FACTORS AFFECTING OXIDATION-REDUCTION PROCESSES IN AN OXISOL WITH A SEASONAL WATER-TABLE

The weekly fluctuation of the water table was recorded for a period of 2 yr at five sites along a transect of a high plateau area in the Cerrados of Brazil characterized by minimal variation in soil morphology, color, and texture. The soil studied was a Red-Yellow Latosol, a member of the clayey, oxidic, isohyperthermic family of Typic Acrustoxes. The soil profile to a depth of 200 cm did not show mottling or color chromas of 2 or less at any site. However, the water table was within 0.4 to 2.0 m of the soil surface at some sites for more than 90 d yr-1. Bright platinum electrodes installed at three sites showed only a moderate range of Eh values (700-500 mV) indicating that no reduction took place in the soil at any depth, in spite of the standing water table. Soil samples taken at the 0- to 20-, 20- to 40-, 40- to 60- and 60- to 80-cm depth, and subjected to different treatments, were incubated for 90 d at approximately 30 kPa water tension or under waterlogged conditions. Treatments consisted of nutrients, nutrients plus lime, and sucrose at two rates, 0 and 10 g kg-1 of soil. The Fe(II) content measured in moist samples at the end of the incubation period showed that intensive reduction took place under all treatments in soil samples taken from the 0- to 20- and 20- to 40-cm depths. Samples taken below 40 cm contained a high Fe(II) content only when sucrose was applied. These data suggest that reduction does not occur in the field in the lower part of the soil profile even when a high water table is present due to the lack of a source of energy for microbial activity. It is suggested that the organic matter present in the lower part of the soil profile is too stable and/or too low in available nutrients for the reduction process to take place during the water logging period.