The predominance of root- and salt-marsh-derived soil organic carbon in a mangrove poleward range expansion front

• Leaf- and root-derived OC were differentiated by lipid and lignin biomarkers. • Mangroves stored leaf-OC in shallow soils and provided root-OC to subsurface soils. • Soil OC sources have shifted from salt marsh roots to mangrove roots. • The majority of soil OC in recent mangroves was derived from previous salt marshes. Due to global warming, temperate salt marshes (e.g., Spartina alterniflora and Juncus roemerianus ) are being overtaken by poleward migrating mangroves (e.g., Avicennia germinans and Rhizophora mangle ). While bulk soil organic carbon (SOC) stocks have been widely compared across mangrove and salt marsh habitats, differentiation of SOC derived from leaves and roots of each mangrove and salt marsh species remains a challenge. Hence, we used multiple biomarkers and proxies (stable isotopes, lignin oxidation products, n -alkanes, sterols, and triterpenoids) to quantify the relative contribution of leaves and roots of each plant taxon to bulk SOC in a mangrove-salt marsh ecotone in Apalachicola (Florida, USA). The shallow peaks of mangrove leaf biomarker (α-amyrin for A. germinans , taraxerol for R. mangle ) suggested the deposition of mangrove leaf-OC over soil surface after the initial mangrove establishment, while the abundance of betulin and 3,5-dihydroxy benzoic acid in A. germinans and R. mangle deep soils, respectively, indicated subsurface contribution of mangrove root-OC, down to 45-cm depth. Based on mixing models, the principal source of SOC in mangrove habitats has shifted from S. alterniflora roots to mangrove roots. The total contribution of roots to the SOC pool in mangrove habitats was 69.0–86.1 %, highlighting that SOC was preferentially formed belowground. Interestingly, within ∼34 years after initial mangrove establishment, the majority (62.3–74.0 %) of SOC in mangrove habitats continues to be derived from pre-existing salt marshes. This emphasizes that comparing bulk SOC without considering their actual sources could result in an overestimation of the contribution of encroaching mangroves to SOC stocks.