The reduction mechanism of manganese in paddy soils

Hydroquinone method manganese (soluble in pH 7.0, 1 N-ammonium acetate solution containing 0.2 percent hydroquinone) and microbially active manganese (soluble in pH 7.0, 1 M-magnesium sulfate solution after flooding soils with or without Chinese milk vetch for 12 or 20 days respectively at 30°C) of 22 paddy soils were determined. The amounts of manganese reduced with sodium oxalate under acid conditions (oxalate method manganese (a) and (b), the former was determined under more rigorous conditions than the latter) were also determined and compared with hydroquinone method manganese and microbially active manganese.

Their levels of many soil samples representing soil groups were also determined to examine the dlfferences In amounts of active manganese among soil groups. The results obtained are as follows.

The relationship between microbially and chemically active manganese. 1) The amounts of microbially active manganese in soils were 48 to 68 mg Mn per 100 g oven-dried soil and these were increased by the addition of Chinese milk vetch. 2) The amounts of hydroquinone method manganese were less than microbially active manganese, and the amounts of oxalate method manganese (b) were larger than microbially active manganese. The amounts of oxalate method manganese (a) were the largest of all the types of manganese. 3) There were high correlations between the amounts of various types of active manganese described in 2). The levels of microbially and chemically active manganese. 1) The amounts of microbially active manganese lay between the amounts of chemically active manganese determined by the hydroqulnone method and by the oxalate method (b) In all soil samples representing soil groups. High correlations were found between these types of active manganese. 2) The hydroquinone method was considered to be unsuitable for quantitatively determining the amounts of chemicallY active manganese in soils of high organic matter content. 3) In both cases of microbially active manganese and chemically active manganese, tha widest range and the largest amount determined were both observed in strongly gley soila. The averages of theae types of active manganese were high in strongly gley soils, pea, and muck soils, and black soils. The differences among soil groups were smalle1 than the differences among soil samples, and little tendency was observed in the differences among soil groups.

From these findings described above it is suggested that the oxalate methoo (b) is more appropriate than the hydroqulnone method for determining chemically active manganese as an index of microbially active manganese.