Contaminant trends in soils and crops.

The evidence for long-term changes in the contaminant content of soils and crops is reviewed. Heavy metals, acidification and trace organic contaminants are each considered separately, with the emphasis placed on changes brought about by inputs from long-term atmospheric deposition. It is argued that effectively all soils in industrialised countries have become contaminated with selected trace substances (notably Pb, Cd, polynuclear aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and polychlorinated dibenzo-p-dioxins and -furans) above their true (historical) background level by aerial inputs. In the case of Pb, for example, cumulative deposition inputs over several centuries have probably roughly doubled the Pb burden of contemporary UK surface soils. Despite this, there is little evidence for a concomitant rise in the contaminant content of UK crops through the twentieth century. In fact, the burden of many contaminants in vegetation may well be somewhat lower than in earlier decades of this century because (1) crop composition is strongly influenced by direct deposition onto the above-ground portion of the plant, (2) root uptake and translocation of soil-borne heavy metals and recalcitrant organic contaminants are inefficient processes, and (3) UK air quality has generally improved over the last 10-20 years. Acidification of soils is of considerable regional significance. The evidence for changes in soil pH over this century is reviewed, and it is concluded that atmospheric inputs have enhanced the rate of soil acidification over large areas of Scandinavia and elsewhere.