Abstract: Aims Mangrove ecosystems are the vital components of typical blue carbon wetlands with a strong carbon sequestration capacity, where the sediment carbon pool constitutes the majority of the total carbon storage. Within the sediment carbon pool, humic carbon is the primary form of sediment organic carbon, typically accounting for more than half of sediment organic carbon content. However, the distribution characteristics and primary driving factors of humic carbon components in different mangrove types remain poorly understood. Methods This study focused on four typical mangrove species, i.e., Bruguiera gymnorrhiza, Kandelia obovata, Rhizophora stylosa, and Avicennia marina, from the Guangdong Zhanjiang Mangrove National Nature Reserve in South China. We quantitatively analyzed the distribution patterns of three humic carbon fractions, i.e., humic acid carbon, fulvic acid carbon, and humin carbon, in sediments of different mangrove types and at sediment depths. We also employed correlation analysis, random forest models, and hierarchical partitioning to identify their main environmental driving factors. Important findings (1) The significant differences in the three humic carbon components were found among different mangrove types and sediment depths. Among four different types, R. stylosa exhibited the highest humic carbon content and A. marina had the lowest humic carbon content. Vertically, the contents of three humic carbon fractions were significantly higher at the 0-40 cm layer than that at 40-100 cm layer, with no significant differences observed at 40-100 cm layer. (2) Sediment nutrients had the strongest influence on humic carbon, particularly effective nitrogen and carbon-to-nitrogen ratio, which were key driving factors of the humic carbon fractions. (3) Chromium had a prominent impact on fulvic acid carbon and humin carbon, and other metal elements also showed correlations with humic carbon fractions. Overall, the variation of spatial distribution of humic carbon in mangrove sediments is primarily determined by nutrient availability, while the influence of microbial indicators is relatively weak.

Key words: mangrove, humic carbon, humic acid carbon, fulvic acid carbon, humin carbon, sediment nutrients