| Phosphorus loss and heavy metal contamination are the major factors causing soil degradation.In this regard,a simultaneous decrease of phosphorus loss and heavy metal bioavailability is emergently required to improve soil productivity and meet future food needs,which is still a thorny issue.Over the past few decades,biochar application is a very popular method to decrease the eco-risk of contaminants due to its excellent absorption capacity.Besides,biochar has been recognized as an eco-friendly,and low-cost material for the removal of contaminants,which is a current research focus.Base on this situation,swine manure derived biochar(MB),maize straw derived biochar(SB),and pine derived biochar(PB)were chosen in this study.A batch adsorption experiment was conducted to explore the absorption characteristic and underlying adsorption mechanisms of phosphorus(P),arsenic(As),and chromium(Cr)to biochar,and the interactive effects among P,As and Cr.Moreover,a pot experiment was conducted to analysis the effects of biochar addition on the distributions of P,As and Cr in the water-soil-rice system,and the response of rice growth and bacterial community structure to demonstrate the P,As and Cr distribution characteristic and the major driver of the contaminants translocation in the system.Hence,Practical implementation of biochar to remediate soil cocontaminated with P,As and Cr has been critically discussed.The results were important to maintain sustainability in agricultural soils and contaminants remediation.The results are as follows:1)Biochar properties were strongly dependent on pyrolysis temperatures and feedstocks.Specifically,the pH values and ash concent significantly increased with increasing temperature,while volatile and cation exchange capacity(CEC)significantly decreased.Moreover,C content was increased whereas H,O,N were reduced with increased pyrolysis temperature;as such,polarity,aromaticity,and hydrophobic of biochar were increased,the number of functional groups was reduced.Furthermore,the porosity of biochar was increased first and then reduced.The availabilities of K,Zn,and Fe were increased,while the availabilities of P,Ca,Mg were reduced with increasing temperature.In addition,compared with SB and PB,MB had higher value of pH,and higher amounts of ash,P,K,Cu,Fe,and Zn.However,PB had more abundant porosity,higher C concent.Overall,CEC was highly variable depending upon the pyrolysis temperature,whereas pH,ash,and mineral compositions were significantly impacted by feedstocks.2)The orders of adsorption capacities of P,As and Cr to different biochar were MB<SB=PB(p<0.05),MB>SB=PB(p<0.05),and MB>SB>PB(p<0.05),respectively.The highest adsorption capacity for P was found in PB700 treatment with adsorption capacity of 5.64 mg/g;while MB700 has the highest adsorption capacity for As with adsorption capacity of 0.3 mg/g,and MB500 has the highest Cr adsorption with the adsorption capacity of 6.57 mg/g.Electrostatic adsorption and functional group complexation were the dominant mechanisms for P absorption to MB,while complexation with ash/mineral,and electrostatic adsorption were the dominant mechanisms for P absorption to SB and PB.Arsenic absorption was closely associated with ash and minerals.Minerals complexation was the dominant mechanism for Cr absorption,electrostatic adsorption contributed a secondary effect for Cr absorption.There were strong interactive effects among P,As and Cr.Specifically,the competitive adsorption between P and As inhibited their adsorptions to biochar.In the binary system,both As and Cr adsorptions were increased due to the formation of As-Cr complex mineral with 0.6:1 atomic ratio of As/Cr.The interactive effect between P and Cr was weak.Compared with single system,P,As and Cr absorption capacities increased by 9-83%,71-73%,42-91%in the multimetal system using biochar,suggesting biochar exhibits a great potential to act as a applicable material for co-present P,As and Cr removal.3)Biochar addition significantly increased soil organic matter(SOM),total nitrogen(TN),available phosphorus(AP),total phosphorus(TP),available potassium(AK)concentrations,while significantly reduced available nitrogen(AN)concentration.The increases of SOM in different biochar treatments ranged in order from PB>MB>SB(p<0.05),while the increases of soil N,P,and K concentrations in different biochar treatments ranged in order from MB>SB=PB(p<0.05).There is no significant difference between Cr fractions under different treatments.Degree of P saturation(DPS)and CaCl2-Cr exhibited significant correlations with crystalline Fe,indicating Fe affected P and Cr release,while CaCl2-As was correlated positively with SOM,suggesting the increase of SOM induced As release.Therefore,SOM and Fe oxides play important roles in P,As and Cr immobilization.4)Compare with control,5%-SB addition significantly increased rice biomass,while 5%-PB addition significantly reduced rice biomass,the trends for rice biomass under the other treatments were not remarkable.Compare with control,biochar addition significantly reduced malondialdehyde concentration and increased root reductive activity.5%-MB addition increased root P by 43%,while reduced root As by 93%,indicating that the increase of P in 5%MB treatment inhibited As uptake.SB addition at dosages of 1%and 5%reduced shoot As concentration.5%-PB addition reduced root As,and 1%-PB addition reduced root Cr by 97%.The significant relationships between shoot P/As/Cr and shoot Fe indicated they might share the same uptake mechanism in rice.The results showed that Fe plays a centrol role in P,As and Cr translation in rice.5)DCB-Fe(dithionite-citrate-bicarbonate extractable Fe)exhibited significantly positive correlations with DCB-P/DCB-As/DCB-Cr,with values of correlation coefficients of 0.54,0.66 and 0.69,respectively.Moreover,pore water Fe exhibited significantly positive correlations with pore water As and Cr concentrations,with values of correlation coefficients of 0.86 and 0.79.The results indicated that biochar addition affected P,As and Cr accumulations in rice tissuse and rice growth through the formation of iron plaque(IP)and Fe cycling.On one hand,iron plaque formation increased P,As and Cr concentrations in iron plaque,decreasing their uptake.On the other hand,the release of Fe reduced the immobilization of As and Cr,and thus increasing their uptake by root.Compared with control,the addition of PB inhibited iron plaque formation and stimulated Fe reduction,subsequently increasing As and Cr release,thus increasing the toxicity of As and Cr,suppressing rice growth.6)Compared with control,MB and SB application significantly increased microbial diversity(Shannon)and richness(Observed species,Chao1)indexes,whereas PB application significantly reduced microbial diversity and richness indexes.At the phylum level,biochar addition significantly increased the relative abundance of Actinobacteria,Acidobacteria,Chloroflexi,Chlorobi,and Gemmatimonadetes.At the genus level,biochar addition significantly increased the relative abundance of plant growth-promoting rhizobacteria(PGPR)by 17-53%,while reduced P-solubilizing bacteria(PSB)and iron-reducing bacteria(IRB)relative abundance by 33-57%and 34-57%,respectively.Pore water Fe exhibited significantly positive correlations with Geobacter and SOM.It suggested that Geobacter was the key factor controlling Fe cycling,influencing As and Cr release.Besides,SOM was participated in and stimulated Fe reduction.Therefore,suppressing IRB growth especially for Geobacter and decreased Fe reduction can be an effective pathway to reduce As and Cr release. |
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