Study On The Transfer Characteristics Of Cobalt And Nickel From Soils To Plants And Their Toxicity To Plants

The highly sensitive vegetable to the toxicity of Co (cucumber) and to Ni (water spinach) and rice were cultivated by soil culture experiments to study the transfer and distribution of Co and Ni in the soil-plant system and the effects and toxicity thresholds of soil Co and Ni to the crops. The influence of soil properties on the toxicity thresholds of Co and Ni were also studied. The main results were as follows:1. In general, the bioconcentration factors (BCF) of Co from soil to different parts of cucumber and rice were increased with the increase in added Co to the soil. BCFs of Co of all parts of cucumber were greater than those of rice. At low Co addition, the transfer factors (TF) of Co from root to shoot for cucumber and rice were increased with increasing Co addition. At high Co addition, TF of Co in rice tended to decrease with increasing Co addition. Co promoted the uptake of Mn, Zn and Cd by cucumber, promoted the absorption of Mg but inhibited the uptake of P by cucumber shoot. Co favored the absorption of Cu, N, P and K by rice.2. The BCF of Ni in different parts of water spinach and rice were increased with the increase of Ni addition into the soil. The BCF of Ni of water spinach was greater than those of rice. For water spinach, TFs of Ni from root to shoot increased first and then decreased with increasing Ni addition. For rice, TFs from root to each part of shoot decreased first and then increased with the increase of Ni addition, all being smaller than the TF of the control. Ni addition promoted the uptake of Mn, Zn, Mg and N while inhibited the absorption of K by water spinach. Ni favored the absorption of Fe by rice root and stem but inhibited the transfer of Fe to rice husks and unpolished rice, repressed the uptake of Cu by rice root while promoted the transfer of Cu to rice shoot, favored the absorption of P by rice root and the transfer of P to rice grain but inhibited the transfer of P from root to stem and leaf.3. Under different dosage of added Co, the shoot mass of cucumber decreased from 21.97% to 95.36%, and the rice yield decreased from 4.59% to 76.14% compared with the control. Under different dosage of added Ni, the shoot mass of water spinach reduced from 29.59% to 93.84%, and the rice yield decreased from 4.24% to 77.49% compared to the control.4. The thresholds of soil Co for 10% reduction of the shoot mass of cucumber were 2.74 mg/kg (DTPA-extractable Co) and 97.98 mg/kg (total soil Co). The thresholds of soil Ni for 10% reduction in the shoot mass of water spinach were 2.20 mg/kg (DTPA-extractable Ni) and 43.67 mg/kg (total soil Ni).5. Single regression was used to determine the relationships between toxicity threshold values and some selected soil properties. The contents of clay and free Fe oxides showed the most significant correlations with EC10 based on total soil Co followed by exchangeable Mg and CEC. The relationships between pH and exchangeable Mg with EC10 based on DTPA-extractable Co were the most significant among the soil properties, and exchangeable Ca and free Fe oxides were also significant with the EC10 based on DTPA-extractable Co. The organic matter was the best predictor of the variation of EC10 based on total Ni followed by exchangeable Ca, CEC and free Fe oxides and exchangeable Mg. The correlation between pH and EC10 based on DTPA-extractable Co was the most significant followed by exchangeable Ca and exchangeable Mg.Stepwise multiple regression analysis between the ECs and the selected soil properties resulted in the following relationships: EC10 (based on the total Co) = 5.442* [free Fe oxides] + 0.329*[clay content] + 0.028 EC10 (based on DTPA-extractable Co) = 12.765 - 1.642*pH– 0.113*CEC...