Redox function and dynamics of manganese in soils

Introduction Manganese oxides exhibit oxidative activity to certain heavy metals and organic matter in soil, such as chromium and phenolic acids. These reactions are very important from the viewpoints of heavy metal pollution, micronutrient availability, and humus accumulation in soil. In most studies, however, synthetic manganese oxides, rather than soil itself, were used. On the other hand, manganese excess symptom have been observed in crop plants under soil drying conditions. Soil chemical factor on the excess symptom, however, has not been clarified. The purpose of the present study was to elucidate these unsolved subjects in relation to redox function and dynamics of manganese in soil. (1) Relationship between soil oxidative activity and manganese oxides The oxidation of chromium (3) to chromium (6) (Cr oxidation) in soils is caused by manganese (Mn) oxides and represents a soil oxidative activity derived from Manganese oxides. Two Andisols (surface and subsurface horizon) and an Inceptisol were analyzed to determine optimal Cr oxidation conditions based on the following factors: coexistent anion, reaction pH, added Cr (3) concentration and reaction time. The determined optimal Cr oxidation condition was applied to 26 soils, including Andisols, Inceptisols and Entisols. Extremely low Cr oxidation, i.e. soil oxidative activity, was observed in Andisols compared with other soils, regardless of easily reducible Mn content. This suggested that the dangers posed by soil oxidative activity such as Cr oxidation are lower in Andisols than other non-Andisols. (2) Oxidation of phenolic acids by soil manganese oxides Relationships between recovery of phenolic acids added to soils and soil properties were investigated by using 32 soil samples and ferulic, vanillic and p-hydroxybenzoic acids.