A. E. Johnston

Effect of phosphate fertilizers and atmospheric deposition on long-term changes in the cadmium content of soils and crops.

Trends are reported in the Cd content of herbage collected and stored from the Park Grass Experiment at Rothamsted Experimental Station, a semirural location in the U.K. Samples from 1861 to 1992 were bulked for 5-year periods from two phosphate fertilizer-treated plots, now with soil pH 4.9 (unlimed) and 6.5 (limed), respectively. Analysis of Cd was by Zeeman correction GFAAS following a nitric acid digestion. The data are compared with those from control plots which have not received superphosphate. On the unlimed plots, levels of Cd in the herbage from the phosphate-treated plot were very similar to those on the control for the years to 1930; whereas from 1940 to the present, the Cd concentrations in herbage were considerably greater on the phosphate-treated plot. On the limed plots, Cd concentrations in the herbage from both the phosphate-treated and control plots have remained similar to each other, although both plots exhibit a small increase of about 1 Mg of Cd kg-1 year-1 since the beginning of the liming treatment in 1903

Contamination of Environmental Samples Prepared for PCB Analysis

Archived air-dried soil that had been collected, stored, and sealed in 1914, before the commercial manufacture of PCBs, together with wet freshly sampled ancient peat, which contained little or no PCBs, were exposed to contemporary air in a laboratory. Measurable increases in the concentration of PCBs, particularly the low molecular weight congeners, were detectable after exposure for as little as a few hours. Concentrations after a few days exposure to laboratory air were similar to those measured in contemporary field surface soils (-20-30 pg of CPCB kgl). Laboratory air concentrations ranged between 4.7 and 8.2 ng of CPCB m-3 during the period of exposure, markedly higher than routinely detected in outdoor U.K. urban air. The calculated average net dry deposition flux from air-soil in the laboratory over 25 days was 5 pg of CPCB m-2 day1. Indoor air concentrations might be expected to be higher than those routinely measured outside, exacerbating the potential problems of sample contamination. Extreme caution is needed in the preparation and handling of samples which contain inherently low concentrations of PCBs and before ascribing the presence of these compounds in certain samples to ‘natural production’ mechanisms.

The significance of the retention of atmospherically deposited cadmium on plant surfaces to the cadmium content of herbage

Samples of mixed herbage were collected in 1993 and 1994 from the Park Grass Experiment at Rothamsted Experimental Station in the UK. The samples were separated into grasses and other species and the material in each group was then halved; one half was washed and the other was left unwashed. Analysis of Cd was by Zeeman correction GFAAS following a nitric acid digestion. The results showed that washing the samples prior to analysis made no difference to their Cd concentrations and that there was almost the same concentration of Cd in grasses and in other species. This implies that Cd remaining on leaf surfaces and changes in sward composition have not made a significant contribution to the previously reported underlying changes in herbage Cd over the last 130 years. Herbage Cd content has changed in response to either an underlying increase in soil Cd, increases in absorbed atmospherically derived Cd and/or changes in soil chemistry.