Fe-Mn concretions and nodules formation in redoximorphic soils and their role on soil phosphorus dynamics: Current knowledge and gaps

Over the last few years, phosphorus (P) has attracted the global interest because it is finite and non-renewable resource. However, some aspects of P dynamics in soils are still unclear and novel insights are necessary to better understand the very complex geochemistry of P and to conclusively evolve the traditional soil P management practices. An explicit example is the very limited research on P behavior in redoximorphic soils. These soils are characterized by specific morphological features, such as Fe-Mn concretions and nodules, and cover large areas around the world. In redoximorphic soils, biogeochemical processes change dramatically as a result of oxygen being periodically excluded from the soil environment affecting the geochemistry of nutrients and trace elements. Fe-Mn concretions and nodules are significant pedogenic components containing high amounts of Fe and Mn oxides and show high sorption capacity for various elements. This paper summarizes the current knowledge on the processes determining the formation of Fe-Mn concretions and nodules in redoximorphic soils and on their impact on soil P dynamics. Depending on soil properties, during Fe-Mn concretions and nodules formation and growth following repeated redox cycles, P can be trapped in the concretions/nodules which act as a sink for P and as a barrier to P movement from land to water bodies. Since very little scientific work on this issue has been done, intensive future work is needed to figure out the complex role of Fe-Mn concretions and nodules on P dynamics in redox-sensitive soils.