Rhizophora stylosanAvicennia marina, Lignin Neutral carbohydrates, Rock-Eval, Stable isotopes, Coastal ecosystem
Organic matter (OM) dynamics in mangrove forests have been studied extensively in terms of the capacity of their soils to store organic carbon. While δ13C, δ15N, and C/N values for mangrove soils and sources are well reported, other indicators of OM maturity and composition are lacking. In this study, soil OM decomposition processes were investigated for a semi-arid bay head mangrove forest in New Caledonia. Mangrove tissues and 20-cm soil cores were collected in monospecific stands of Avicennia marina and Rhizophora stylosa. The isotopic compositions of the samples were assessed, along with their molecular compositions (lignin-derived phenols and neutral carbohydrates). Rock-Eval analysis was also performed on the samples to investigate OM characteristics. Results showed that stable isotope ratios and Rock-Eval parameters followed similar vertical trends beneath both species indicating the influence of depth on OM state. However, the more anoxic conditions beneath R. stylosa limited OM decomposition as shown by the lower TpS2 values (indicator of OM thermal stability). Neutral carbohydrates and, surprisingly, lignin-derived phenols, were lost at higher rates than bulk organic carbon beneath both mangrove species. Selective degradation of individual compounds was observed, and species-dependent variations associated with the redox conditions and the OM sources were identified. We suggest that lignin was degraded, even in anoxic environments, because of the amount of labile lignocellulosic components in the soil. These findings enhance our understanding of OM dynamics in mangrove ecosystems, shedding light on the mechanisms underlying carbon cycling and their implications for global carbon storage and ecosystem management.
