Carbon recalcitrance and stabilization processes vary across mangrove eco-geomorphologies

Mangrove ecosystems are one of the most carbon dense ecosystems worldwide. Yet, the stabilization and recalcitrance of carbon (C) and organic matter (OM) are little understood in mangroves, especially across eco-geomorphological settings and depths. Here, we characterized the sediment C and OM of Indo-Pacific mangroves, located in four distinct eco-geomorphological settings (i.e., delta, estuary, non-carbonated open coast, carbonated open coast) and at two different depths (i.e., 0-20 cm and 80-100 cm). We quantified the fraction of C within (i) mineralized associated organic matter (MAOM), and (ii) within particulate organic matter (POM). We coupled these analyses with lignin quantity and composition, as well as stable C isotopes analysis in mangrove sediments.

We found significant variation in the quantity of MAOM and POM across mangrove eco-geomorphological settings, but not across mangrove sediment depths. The terrigenous deltaic mangrove exhibited up to three times more MAOM than the carbonate open coast mangrove, which was dominated by POM. Mangroves of the carbonate coast type had higher C content than other eco-geomorphic types. The  was not different across mangrove eco-geomorphologies, but was different across mangrove sediment depths. Regarding OM recalcitrance, the lignin content displayed strong variations across the different eco-geomorphologies, however, there was no clear pattern of lignin degradation stage across depths. Finally, an inverse correlation between sediment C recalcitrance (i.e., lignin content) and stabilization (MAOM) processes were determined across mangroves.

Our findings suggest that the processes leading to OM preservation differ among mangroves in various eco-geomorphological settings. Those results have important implications to guide mangrove restoration for carbon persistence and to model carbon pools across mangrove areas.