Solid‐Phase Manganese Fractionation Changes in Saturated Arid‐Zone Soils: Pathways and Kinetics

Short‐ and intermediate‐term transformations of Mn among the solid‐phase components of two arid‐zone soils were studied under saturated paste incubation. Manganese was fractionated by a selective sequential dissolution procedure into several empirically defined fractions. Before incubation, Mn was predominantly in the easily reducible oxide (ERO) fraction, followed by the carbonate (CARB) fraction and the residual (RES) fraction, with almost negligible amounts present in the exchangeable (EXC) fraction. During saturation incubation, Mn in both soils was transformed from the ERO fraction to the EXC and CARB fractions. However, the pathways differed between the two soils. The rates of the transformations were initially very high: major changes occurred during the first 6 d in the sandy soil and the first 12 d in the loessial soil. Following the initial rapid changes, both soils showed only subtle changes for the rest of the period (1 yr). The transformation (reduction) of Mn from the main source, the ERO fraction, was controlled by, and synchronized with, changes of soil pe + pH. The initially measured rates of reduction in ERO Mn were 1.43 × 10⁻⁴ and 0.828 × 10⁻⁴ mg kg⁻¹ s⁻¹ in the sandy and loessial soils, respectively. A parameter, U_ts, measuring the departure from the initial state of equilibrium in the soil showed that Mn initially departed rapidly from the native state of quasi‐equilibrium in the oxidized soils, but tended to return toward it during prolonged incubation. The flux of Fe to the EXC and CARB fractions during the early stages of incubation was very small compared with that of Mn, suggesting increased Mn availability.