A Critical Review on the Multiple Roles of Manganese in Stabilizing and Destabilizing Soil Organic Matter

Manganese (Mn) is
a biologically important and redox-active metal
that may exert a poorly recognized control on carbon (C) cycling in
terrestrial ecosystems. Manganese influences ecosystem C dynamics
by mediating biochemical pathways that include photosynthesis, serving
as a reactive intermediate in the breakdown of organic molecules,
and binding and/or oxidizing organic molecules through organo-mineral
associations. However, the potential for Mn to influence ecosystem
C storage remains unresolved. Although substantial research has demonstrated
the ability of Fe- and Al-oxides to stabilize organic matter, there
is a scarcity of similar information regarding Mn-oxides. Furthermore,
Mn-mediated reactions regulate important litter decomposition pathways,
but these processes are poorly constrained across diverse ecosystems.
Here, we discuss the ecological roles of Mn in terrestrial environments
and synthesize existing knowledge on the multiple pathways by which
biogeochemical Mn and C cycling intersect. We demonstrate that Mn
has a high potential to degrade organic molecules through abiotic
and microbially mediated oxidation and to stabilize organic molecules,
at least temporarily, through organo-mineral associations. We outline
research priorities needed to advance understanding of Mn–C
interactions, highlighting knowledge gaps that may address key uncertainties
in soil C predictions.