SEASONAL REDISTRIBUTION OF MANGANESE IN SOIL AGGREGATES OF A COSTA-RICAN COFFEE FIELD

Previous research has indicated that heavy metals, including Mn, are d istributed 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 p aper is to examine differences in redox dynamics of Mn between aggrega te 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 w ith permanently installed Pt-electrodes. Aggregates and bulk soil were collected every 2 weeks at depths of 0-10 and 30-40 cm. Soil aggregat es were separated mechanically into interior and exterior. In the aggr egate fractions we measured Mn concentrations, which were speciated wi th a 4-step sequential extraction procedure. Although the redox potent ial 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 conce ntrations in the topsoil were 193 mg kg(-1) in the exterior. The bulk soil, consisting of soil aggregates and nonaggregated soil material be tween aggregates, showed a decrease in mobile Mn during the whole stud y period. This was probably attributable to increased leaching and pla nt uptake. These seasonal Mn dynamics are important for plant Mn nutri tion and may also play a crucial role in heavy metal mobilization caus ed by the high heavy metal sorption capacity of Mn oxides.