Transformation of lignin and lipid biomarkers through degradation of plant tissues and soil organic carbon in a salt marsh-mangrove ecotone

Abstract

The poleward expansion of mangroves into native salt marshes, because of global warming, has the potential to alter coastal soil organic carbon (SOC) stocks and composition. However, its effects on organic carbon decomposition remain largely unknown. This study compared concentrations and degradation indices of lignin, sterol, triterpenoid, and n-alkane biomarkers between live-and-dead leaf-and-root tissues and SOC down to 45-cm depth across habitats dominated by two mangrove and two salt marsh species in Apalachicola coastal wetlands, Florida, USA. A significant increase in n-alkanes and lignin oxidation products was observed following the senescence of Avicennia germinans mangrove tissues. Dead leaves of all four plant species, especially Spartina alterniflora and Juncus roemerianus salt marshes, became hotspots of fungal colonization as indicated by elevated ergosterol content. Interpretation of biomarker degradation ratios was complicated by their large difference between living root and leaf endmembers. In soil cores from all plant habitats, the predominance of tetracyclic triterpenoid degradation products (tetracylic:pentacylic ratio = 0.6–518) indicated prevalent anaerobic decomposition, agreeing with higher cholestanol/cholesterol and stigmastanol/stigmasterol ratios in SOC (0.53 ± 0.29 and 1.14 ± 0.83), relative to plant tissues (0.16 ± 0.17 and 0.57 ± 0.44, respectively). In contrast, the presence of pentacyclic degradation products in deep mangrove and salt marsh soils suggested localized aerobic degradation, likely due to subsurface oxygen release from plant roots. In conclusion, salt marsh-to-A. germinans mangrove replacement modified degradability of surface litter, yet degradation of deep SOC was more dependent on biogeochemical characteristics of the soils.