MANGANESE DISTRIBUTIONS IN ACID SOILS OF THE NORTH-CAROLINA PIEDMONT

Manganese is a dynamic component of soils that can be present in a variety of forms. This study was initiated to examine the distribution of Mn in Ultisols and Alfisols formed in gneisses of the North Carolina Piedmont. Secondary Mn (Mn(d)) and Fe (Fe(d)) depth distributions were evaluated using a citrate-bicarbonate-dithionite extract; the distribution of Mn among organic and various inorganic soil fractions was determined usiing a sequential-extraction procedure. Results indicate that accumulation of Mn(d) is related to interactions between landscape position and associated redox potential (Eh)-pH environments rather than mineralogy of soil parent material. Spatial relationships between Mn(d) and Fe(d) within soils reflect Eh-pH-potential gradients in these acid systems. Accumulations of secondary Mn occur in well-drained surface horizons of soils occupying lower lying landscape positions. These accumulations occur in weathered hillslope sediments and are not related to mineralogy of the underlying residuum. As much as 50% of this Mn is associated with the organic-matter fraction and the inorganic exchange complex. Secondary accumulations of Mn also occur in B/C horizons and saprolite residuum in soils of actively down-wearing geomorphic positions. Most of this Mn is in the crystalline Mn oxide fraction. Data indicate a significant correlation (P < 0.01) exists between quantities of Mn found in the amorphous, exchangeable, and crystalline Mn oxide fractions.