We collected materials partially filling or coating the walls of soil macropores to compare their sorptivity for cadmium (Cd) and manganese (Mn) with that in homogenized soil samples from the same soil profile. Organic-rich linings and fillings in root-derived soil macropores exhibited higher sorptivities for Cd and Mn than homogenized soil, particularly at lower solution pH values (pH 4 to 5). Due to the higher density of pH-dependent sorption sites on organic matter, sorption on root-derived macropore materials increased more rapidly as pH increased than did sorption on inorganic macropore materials. Inorganic linings and fillings in fracture-derived macropores exhibited sorptivities for Cd and Mn that were very similar to homogenized soil sorptivities. For both organic and inorganic macropore materials, solution pH was generally far more important in determining sorptivity than the nature of the macropore material. Overall, the results of the study suggest that the migration of Cd and Mn will likely be more highly retarded in root-derived macropores than in other types of macropores. The absence of significant differences in sorptivity between inorganic macropore materials and homogenized soil suggests that factors (e.g., hydrology) other than sorptivity will be more important in determining whether retardation of Cd and Mn is greater or less in the whole soil matrix than that in non-root-derived macropores.
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