Biogeochemistry of manganese oxide coatings on pebble surfaces in the Kikukawa River System, Shizuoka, Japan

The biogeochemistry of Mn-oxide coatings formed over submerged pebble surfaces on the streambed of the Kikukawa river system has been investigated. Located in central Shizuoka, Japan, this system drains strongly acidified soils under tea plantations. Besides containing high amounts of Mn (up to 450 μg/cm2), the coatings are capable of scavenging and accumulating other elements including Ba, Zn, Ni, Co, W, Mo and Sb. When suspensions of the coating material were incubated with 0.2 mM Mn2+, the Mn(II) ion was microbially transformed into Mn-oxides. When the same suspensions were spread on agar plates containing acetate, yeast-extract, and 1.0 mM Mn2+ (AY agar medium) both Mn-oxidizing bacteria and fungi appeared, indicating the existence of a diversity of Mn-oxidizing microorganisms in the system. Plate counts using two agar culture media with varied nutrient levels indicated that the ability of these microorganisms to oxidize Mn(II) was strongly dependent on nutrient supply. The relatively nutrient-poor AY agar medium was more conducive to microbial growth than the K1 agar medium with a higher organic nutrient content. Concentrations of Mn dissolved in the stream waters did not correlate well with the amounts of solid Mn on submerged pebbles. Thus, factors other than dissolved Mn concentration (e.g., organic nutrient supply and pH) determined the ability of microorganisms to oxidize Mn in the streambeds. A survey of dissolved Mn in streams and water draining tea plantations combined with chemical analysis of Mn in the underlying soils indicate that the soils have been strongly acidified through excessive applications of N-fertilizers. As a result, Mn was leached from the soil column into the Kikukawa river system. Biogenic Mn-oxide coatings on streambeds can therefore serve as an indicator of soil acidification and metal leaching from soils of the corresponding watershed.