Organo-metal coprecipitation contributes to stable organic carbon fraction in mangrove soil

Prediction of the impact of anthropogenic disturbances and global change on Organic Carbon (OC) pools in mangrove soils requires a detailed understanding of the mechanisms underlying OC stabilization. Using density fractionation to physically separate OC fractions with varying degrees of mineral association and protection, this study aimed to assess distributions of these fractions and the geochemical factors influencing the most dominant and refractory mineral-associated, High-density Fraction (HF). We conducted forest-wide soil sampling in the Gaburumata mangrove forest on Ishigaki Island, Japan, along three transects (upstream, midstream, downstream) and at five depths (until 100 cm). The OC in HF (OC_HF) was the oldest (median Δ¹⁴C value of −13.81 ‰) and contributed most significantly to bulk soil OC (43 %-63 %) and total nitrogen (64 %-85 %). Among the extractable metals analyzed (aluminum [Al], iron [Fe], calcium [Ca], and magnesium [Mg]) with different crystallinity, only organically complexed Al and Fe showed strong positive correlations with OC_HF. Together with high OC_HF:Fe ratios that surpassed the maximum sorptive capacity of Fe oxides, these results indicate that co-precipitation of OC and Fe was the dominant mode of organo-mineral associations. The low clay content reduced the importance of Ca and Mg on OC_HF, as these divalent cations typically facilitate OC stabilization through cation bridging between negatively charged clay surfaces and organic matter. Furthermore, the Δ¹⁴C–OC relationship suggested efficient incorporation of mangrove-derived modern C into HF, in addition to the pre-existing old C. Thus, mangrove expansion may enhance stable soil OC pools as well as increase plant biomass and litter. Overall, this study proposes a biogeochemical mechanism for how stable mangrove OC is newly formed, as well as maintained, with ramifications for global mangrove expansion and plantation efforts.