Variability of the optical signatures of dissolved organic matter in soils of different mangrove stands (Ouvéa, New Caledonia)

Abstract

Mangrove ecosystems are known to play a key role in the global carbon cycle, due to their productivity and their ability for carbon sequestration both in the biomass and in the soil. In the latter, various geochemical processes lead to the production of dissolved organic matter (DOM) that can be exported through tidal pumping and then constitute an important source of organic carbon for adjacent ecosystems. DOM characteristics, and their variabilities, within mangrove soils depend on several factors, including the mangrove species, yet these variations and their origin still need to be precisely constrained. This study examined DOM sources in soils of a carbonate atoll mangrove (Ouvéa, New Caledonia), focusing on two tree species, Rhizophora apiculata and Bruguiera gymnorhiza, at different growth stages. We analysed porewater properties and DOM optical characteristics through spectroscopic and EEM-PARAFAC methods. Our results indicate distinct TOC and DOC concentrations across species, with mature B. gymnorhiza soils showing the highest TOC content (~ 30%) but the lowest DOC content (32 mg L⁻¹). These differences seem not to be directly related to site physicochemical conditions (redox, pH, salinity) but may rather reflect differences in DOM sources and production, notably due to different symbiotic relationships with mycorrhizal fungi, which influence microbial activity and organic matter diagenesis. DOM absorbance patterns also varied significantly between species: Beneath R. apiculata, DOM had higher protein-like and fulvic-like fluorescence, indicating fresher organic matter, while beneath B. gymnorhiza, especially in mature stands, DOM was more humified, suggesting older OM because of a possible long-term accumulation due to the basin-like morphology of this site.