| Heavy metals and polycyclic aromatic hydrocarbons?PAHs?which poses a huge threat to human beings and natural ecosystems.River,lake and agricultural soils,mainly rice-producing provinces are polluted by heavy metal and particularly by Cd which presently occupies the first rank of the most serious pollutant in China.Cd concentration in soil increase more and more compared to the background value in the soil in 1990.Around 13 330 ha of farmland in different provinces have been contaminated by Cd.Similarly,the emulsion of phenanthrene,a three-fused ring compound commonly present in PAH-contaminated soils,in China was estimated to about 25.300 t in 2010,due to straw burning,coke production coal/biomass combustion and anthracene oil production.Both chemicals are hazardous not only to plant and soil microorganism but pose health risks for animal and human.To handle with these combined pollutions,soil culture experiment was conducted with rice straw of Oryza sativa?Taichung Native-1 variety?,two ecological earthworms' species Eisenia fetida?epigeic species?and Aporrectodea caliginosa?endogeic species?and sunflower plant?Helianthus.annuus L?were used as a novel strategy.This strategy in one hand consisted of cleaning up Cd by phytoremediation process assisted by earthworms,minimizing and alleviating its effects by immobilizing it in soil and decreasing its accumulation in plants' edible parts.On the other hand,it permit to promote the metabolism of phenanthrene by stimulating the development of indigenous phenanthrene-degrading bacteria in soil and inducing the increase of some specific enzymes which could catalyze the metabolic activation step of phenanthrene.The main results are followingFirst,five concentrations in a geometric series of 0.5,1,3,5 and 10 mg Cd kg-1of soil were used to get the concentration value that exhibited 50% mortality of earthworms andii significantly reduced plant height,at 95% confidence interval.The concentrations ofphenanthrene used were 5,10,15,20,25 and 30 mg kg-1.The interactive effect of chemicals was scrutinized by using the acute toxicity test with the concentrations of 2.51 mg kg-1 and3.74 mg kg-1,respectively,being lethal for 50% of E.fetida and A.caliginosa.It was found that in the mixture treatment,phenanthrene at 5,10,15 and 20 mg kg-1 significantly mitigated both earthworm's species mortality and body-mass loss.The factor of Cd accumulated in E.fetida and A.caliginosa tissues was significantly decreased by about 12% and 16%,respectively.The reduction of both earthworm species mortality was negatively and significantly correlated?r2 = 0.98 ± 0.40 and 1 ± 3.9p < 0.001?with phenanthrene concentration in soil.However,over 20 mg kg-1 of phenanthrene,both organisms mortality rate increased again,as was the bioaccumulation factor of phenanthrene.The combined dose at25 and 30 mg kg-1 of phenanthrene was found to significantly reduced both plants height,root biomass and root length,while remaining lower than the inhibition effect induced by phenanthrene in single treatment at these concentrations.Additionally,the effect observed at3.47 mg Cd kg-1 was slightly higher compared to that of 2.51 mg Cd kg-1.This would indicate that the combined effect of phenanthrene and Cd could be concentration dependent than joint response effect.These findings showed that at a degree of concentration,phenanthrene could antagonistically affect Cd on different ecological species of earthworms and plant in soil and could render Cd less-toxic.Secondly,Cd toxicity is related to its bioavailability in soil.The evaluation of the impacts of earthworms and crop straw on Cd mobile form changes in soil and their effects on Cd uptake by plant revealed that earthworms,rice straw,and plant interactions change the Cd mobile forms in soil.The sequential extraction of Cd which was divided into exchangeable Cd,bound to organic matter,bound to iron-manganese oxide and residual fraction was ranged followingthe order:?i?in earthworms treatment: exchangeable > residual > bound to Fe-Mn oxide >bound to organic matter,and?ii?in rice straw treatment: exchangeable > bound to organic matter > residual > bound to Fe-Mn oxide.The accumulation of Cd in plants was increased by earthworms while decreased by rice straw.These findings highlighted that earthworms' activities and crop straw can modify soil properties and structure and promote the remediation of heavy metal.Thirdly,the sequencing 16 S r RNA genes and the result about functional genes information revealed that carbohydrate metabolism including bisphenol,benzoate and ethylbenzene degradation were more enriched in soil samples,indicating that both rice straw and earthworm enhanced phenanthrene degradation.Subsequently,both Shannon diversity index and bacterial16 S r RNA genes negatively correlated with the remained phenanthrene concentration in soil.The lowest soil remained phenanthrene concentration ratio,the highest Shannon diversity index and the highest bacterial 16 S r RNA genes,in the dual treatment designated that both earthworms and rice straw might improve the phenanthrene metabolism by increasing soil bacteria community and diversity.The abundance of genus Pseudomonas,Luteimonas,Rhodanobacter,Sphingomonas,Gemmatimonas,Flavobacterium,and Leifsonia,which was found to negatively correlate with the remained phenanthrene concentration in soil and was significantly increased in the presence of both earthworms and rice straw.Based on the negative correlation between remained phenanthrene concentrations in soil,the abundance of soil bacteria,bacteria Shannon index and the increase of some specific bacteria as well,this study proposed that the positive interaction between earthworms and rice straw could change soil microbial community and promotes phenanthrene degradation in soil.Fourthly,the modified rice straw by Na OH solution to produce a carbonaceous for the phenanthrene degradation activation and provide more binding sites for Cd was revealed to beeffective,since the alkali treatment not only led to expose certain anionic matters groups on the modified rice straw surfaces,which can complex with Cd and reduced its availability in soil,but also subsequently,the prediction of the functional profile of soil samples using COGs and KEGG data-base revealed that,it increased ion transports which might be involved in Cd management,and signaling molecular including cytochrome P450 s categories.The exposed functional group might serve as external subtract,while the monoxygenase cytochrome P450 s as catalyzer to activate and initiate phenanthrene metabolism process.These finding offer confirmation that modified straw could promote the reduction of heavy metal and the degradation of PAHs in soil.In conclusion,this study enlightened that the ecological context of remediation instead of being limiting on chemical and physical approaches,should integrate the natural materials forsaken that are available in large quantity such as rice straw which might have potential as inexpensive sorbent or immobilizer for both organic and inorganic pollutant,positive influence on soil microbial growth,and earthworms as soil enginer which can provide a positive impact on both plant health,soil microbial and the remediation of both organic and inorganic contaminant in soil. |
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