SECONDARY MANGANESE IRON RATIOS AS PEDOCHEMICAL INDICATORS OF FIELD-SCALE THROUGHFLOW WATER-MOVEMENT

Thermodynamic models predict that Mn is a more mobile component of soil systems than is Fe and therefore subject to more extensive redistribution within soils and landscapes. This study was initiated to examine the accumulation of secondary Mn (Mn(s)) relative to that of secondary Fe (Fe(d)) as influenced by field-scale water movement. Secondary Mn and Fe were measured using a citrate-bicarbonate-dithionite (CBD) extract of genetic horizons of 60 soils sampled in an Ultisol-dominated landscape of the North Carolina Piedmont. Soil water pressure was measured using tensiometers installed at each soil sampling site to infer water flow patterns. For all well-drained soil horizons, Fe(d) contents correlate well with clay content (r = 0.878***), suggesting that relatively little redistribution of Fe released through weathering has occurred within the landscape. In contrast, Mn(d) contents are extremely variable among genetically similar horizons and are not well correlated with clay content (r = 0.327***). These differences serve as the basis for using secondary Mn/Fe (Mnd/Fed) ratios as relative indicators of Mn accumulation. Total soil water potential contours for Ap and bt horizons closely parallel elevation contours, indicating the strong influence of slope gradient and configuration on water movement. Secondary Mn/Fe ratios are highest in Ap horizons of soils occupying lower slope positions and concave areas of the landscape. Total soil water potential data indicate convergent throughflow occurs in these wetter areas of the landscape. In contrast, lower Mnd/Fed ratios are found in soils occupying more convex landscape positions where soil water potential data show that divergent throughflow occurs. Results indicate that the difference in mobility between Mn and Fe in these acid soil systems allows secondary Mn/Fe ratios to be used as pedochemical indicators of field-scale water movement.