Rainfall defines patterns in manganese oxidation state and redox cycling along a Hawaiian rainfall gradient

This is the authors' abstract. We don't add key points for this paper.

Manganese (Mn) is a largely unexplored but potentially critical element to terrestrial biogeochemical cycles. It exists in three oxidation states in soils—Mn2+, Mn3+, and Mn4+—each possessing distinct reactivities that shape their biogeochemical roles. It is unclear what soil processes drive Mn abundance and the distribution of its oxidation states. We investigated Mn dynamics on a Hawaiian rainfall gradient to evaluate how mean annual rainfall, mean annual evapotranspiration, soil moisture, soil pH, and soil redox conditions influence Mn oxidation states, potential reactivity, and plant uptake. We found that mean annual rainfall is the most consistent variable predicting the distribution of Mn and its oxidation states along the gradient. Grass Mn and soil Mn2+ increased with rainfall, and total soil Mn and soil Mn4+ generally decreased with increasing rainfall. Mn3+ trends effectively split into two groups: actively cycling Mn3+ and persistent soil Mn3+. Mn3+ pools in soils were most abundant in the driest and wettest sites; in contrast, actively cycling Mn3+ was most abundant in the middle of the gradient, from ~850–2400 mm mean annual rainfall. These findings illustrate that rainfall defines where Mn is most concentrated, bioavailable, potentially reactive, and—accordingly—most likely to influence terrestrial biogeochemical cycles.