Seasonal Redistribution of Manganese in Soil Aggregates of a Costa Rican Coffee Field

Previous research has indicated that heavy metals, including Mn, are distributed unevenly within soil aggregates. Reducing conditions at the onset of the rainy season in Costa Rica may lead to a redistribution of Mn between aggregate interior and exterior. The objective of this paper is to examine differences in redox dynamics of Mn between aggregate interior, exterior and bulk soil in a Typic Haplustalf. From April 4 to June 26, redox potential was measured in depths of 10 and 35 cm with permanently installed Pt-electrodes. Aggregates and bulk soil were collected every 2 weeks at depths of 0-10 and 30-40 cm. Soil aggregates were separated mechanically into interior and exterior. In the aggregate fractions we measured Mn concentrations, which were speciated with a 4-step sequential extraction procedure. Although the redox potential dropped only slightly from April to July, Mn was reduced first in the aggregate exterior and later in the aggregate interior. The ratio of mobile Mn in the exterior to that in the interior reached 7 in the topsoil and 15 in the subsoil on May 16. At that time, mobile Mn concentrations in the topsoil were 193 mg kg−1 in the exterior. The bulk soil, consisting of soil aggregates and nonaggregated soil material between aggregates, showed a decrease in mobile Mn during the whole study period. This was probably attributable to increased leaching and plant uptake. These seasonal Mn dynamics are important for plant Mn nutrition and may also play a crucial role in heavy metal mobilization caused by the high heavy metal sorption capacity of Mn oxides.