Influence of mangrove zonation on dissolved trace metal dynamics in soils

Mangrove soils play a key role in the cycling of trace metals (TM), yet the drivers controlling dissolved metal concentrations and distribution in porewaters remain poorly studied. Here, we investigate the distribution, dynamic, and controlling factors of dissolved TM along a land-sea gradient in the semi-arid mangrove of Bouraké (New Caledonia). Dissolved and solid-phase TM concentrations were measured in soils (0-30 cm), together with porewater chemistry, redox conditions, bulk density, and mineralogical composition (SEM-EDX). Solid metal stocks and dissolved to solid ratios were calculated to assess relative metal mobility. Dissolved TM concentrations exhibited strong spatial heterogeneity along the land-sea gradient, reflecting zonation-specific geochemical conditions. The landward salt-flat is the most oxidizing zone and contains abundant Fe-Ti oxides, resulting in the largest solid-phase metal stocks. The intermediate Avicennia marina stand is characterized by suboxic redox conditions and exhibits the highest dissolved Fe, Mn, Ni, and Co concentrations, indicating active Fe-Ti oxide dissolution. In contrast, the seaward Rhizophora stylosa stand is characterized by strong anoxia, pyritization, low dissolved metal concentrations, and the lowest solid-phase stocks, consistent with metal sequestration into sulfides. Overall, our results show that dissolved TM dynamics in mangrove soils is governed by the combined influence of redox dynamics, organic matter quantity, and mineralogical transformations. Because dissolved metals represent the most mobile and exportable fraction, this study highlights the necessity of considering porewater chemistry to assess metal cycling and export in mangrove.