Manganese oxidation states and availability in forest weathering profiles of contrasting climate

The abundance and oxidation states (II, III and IV) of manganese (Mn) in a weathering profile encompassing both the soil layers (A and B horizons) and the underlaid saprolite (C horizons) determine the availability of Mn as a plant nutrient and regulate its role in cycles of other elements in Earth's critical zone. However, it remains unclear how the abundance and oxidation states vary with depth under different climates, and how the soil forming processes and soil properties control the variations. We examined four forest granite weathering profiles developed under climates ranging from temperate to tropical climate. Regardless of climate types, all four profiles showed similar vertical variation patterns of Mn concentration and oxidation states. The major features of the patterns can be understood from the perspective of soil forming processes and soil properties. Climate affected the Mn oxidation states in the fine fraction (< 2 mm; i.e., the soil fraction) of the poorly weathered saprolite by controlling the weathering degree of Mn-bearing primary minerals. The weathering released Mn(II) and Mn(III) in the primary minerals to the circumneutral environment where it was subsequently oxidized by O₂. In contrast, climate affected the Mn oxidation states in the soil layers poor in parent materials largely by controlling soil redox conditions and pH because most of the Mn in soils was reactive. As the climate became warmer/wetter, the weathering intensified and soils became more reducing and acidic, resulting in more reduced Mn in the soil layers but more oxidized Mn in the fine fraction of saprolite. Moreover, relative to Mn(II) and Mn(IV), Mn(III) preferentially accumulated in the subsoil (B horizons), likely as Mn(III) oxyhydroxides in the colder and drier climates, and as a substitute ion in well-crystallized Fe(III) oxides in the warmer and wetter climates. These findings improve our understanding of Mn availability and cycling and its role in biogeochemical cycles of other elements in Earth's critical zone.