THE INFLUENCE OF HEAVY-METALS ON CARBON-DIOXIDE EVOLUTION FROM A TYPIC XEROCHREPT SOIL

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 1000 mu g g(-1) were determined. Differences in toxicity with respect to t he 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 we re 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 o f the available metals weighted to their relative toxicity with respec t 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 b e positively related to the percent inhibition of soil respiration. Ch lorides and sulphates appeared to depress soil respiration more than n itrates, the latter counter-balancing the toxic effect of the heavy me tals.