From the Top: Surface-derived Carbon Fuels Greenhouse Gas Production at Depth in a Neotropical Peatland

Abstract. Tropical peatlands play an important role in global carbon (C) cycling but little is known about factors driving carbon dioxide (CO₂) and methane (CH₄) emissions from these ecosystems, especially production below the surface. This study aimed to identify source material and processes regulating C emissions from deep in a Neotropical peatland on the Caribbean coast of Panama. We hypothesized that: 1) surface derived organic matter transported down the soil profile is the primary C source for respiration products at depth and 2) high lignin content results in hydrogenotrophic methanogenesis as the dominant CH₄ production pathway throughout the profile. We used radiocarbon isotopes to determine whether CO₂ and CH₄ at depth (measured to 2 m) are produced from modern substrates or ancient deep peat, and we used stable C isotopes to identify the dominant CH₄ production pathway. Peat organic chemistry was characterized using ¹³C solid state nuclear magnetic resonance spectroscopy (¹³C-NMR). We found that deep peat respiration products had radiocarbon signatures that were more similar to surface dissolved organic C (DOC) than deep solid peat. Radiocarbon ages for deep peat ranged from 1200 – 1800 yrBP at the sites measured. These results indicate that surface derived C was the dominant source for gas production at depth in this peatland, likely because of vertical transport of DOC from the surface to depth. Carbohydrates did not vary with depth across these sites, whereas lignin, which was the most abundant compound (55–70 % of C), tended to increase with depth. These results suggest that there is no preferential decomposition of carbohydrates, but preferential retention of lignin. Stable isotope signatures of respiration products indicated that hydrogenotrophic rather than acetoclastic methanogenesis was the dominant production pathway of CH₄ throughout the peat profile. These results suggest, even C compounds that are typically considered vulnerable to decomposition (i.e., carbohydrates) are preserved deep in these tropical peats, highlighting the importance of anaerobic, waterlogged conditions for preserving tropical peatland C.