Is manganese(III) a ubiquitous part of the total dissolved manganese in soil solutions?

• Applicant: Professor Dr. Tim Mansfeldt
• Subject Area: Soil Sciences
• Term: from 2019 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 426549767

The number of possible valence states of manganese (Mn) renders the environmental behavior of this essential trace element complex. Until recently, it was assumed that natural waters are dominated by the divalent form, Mn(II), and its related aqueous species, whereas the trivalent, Mn(III), and tetravalent, Mn(IV), forms occur exclusively as colloids. The latest field studies of marine and estuarine environments, however, revealed that Mn(III) is a major part of the total dissolved Mn pool. Mn(III), which is generated by both oxidative and reductive pathways, is stabilized by natural organic ligands in these waters. We postulate that Mn(III) is a ubiquitous component, even in the aqueous phase of soils, because both pathways take place in soils and the omnipresent dissolved organic matter (DOM) can act as a ligand for Mn(III). However, knowledge concerning dissolved Mn(III) in soils is extremely limited. The objectives of this proposal are: (i) to clarify the influence of different reduction-oxidation (redox) conditions on Mn speciation in soils with different properties; (ii) to reveal the effects of various natural organic ligands on the dissolution of synthetic Mn oxides and the subsequent speciation of released Mn; and (iii) to prove the existence of Mn(III) in soil solutions and forest floor leachates in field conditions. Speciation of Mn will be achieved using a spectrophotometric kinetic method which differentiates Mn(II), as well as weak and strong organic complexes of Mn(III). The speciation of Mn under different redox regimes will be studied using microcosm experiments with controlled redox potential in the laboratory. Each of two soil horizons from three soils (one Stagnosol and two Gleysols) which differ in their water regime, land use and physicochemical properties will be investigated. Soil suspensions will be incubated in a bioreactor using representative redox cycles. The influence of DOM obtained from moder and mor on the dissolution of birnessite and manganite and the speciation of the released Mn will be determined in batch experiments, with special focus on the reaction kinetics and the effects of pH. To determine whether Mn(III) is a component of soil in field conditions, soil solutions and forest floor leachates will be investigated in four mineral soils (two Gleysols, a Stagnosol and a Cambisol) and one organic soil (Histosol). Our findings will provide the first insight into the occurrence and behavior of Mn(III) in soil solutions. Neglecting the intermediate Mn species would underestimate the redox capacity of the soluble Mn pool in soil solutions. The environmental significance of the strong oxidant, Mn(III), is attributed to, among other factors, its critical involvement in the breakdown of soil organic matter, oxidation of particulate organic carbon to the greenhouse gas carbon dioxide, and degradation of organic contaminants.

• DFG Programme: Research Grants