Manganese Oxidation States Reveal a Continental-Scale Climate-Driven Redox Gradient in Upland Soils

Ke Wen , Eric W. Slessarev, Oliver A. Chadwick, Aaron Thompson, Hongfei Liu, Mengqiang Zhu

Soil redox conditions regulate soil organic carbon (SOC) persistence and nutrient availability, yet macroclimatic controls in uplands remain unclear because O2 depletion reflects the interactions of moisture, labile carbon, soil texture, and microbial demand. Here we pair manganese (Mn) oxidation state, an integrative redox proxy, with effective water balance (mean annual precipitation minus potential evapotranspiration; MAP–PET) across mineral soils from 20 US ecoclimatic domains. Mn oxidation states shift predictably with MAP–PET: Mn(IV) dominates when MAP–PET < –1 m, Mn(III) at intermediate values (–1 to +1 m), and Mn(II) when MAP–PET > +1 m. These thresholds define a net average oxic–suboxic–anoxic continuum governed mainly by SOC concentration and water availability. Global extrapolation suggests suboxic uplands, the most prevalent state, store 78 – 91% of SOC, highlighting widespread oxygen limitation as a key stabilization mechanism across most upland ecosystems. Incorporating climate-driven redox thresholds into Earth system models should improve carbon–climate feedback predictions under rising hydroclimatic variability.