The influence of heavy metals on carbon dioxide evolution from a typic xerochrept soil

Abstract

In a laboratory study the effects on soil respiration of trace metals (Ni, Cd, Cu, Mn, Pb, Zn) added at loading rates ranging from 0 to 1000μg g⁻¹ were determined. Differences in toxicity with respect to the type of metal salt added were also evaluated. The inhibitory effect on soil respiration differed considerably among the heavy metals and increased with the increasing loading rate. No linear relationships were found between the degree of inhibition and the levels of total and available metals. Toxicity evaluation at 20 and 50% inhibition of soil respiration showed Cu as the most toxic and Mn as the most tolerable metal. A ‘metal equivalent’ was calculated as the sum of the amounts of the available metals weighted to their relative toxicity with respect to the least toxic one: Mn equivalent=Mn+1.9Pb+2.1Ni+2.5Zn+6.7Cd+6.7Cu. The ionic potential of the heavy metals was found to be positively related to the percent inhibition of soil respiration. Chlorides and sulphates appeared to depress soil respiration more than nitrates, the latter counter-balancing the toxic effect of the heavy metals.