Cadmium enrichment in topsoil: Separating diffuse contamination from biosphere-circulation signals

Abstract

Eight regional to continental scale datasets providing Cd concentrations in subsoil (C horizon or mineral soil collected at depth) and topsoil are used to compare the statistical distribution of Cd in the two soil layers. Topsoil is invariably enriched in Cd when compared to subsoil. When both horizons are mineral soil the concentration ratio CdTOP/CdSUB is 1.3–2.2. This ratio is substantially larger (6.6–16.5) when mineral subsoil is compared to an organic topsoil O horizon. Data from regional multi-media transects underline that Cd, despite of toxicity, plays an important role in the biosphere, and several plants and a mushroom not only accumulate but also adjust their Cd content. Because organic topsoil is derived from local vegetation residues, its Cd cumulative distribution function (CDF) reflects also Cd accumulation related to local plant diversity. This is a major difference to Pb which is not usually actively taken up by plants, whereby a linear concentration shift between mineral soil and organic soil dominates the CDFs. To estimate the amount of excess Cd due to diffuse contamination, the low-concentration ends of the CDFs from the regional datasets are studied. For two datasets a diffuse Cd contamination below 0.03 mg/kg emerges, a reasonable value when compared to either the median concentration of 0.15 mg/kg Cd in topsoil, or to published Cd fluxes. For the other datasets the apparent diffuse Cd input is between 0.05 and 0.28 mg/kg. In one data set this seems to indicate a true contamination blanket due to several large-scale regional anthropogenic sources at the single country scale. In many surveys, the low end of the subsoil Cd concentration is difficult to assess due to analytical limitations. The results suggest that hitherto neglected natural processes selectively accumulate Cd and substantially change its distribution characteristics in the biosphere and the organic topsoil.