Retrospective analysis of an archived soil collection. II. Cadmium

Soil samples collected and stored since the mid-1800s to the present day have been analysed recently for Cd. The samples from long-term experiments under permanent grassland or arable crops at Rothamsted Experimental Station (U.K.) were selected to investigate time trends in elemental composition, due either solely to atmospheric deposition or to a combination of atmospheric deposition and various soil treatments. Increases in soil Cd of 27-55% since the 1850s due to atmospheric deposition were observed. This corresponds to an increase in the soil plough layer Cd concentration of between 0.7 and 1.9 micrograms kg-1 year-1 and is equivalent to an increase of 1.9-5.4 g Cd ha-1 year-1. The changes in soil Cd concentrations since 1846 at one control site corresponded well to predicted increases in the plough layer Cd burden based on assumptions about the temporal trends in atmospheric Cd emissions. In addition, sub-samples of a selection of rock phosphates of known origin and superphosphates, mainly from one supplier, collected and stored in the archive from 1925 onwards were also analysed for Cd. The concentrations ranged from 3.6 to 92 (mean 36) mg Cd kg-1 for rock phosphates and from 3.3 to 40 (9.7) mg kg-1 for superphosphates. On the basis of these data and known application rates the estimated input of Cd to P-treated plots at Rothamsted was 2 g ha-1 year-1, but there was little further increase in soil Cd due to this addition in three long-term arable experiments where soil pH was greater than 6.5. On P-treated plots the mean increase in soil Cd was 1.2 micrograms kg-1 year-1, which is equivalent to an increase in the plough layer burden of 3.1 g Cd ha-1 year-1. By contrast, P-treated soils under permanent grassland with a higher organic matter content and lower pH have increased their Cd content by 7.2 g ha-1 year-1. When permanent grassland soils ranging in pH from 5 to 7 were examined it was found that organic matter had a larger effect on Cd concentration than pH and the effects of pH were not consistent. Farmyard manure applied to some experimental plots at Rothamsted appears to have been a more significant source of Cd than combined atmospheric and phosphate fertiliser inputs.