Extreme drought drives contrasting fates of labile and recalcitrant mangrove soil organic matter.

Mangroves are globally important blue carbon ecosystems, yet increasingly threatened by extreme climate events. We investigated sedimentary lipid biomarkers (n-alkanes, n-alcohols, sterols, triterpenoids, triterpenones) across tidal flat, Rhizophora mangle, and Avicennia schaueriana zones in the Piraquê-Açu and Piraquê-Mirim estuarine system (SE Brazil) during the 2015-2016 M-drought. High molecular weight n-alkanes (C29 dominance) and alcohols (C28-C30) indicated persistent vascular plant inputs, supported by elevated Carbon Preference Index values (CPILC = 2.0-4.6; CPIOH = 12.8-16.9). However, the Terrestrial/Aquatic Ratio (TARHC) dropped markedly during drought (wet: 34.2-11.9; dry: 6.6-3.8), reflecting enhanced marine-derived inputs to Tidal Flat sediments. In contrast, Rhizophora and A. schaueriana zones maintained the dominance of terrestrial lipids with high concentrations of sterols triterpenoids and ketones, suggesting higher litterfall under physiological stress. Principal Component Analysis confirmed divergent responses: homogenization of organic matter (OM) sources in lower zones under saline intrusion, and strengthened autochthonous signatures in upper zones. These results demonstrate that extreme drought reshapes mangrove sedimentary OM composition, promoting labile marine inputs in lower zones while intensifying local mangrove contributions in higher zones. Such spatially heterogeneous responses weaken the resilience of blue carbon pools by enhancing OM cycling and constraining long-term carbon storage.