remain poorly understood. Here we explore the variability of peat chemical composition in three cores from a coastal tropical peatland: Campinas do Encantado (Ribeira Valley, São Paulo State, Brazil). The cores, with a depth of 4-5 m, were sliced at regular depth intervals of 2 cm. The samples were analyzed for C and N elemental and isotopic composition, and molecular composition (pyrolysis-GC/MS). The results are interpreted over time using ¹⁴C. The C content exhibits a sharp increase at approximately 3,800 cal yr BP (at 150, 275, or 220 cm depth), reflecting a transition from mineral (deeper section, average 7.1%) to peat (upper section, 50.6%) soil. Although with large variation within each section, The peat horizons display lower δ¹³C (−28.86‰) and δ¹⁵N (0.90‰), and higher C/N (41.5), while the mineral horizons present relatively enriched δ¹³C (−27.1‰) and δ¹⁵N (4.8‰), and lower C/N (23.4). The δ¹³C values indicate a predominance of C₃ plants in both the organic and mineral horizons. The higher isotopic values together with lower C/N in the mineral horizons, suggest that the organic matter is more decomposed compared to the peat, or has a contribution from algae. Pyrolysis results support elemental and isotopic data, showing higher contributions from compounds derived of lignin and cellulose in the peat horizons, while the mineral horizons were dominated by either aromatic, aliphatic or N-compounds.