Redoximorphic Macropore Environments in an Agrudalf

Soil materials in fracture walls may strongly interact with solutes and colloidal particles during preferential flow. Wall coatings rich in metal oxides, clays, and organic matter may increase sorption capacities, whereas coatings devoid of these constituents have the opposite effect of increasing the risk of leaching of otherwise strongly sorbing solutes. The contrasting compositions between bulk horizon and fracture wall materials of a Typic Agrudalf excavated at Flakkebjerg, Denmark, were studied by using chemical and micromorphological methods. In the upper 220 cm of the profile, the predominant desiccation and shear fractures had 2-30 mm thick hypocoatings depleted of Fe-oxides with adjacent 5-20 mm thick quasi-coatings containing 5-6 times as much Fe-oxide. Thin hypocoatings covering walls of smaller voids and surfaces of sand particles and with strong enrichments of Fe- and Mn-oxides occurred throughout the profile, but were most abundant below 220 cm. Fracture walls, commonly with distinct laminas of clay, silt, and organic matter, generally had slightly coarser texture, but were enriched in smectite compared with horizon materials. Higher contents of organic C in fracture coatings were attributed to root growth and deposition of A-horizon materials. Despite removal of Fe-oxides from depletion hypocoatings, no corresponding depletion of P was observed. However, calculations demonstrated that, in the case of macropore transport only, P sorption capacity would be at least 5 times less than during piston-like matrix flow. For adequate estimations of solute leaching from macroporous soils there is a strong need to properly take into account sorption properties of macropore wall materials!