Intercropping Remediation Of Legume And Gramineae In Phenanthrene Contaminated Soil And Its Mechanism

Polycyclic Aromatic Hydrocarbons (PAHs) have been the subject of intense research due to their hydrophobic, recalcitrant, persistent, potential carcinogenic, mutagenic and toxic properties. Over90%of PAHs in the environment reside in surface soil. Therefore, more attention has been received in remediation of PAH-contaminated soil. Compared to single cropping, intercropping has more advantages in plant growth and phytoremeadation of polluted soil. In this paper, intercropping between gramineae and legume was employed to remediate phenanthrene-contaminated soil. The effectiveness of intercropping and relative mechanisms were also investigated. The results showed as follows:Plants grew well in the phenanthrene range of0-50mg·kg-1. Phenanthrene in soil decreased significantly over the experiment period because of biodecomposition and plant root absorption. The effectiveness of intercropping remediation is better than that of single cropping remediation. Removal rate of phenanthrene was up to99.50%in intercropping remediation. Wheat/clover intercropping enhanced the absorption of phenanthrene by wheat and clover roots. Hence, wheat couldn’t be produced safely in this intercropping phytoremediation system. Ryegrass/clover intercropping also promoted the absorption of phenanthrene by ryegrass and clover roots, and it’s a practical intercropping phytoremediation system.Soil physicochemical properties play an important role in the efficiency of phytoremediation and soil arability after phytoremediation. Soil pot experiments were conducted to investigate the dynamic changes of physicochemical properties in phenanthrene-contaminated soil under wheat and clover intercropping. Plants improved the pH values of phenanthrene-polluted soil with a maximum△pH of0.61. The difference in pH between wheat/clover intercropping and wheat/clover single cropping was not significant. Soil organic matter, total nitrogen, available nitrogen, total phosphorus, available phosphorus, cation exchange capacity and available potassium decreased over the experiment period due to biodecomposition and plant root absorption. The intercropping of wheat and alfalfa accelerated the consumption of the above-mentioned nutrients. And the decrease percentages ranged from5.24%to57.85%, more than those of wheat or alfalfa only planted with decrease percentages between6.29%and39.09%. In particular, soil available nitrogen and availiable phosphorus decreased more than the other nutrients with a maximum reduction percentage of57.85%. Therefore, the application of nitrogen and phosphorus must be paid more attention during phytoremediation of polycyclic aromatic hydrocarbon-contaminated soil in the wheat and alfalfa intercropping system. Electrophysiological results indicated that the phenanthrene-triggered electrical response of wheat roots was comprised of two sequential phases:a depolarization of the membrane potential, followed by a. repolarization. Phenanthrene concentration-dependent depolarization of membrane potential had features of saturating kinetics with an apparent Km of10.8μM. Moreover, wheat root uptake of phenanthrene depended on external solution pH. Lower pH favored phenanthrene uptake. Simultaneously, phenanthrene uptake could cause an increase in external solution pH. Plasmalemma H+-ATPase activity positively correlated with phenanthrene concentration in solution. Both vanadate and2,4-dinitrophenol suppressed the electrical response to phenanthrene and phenanthrene uptake. Membrane depolarization, pH changes, and pH dependence of uptake demonstrated that phenanthrene actively entered wheat root via a phenanthrene/H+symporter driven by electrochemical proton gradient.Relationship between plant root components and morphology and PAH uptake by plant roots under hydroponic conditions was also studied. The greater the root morphological parameters, the more uptake of phenanthrene was. Soybean root morphological parameters were higher than those of ryegrass, wheat and carrots, and it absorbed more phenanthrene. This displayed that there was a good correlation between root morphological parameters and root uptake of phenanthrene. Lipid content in soybean and carrot roots was significantly greater than in wheat and ryegrass roots. Absorption of phenanthrene by soybean and carrot root was also higher than by ryegrass and wheat root. Therefore, phenanthrene uptake by plant roots is affected by root components.