Manganese (Mn) exists as Mn(II), Mn(III), or Mn(IV) in soils. The Mn oxidation state controls its role in numerous environmental processes, yet its variations with climate remain poorly known. We determined the Mn oxidation states in volcanic soils across two rainfall gradients covering mean annual precipitation (MAP) of 0.25 – 5 m. With increasing MAP, the soil redox conditions shifted from oxic to suboxic and to anoxic; concurrently, the proportions of Mn(IV) and Mn(II) decreased and increased, respectively. Mn(III) was low at both low and high MAP but accumulated substantially, up to 80% of total Mn, in suboxic soils at intermediate MAP. The Mn redox chemistry was at disequilibrium, correlating with redox soil potentials better than with soil pH. Mn(III) was likely hosted in Mn and iron(III) oxides or complexed with organic matter, and its composition varied with soil redox potentials and soil pH. The soil redox conditions and rainfall leaching jointly controlled the exchangeable Mn(II) in soils, with its concentration peaking at intermediate MAP. The soil redox conditions likely fluctuated more frequently at intermediate than at low and high MAP, creating biogeochemical hot spots where Mn, Fe, and other redox-active elements can be actively cycled.

Climate-driven accumulation of trivalent manganese in suboxic soils

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