Studies On Mechanisms Of External Phosphorus-Remediation Of The Soil Contaminated With Cadmium (Cd) And Zinc (Zn)

Content: Soil contaminated with heavy metals is one of the environmental problems that should be resolved urgently, because it is accumulative, lagging, dormant, and irreversible. Aiming at the actuality that the soils contaminated with the complex pollution of Cd and Zn in most cultivated land in China, this paper based on the previous researches and combined with the actual conditions in China, the mechanism of remediation was conducted using the agricultural measure of external P.First of all, we made use of the isotopes tracer techniques of ¹⁰⁹Cd, ⁶⁵Zn together to study the absorption kinetics of complex pollution of Cd and Zn to maize under different concentration of external P, exploring the interaction among P, Cd and Zn.The results showed as follows: (1) Along with the time went on, there two process of absorption in the soil to root system existed, one was the process of fast accumulation, the other was of low accumulation, up to a balance state relatively at last;(2) Whatever contaminated by Cd and Zn in a complex pollution or as a single element of Cd or Zn individual, as the increasing concentration of external P, it occurred in a trend that the specific activity of cadmium and zinc of roots and shoots in maize were increased while external P used as 0.25 to 0.6 mmol/L. When P at 3.0 mmol/L, the specific activity of cadmium and zinc of roots and shoots in maize were decreased. Comparing the quantity of Cd and Zn in roots and shoots respectively, the most of Cd and Zn were accumulated in roots and few of those were conveyed to shoots (p< 0.05);(3) When the concentration of external P keeping under invariability, comparing the quantity of Cd and Zn in roots and shoots by single and combination of Cd and Zn, it was discovered that the quantity of Cd and Zn in roots and shoots with the complex pollution accumulated more than those with a single element pollution. The result showed that there was synergism between Cd and Zn, so it could be more harmful to the plant;(4) when the external P reached the definite concentration, it could decrease the valid quantity of Cd and Zn, thereby the absorption, accumulation and translocation of Cd and Zn in the roots were regulated commendably and reduced the absorption of toxic elements by the shoots.The techniques of scanning electron microscope (SEM) and transmission electron microscope (TEM) were applied in the trial to study the effects of external P on the cell ultrastructure of maize under Cd and Zn stress. Firstly, the chloroplasts of leaf cells and nuclei and cell wall in adventitious roots were observed by TEM analysis. The results indicated that external P could reduce the damage of the cytoarchitecture obviously, at the same time some deposits were found within the chloroplasts and cell walls. Second, the surface of roots were observed through SEM technique, which showed that the deposits of phosphate with Cd and those with Zn could exist on the surface of the root, consequently, which prevented the translocation of Cd and Zn from the roots to the shoots. The external P performed an important role to prevent the translocation of Cd and Zn from the roots to the shoots.The pot experiment was employed to study the interactions of external P with Zn and Cd, as well as the chemical and physiological mechanisms under various levels of P simulating the circumstance of the field. The results indicated that the biomass of the maize was higher than that of the others treated with P at 2500 mg/kg. Exchangeable Cd and Zn in soil and the concentration of Cd and Zn of maize tissue were least, it showed that high concentration of P could make plenty of Cd and Zn be fixed in the soils and the uptake of Cd and Zn reduced in plant. But while the concentration of Cd was 5mg/kg, it occurred in a trend that the pH was decreased in soil, soils phosphatase and the content of chlorophyll of the leaves were reduced. When P at 500mg/kg, the phosphate amendments could decreased the biologic effect of Cd and Zn and reduced Cd and Zn uptake by plant significantly (p<0.05). When P at 50mg/kg, the low concentration of P increased the available form (exchangeable) of Cd in soils and enhance Cduptake by plant, whereas restricted Zn uptake by the plant. In addition, from the results we could find that P could decrease the synergism between Cd and Zn and that the antagonism between P and Zn was more stronger than antagonism between P and Cd: Scanning the surface of soil and the roots of the maize> the deposits which we speculated the phosphate with Cd and those with Zn may exist.Above all the results showed that aiming at the complex pollution with different levels of Cd and Zn concentrations, external P could change the sorption-desorption in contaminated soils and came into being the deposits with P, Cd, Zn to decrease the bioavailability and translocation, accordingly achieved the intention to remediate the contaminated soils in the course of the application of phosphate fertilizer.