Phytoremediation Effects Of Phosphorus And EDDS On Copper And Cadmium Contaminated Soils By Ricinus Communis L.

In recent years,mining activities have led to heavy metal pollution of surrounding farmland soil.Heavy metal pollution can worse the ecological environment and affect safety production of food.Heavy metal pollution of farmland soils in mining area are usually caused by multi-metal elements and the most common is Cu and Cd co-contaminated soil.Cu and Cd can produce oxidative stress on plants,decrease the biomass of plants,and eventually reduce phytoremediation efficiency.The addition of exogenous chelating agents and fertilization can prominently facilitate the uptake of heavy metals by plants,but the current researches are mostly limited to a single strengthening measure,such as fertilization or chelating agents.Thus,the effect of fertilization combined with chelating agents on phytoremediation of heavy metal contaminated soil needs to be further explored.Castor（Ricinus communis L.）was used as the experimental research object.The soil culture and a pot tests were served to investigate the effect of phosphorus fertilizer and EDDS（N,N’-1,2-ethanediylbis-1-aspartic acid）jointly applied on the form and availability of Cu and Cd in heavy metals contaminated soil.The effects of phosphorus,EDDS and the combination of phosphorus and EDDS on the mechanism of uptake,translocation and accumulation of Cu and Cd in castor were analyzed.The main results are as follows:（1）The addition of EDDS significantly increased the availability of Cu and Cd in contaminated soil.In the treatment without phosphorus application,the availability of Cu and Cd increased with the increase of the EDDS concentrations.The addition of EDDS promoted the transformation of Cu and Cd from weak bound state to availability.At the same culture time,the available Cu and Cd in soil reached the maximum with applying 7.5 mmol/kg EDDS.The activation effect of EDDS on Cd and Cu reached the highest on the 15th day.The contents of available Cd and Cu increased by 43.0%-137%and 53.6%-78.6%respectively compared with the control on the 15th day,and then it gradually decreased.However,when the phosphorus application rate was 300 mg/kg P₂O₅,adding 5.0 mmol/kg EDDS was more effectively promoted the activation of Cu and Cd than 7.5 mmol/kg EDDS in Cu and Cd co-contaminated soils.The activation effect of EDDS on soil Cu and Cd was related to phosphorus level.（2）The application of phosphorus increased the biomass of castor in Cu and Cd contaminated soil,reduced the toxicity of heavy metals on castor leaves,and enhanced the antioxidant defense ability.Compared with the NK treatment,the fresh weight of stems and leaves increased by20.5%-31.2%and 23.8%-35.0%respectively.When the phosphorus application amount（P₂O₅）was500 mg/kg,the total Cu and Cd uptake amount were 201μg/plant and 29.9μg/plant,and the accumulation of Cu was 1.52 times that of the NK treatment.Meanwhile,applying 500 mg/kg P₂O₅markedly increased the content of residual Cd,but decreased the content of acid extractable Cd in soil.The reason of applying phosphorus enhanced the castor uptake Cu and Cd was that phosphorus promoted the growth of castor.（3）The addition of EDDS significantly increased the contents of Cu and Cd in aboveground parts of castor,and promoted the transportion of Cu and Cd to the shoot of castor.In the no phosphorus application treatment,adding 5.0 mmol/kg EDDS markedly increased the Cu contents in castor,and the aboveground Cu contents increased by 344%compared with control.EDDS had different effects on the uptake of Cu and Cd,and it more effectively promoted the uptake of Cu by castor.The addition of 2.5 and 5.0 mmol/kg EDDS increased the contents of soil Olsen-P,NO₃^--N and NH₄⁺-N,which was conducive to the increase of nitrogen and phosphorus in the shoot of castor,and increased the contents of photosynthetic pigments.However,EDDS inhibited the growth of roots,increased the content of malondialdehyde（MDA）in castor roots and reduced the root’s biomass.In summary,EDDS enhanced the castor uptake Cu and Cd and enhanced phytoremediation efficiency.（4）A pot experiment was set to investigate the effect of both phosphorus fertilizer and EDDS on the growth of castor and the accumulation of Cu and Cd in castor.The study showed that applying phosphorus significantly promoted the growth of castor;On the other hand,EDDS increased the content of Cu and Cd in shoots of castor,increased the accumulation of heavy metals in shoots,and improved the efficiency of phytoremediation.When the phosphorus application rate was 100 mg/kg and the addition amount of EDDS was 5.0 mmol/kg,the TF of Cu and Cd were 0.530 and 0.250,and the accumulation Cu in aboveground was 12.4 times that of CK,which was more conducive to the removal of Cu from soil.Nevertheless,When the P₂O₅ application rate was 300 mg/kg,it could significantly promote the accumulation of Cd in aboveground of castor.Consequently,when phosphorus and EDDS applied together were more conducive to the uptake and enrichment of Cu and Cd in the aboveground part of castor and improved the phytoremediation efficiency of castor.