Oxidative coupling activity in citrate buffer extracts of soils

Abstract

The catalytic agents which are responsible for the oxidative coupling activity in citrate buffer extracts of paddy and upland soils and the changes in the oxidative coupling activity of soil under different incubation conditions were investigated. The oxidative coupling activity was determined using 2,6-dimethoxyphenol (2,6-DMP) as a substrate. The oxidative coupling activity of 5 paddy soil extracts ranged from 0.2 to 6.2 units, while that of 5 upland soil extracts from 0.1 to 5.5 units. The oxidative coupling activity was highly correlated with the amounts of Mn in the soil extracts (r = 0.818, significance at 1% level). Synthetic Mn(IV) oxide catalyzed the oxidative coupling of 2,6-DMP, but synthetic Fe(III), Al, and Si oxides and Mn(ll) salt had no effect on the coupling. Furthermore, when Mn oxides were selectively removed from the soil, the subsequent extract was inactive. Although the oxidative coupling enzymes contain Cu or Fe as active factor, the metals had no effect on the coupling of 2,6-DMP. From these findings, it was suggested that Mn oxides were the major oxidative coupling agents in the soil extracts studied.

When a soil was incubated under different water regimes and at different temperatures, the oxidative coupling activity decreased more rapidly under the flooded conditions than under the moist conditions and also at low temperatures than at high temperatures.

Key Words:

• Mn oxide
• oxidative coupling activity
• paddy and upland soils
• phenol