Intertidal zonation of mangrove organic carbon fractions driven by vegetation biomass and soil nutrient levels

High net primary production and low soil organic carbon (SOC) decomposition rates ensure that mangroves are important carbon sink likely to be disturbed by rising sea level. The differences in environmental factors along land-sea gradients in mangrove forests influence SOC characteristics. Six typical intertidal zones of mangrove distribution in the subtropical and tropical regions of China were selected for this study. Each intertidal zone was divided into three sections, ranging from land to sea: the landward zone (LW), the middle zone (MZ), and the seaward zone (SW). This classification aimed to investigate the distribution of key driving factors influencing mangrove wetland SOC and its various fractions SOC and its different fractions. The average SOC content ranged between 15.85 and 37.08 g kg⁻¹, and was 31 % and 57 % lower in MZ and SW than LW, respectively. Compared with LW, easily oxidizable-carbon (EOC) content in MZ and SW was 16 % and 58 % lower, and dissolved organic-carbon (DOC) content was 46 % and 67 % lower, respectively. Location affected the characteristics of the distribution of SOC fractions (p < 0.05). Vegetation biomass and total N and P contents were key driving factors affecting SOC content. Mangrove forest age was another important factor affecting SOC. LW environment had the largest total N and P content, leading to larger SOC, EOC and microbial biomass carbon content compared to MZ and SW. We highlight the environmental gradient divergence in the SOC in the intertidal zone found when systematically assessing the SOC pool function of the coastal zone, which warrants research into the SOC cycle in coastal wetlands. Rising sea levels may cover more current tidal mangrove areas and, if there is no space to move inland, current mangrove formations in more tidal areas could disappear. As a result, the C-stored in these mangrove communities could be eroded, sparced and lost.