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

Abstract

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) in mangrove soil. 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 (OCHF) was the oldest (median {Delta}14C value of -13.81{per thousand}) and contributed most significantly to total soil OC (43%-63%) and 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 OCHF. Together with high OCHF: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 OCHF, as these divalent cations typically facilitate OC stabilization through cation bridging between negatively charged clay surfaces and organic matter. Furthermore, the {Delta}14C-OC relationship suggested efficient incorporation of mangrove-derived modern C into HF, in addition to the pre-existing old C. Thus, mangrove expansion is likely to enhance stable soil OC pools in addition to increasing 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.