Effect Of Compound Pollution Of Heavy Metals On Soil Enzymic Activities And Rape

Pot experiment was conducted to study effect of single cadmium, zinc and lead as well as compound pollution of the three elements on soil enzymic activities and rape grown in medium loam meadow cinnamon soil. The four enzymes studied in the experiment include calatase, urease, alkaline phosphatase and invertase. The results showed that the influence of single element Cd on soil enzymic activities was greater than that of single Zn and Pb in general. The inhibiting concentration of Cd on soil urease, alkaline phosphatase and invertase all started from 5 mg. kg-1. Among the four soil enzgmes studied, single element Zn exbihibited obvious inhibiting effect on the other soil enzymic activities in addition to the activating function on phosphatase. In general, effect of single Pb on the four soil enzymes mainly exhibited activating function. The inhibiting effect of the three elements on soil enzymes had a toxicity order: Cd>Zn>Pb. So it is feasible to use urease activity as primary biochemical index to evaluate the polluted leavel of Cd and Zn. Influence of Cd-Zn-Pb compound pollution on soil enzymic activities differed from that of each single element treatment. And effect of compound pollution of the three heavy metals on four soil enzymic activities was different too. Under Cd-Zn-Pb coexisting condition, inhibiting effect of the combination on urease activity was greatest and urease activities of combination of the three elements were obviously lower than those of single elements. Especially under Cd-Zn coexisting condition, the synergistic function of Cd-Zn markedly decreased urease activity. simultaneously, the activating effect of Pb on urease activity was inhibited under compound pollution condition. Therefore, the characteristic of Cd-Zn-Pb compound pollution exhibited synergistic inhibiting neg- ative effect. Cd-Zn-Pb compound pollution for calatase activity exhibited protective effect to some extent. Under combination of Cd-Zn-Pb, mainly with increasing contents of Cd, intertase and phosphatase activitiees decreased very significantly.But effect of Zn and Pb on invertase and phospatase activities wasn't evident. It is thus clear that influence of compound pollution of heavy metals on soil enzymic activities was very complex and the three coexisting elements had a toxicity order on soil enzymes; Cd>Zn>Pb. Meanwhile, the paper also indicated that the rape yields of each single element treatment were different. There was a increasing effect on rape yield when Cd and Zn had low concentration and there was a reducing effect when they were high concentration, and there's a effect with a toxicity order: Cd>Zn>Pb. The combination of Cd-Zn exhibited synergistic decreasing effect on rape yield, whereas the rape yield of Pb element treatment was increased in the experiment. The toxicity sequence of the three elements on rape yield was Cd>Zn>Pb. Whether Cd, Zn and Pb were separate or compound pollution, their accumulation in rape evidently increased with increasing concentration of Cd, Zn and Pb. Cd and Zn were mainly accumulated in leave and root of rape. But Pb was mainly accumulated in root and the amounts of movement of Pb from root to shoot were very low. The accumulation of heavy metals in the edible parts of rape depended on their interaction in addition to their doses and properties by themself with an accumlation rate order: Cd>Zn>Pb, Therefore Cd was absorbed very easily by rape and it had the greatest potential toxicity to human. Zn supplied in soil significantly suppressed Cd accumulation in rape top. The ralation between ionic implusion and rape yield revealed that ionic implusion was proposed as a contral index of heavy metals compound pollution studied in the present experiment. And urease activities significantly decreased with increasing inonic implusion, So it was feasible to use soil urease activity as primary bichemical index to evaluate Cd-Zn-Pb compound pollution in the soil.