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Burned rice straw reduces the availability of clomazoneto barnyardgrass
Chao Xua, Weiping Liub, G. Daniel Shengb,⁎aInstitute of Environmental Science, Zhejiang University, Hangzhou 310029, ChinabResearch Center of Environmental Science, College of Biological and Environmental Engineering, Zhejiang University of Technology, Hangzhou310032, China
Article history:Received 22 May 2007Received in revised form24 November 2007Accepted 26 November 2007
Field burning of crop residue is a common post-harvest practice to dispose of theseagricultural by-products and for land clearing. Burned crop residues may effectively adsorbpesticides and thus influence their bioavailability in agricultural soils. The adsorption ofclomazone by a soil amended with a burned rice straw (BRS) was measured. The availabilityof clomazone to barnyardgrass in the soil in the absence and presence of BRS was tested.The BRS was 1000–20,000 times more effective than soil in sorbing clomazone. The sorptionof clomazone by soil increased with increasing BRS amount in the soil. In a bioassay, theinjury of barnyardgrass 9 days after planting decreased with increasing BRS amount in soilindicating the effect of BRS on clomazone availability. Residual analyses showed higherconcentrations of clomazone in soils receiving higher rates of the herbicide than in soilswith lower application rates suggesting the adsorptive role of BRS. At typical application rateof clomazone (0.3 μg g−1), BRS amounts of 0.02 wt.% and higher caused an appreciablereduction to a complete loss in clomazone availability. Calculations suggest that fieldburning of rice strawmay result in sufficiently high amounts (N0.02 wt.%) of BRS, and hencecontribute to often experienced loss of pesticide availability in agricultural soils. Our resultsmay be extended to field situations where other crop residues and vegetation are burned.Alternative management of crop residues may improve the bioavailability of pesticides inagricultural soils.
Field burning of vegetation for weed control, disposal of post-harvest crop residues, and immediate land clearing is acommon agricultural practice worldwide. While completeburning of vegetation would yield mineral ash, this isgenerally not the case. Burned agricultural residues usuallycontain a significant amount of charcoal and black carbon dueto combustive carbonization. These components have highsurface areas and hence possess adsorptive abilities. Burnedresidues, when incorporated into agricultural soils, may thussignificantly increase the sorptive abilities of the soils. The
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adsorptive property of burned residues was first proposed byHilton and Yuen (1963) who reported that the sorption ofsubstituted ureas and s-triazines on Hawaiian sugarcane(Saccharum officinarum L.) soils that previously received burn-ing of cane trash remained high after the oxidative removal ofsoil organic matter by hydrogen peroxide and was reducedmarkedly on ignition. The authors speculated that thesorptive behavior of the sugarcane soils was a property ofboth easily oxidized soil organicmatter and peroxide-resistantcarbon arising from cane trash burning. However, no directevidence of pesticide adsorption by burned residues waspresented. Recently, Yang and Sheng (2003a) reported that, on
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a unit mass basis, the adsorptivity for diuron of a particulatematter arising from burning of wheat (Triticum aestivum L.)straw was 400–2500 times higher than that of a soilcontaining 2.1% organic matter. The diuron sorption by thesoil amended with the particulate matter increased withincreasing particulate matter content in the soil. Aged in thesoil for up to 12 months, the particulate matter remainedhighly effective in adsorbing diuron, indicating its refractorynature (Yang and Sheng, 2003b). The carbon fraction of theparticulate matter, obtained by repeated washing with diluteHCl–HF solution, was found primarily responsible for its highadsorptivity (Yang and Sheng, 2003a).
Previous studies have established that phytotoxicity ofpesticides is related to their concentrations in the soilsolution (e.g., Lambert 1966; Pillay and Tchan 1971) andhence largely controlled by sorption/desorption of thepesticides in soils. Due to its high adsorptive ability forpesticides, burned crop residues effectively decrease theconcentrations of pesticides in the soil solution. Conse-quences of this behavior may result in not only a loss ofpesticide efficacy leading to the protection of crops frominjuries or poor pest control, but also reduced bioavailabilityof pesticides and thereby increased environmental persis-tence. It has been shown that charcoal dipping or bandingeffectively protects many crops from herbicidal injuries (e.g.,Arle et al., 1948; Jordan and Smith, 1971; Toth et al., 1987;William and Romanowski, 1972). Reduced phytotoxicity ofdiuron applied over burned kangaroo grass (Themeda australis(R.Br.) Stapf), and of thiobencard and molinate over burnedrice stubble, was reported in the literature (Toth et al., 1981,1999). Biodegradation of benzonitrile was substantiallyreduced in soils containing a burned wheat straw (Zhang etal., 2004, 2005). Poor pest control is obviously an economicconcern due to additional costs for pest control or lost profitsfrom reduced yields or poor crop quality. Reduced biode-gradation is a serious environmental concern over sustain-able agriculture and soil quality.
This study examined the herbicidal effectiveness ofclomazone to barnyardgrass (Echinochloa crus-galli (L.) Beauv.)(BYG) seedlings in a soil in the absence and presence of aburned rice straw (BRS). Selection of clomazone and BYG asrespective testing pesticide and plant was based on the factthat BYG, a common rice grass, is the primary target ofclomazone in weed control. It was hypothesized that effectiveadsorption of clomazone by BRS would lead to a significantreduction in clomazone availability to BYG. Our objectiveswere therefore to investigate the impact of clomazoneadsorption to BRS on its availability to BYG and to understand,in a broader context, how burning of crop residues affects thebioavailability of pesticides in agricultural soils.
2. Materials and methods
2.1. Materials
Rice (Oryza sativa L.) straw was collected from a rice fieldlocated in suburban Hangzhou, Zhejiang Province. The straw(10 kg) was air-dried in a greenhouse for 1 week and thenburned in an open field under natural conditions on a calm
sunny afternoon. BRS was then collected and stored in thelaboratory at room temperature (~24 °C).
A soil, classified as a silt loam, was also collected from thesame rice field. The soil contained ~1% organic matter andconsisted of minerals of mostly kaolinite and oxides. The soilwas air-dried in a greenhouse, ground, and sieved through a 2-mm sieve.
Clomazone, a rice herbicide, was purchased from Chem-Service (West Chester, PA, USA) with a purity of N98%, andused as received. The herbicide has a water solubility of~1100 mg L−1 and a log Kow of 0.25 (Howard and Meylan 1997).The Henry's Law constant of 4.13×10−8 atm·m3·mol−1 indi-cates that the herbicide is non-volatile and hence suitable forgreenhouse study.
BRS-amended soils were prepared by placing 200 g of soilinto plastic bags. BRSwas added to the soil in exact amounts of0.01, 0.02, 0.05, 0.1, 0.5 and 1% (w/w). The bagswere sealed, andthe contents were shaken thoroughly to mix the soil with BRS.
2.2. Sorption of clomazone
Sorption of clomazone by soil, BRS, and BRS-amended soilswasmeasured by the batch equilibration technique. The effectof BRS on the sorptive property of soil was evaluated usingsoil, BRS, and two BRS-amended soils. Various quantities(2.58–40.8 µg) of clomazone in 8 mL of 0.005 M CaCl2 solutionwere introduced into 25-mL Corex glass centrifuge tubescontaining 4.34 g of soil, 0.0125 g of BRS, 3.97 g of 0.1% BRS-amended soil, or 2.18 g of 0.5% BRS-amended soil. Themass ofsorbents was chosen to allow for N30% of added clomazone tobe sorbed at equilibrium. The initial concentrations ofclomazone ranged from 0.320 to 5.96 mg L−1. The centrifugetubes were closed with Teflon-lined screw caps and rotated(40 rpm) at room temperature (~24 °C) for 48 h. Previous kineticmeasurements showed that pesticide sorption on soils andburned crop residues reaches equilibrium within 24 h (e.g.,Yang and Sheng, 2003a,b). After establishment of sorptionequilibrium, sorbents and aqueous phases were separated bycentrifugation at 6000 rpm (RCF=5210 g) for 20 to 30 min. Theclomazone concentrations in supernatants were analyzed byhigh performance liquid chromatography (HPLC). Each sorp-tion measurement was made with eight concentrations induplicate; the average data were reported. The amount ofclomazone sorbedwas calculated from the difference betweenthe amount initially added and that remaining in theequilibrium solution. Blanks not containing sorbents werealso run to ensure that glass tubes did not adsorb clomazone.
2.3. Greenhouse bioassay
An initial stock solution of clomazone was prepared in puremethanol. The stock solution was diluted to prepare fiveworking solutions with clomazone concentrations of 3, 6, 12,24, and 120 µg mL−1 in methanol. Five mL of each of thesolutions were added to 200 g of soil or BRS-amended soils inplastic bags to give five application rates of clomazone. Thefive rates were labeled as 0.25X, 0.5X, 1X, 2X, and 10X,respectively, where X=0.3 µg g−1 and represents the recom-mended field rate of clomazone. The five rates were equiva-lent to the soil concentrations of 0.075, 0.15, 0.30, 0.60, and
Fig. 1 –Sorption of clomazone from water by soil, BRS, 0.1%BRS-amended soil, and 0.5% BRS-amended soil.
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3.0 µg g−1, respectively. A control soil receiving 5 mL ofmethanol without clomazone was also prepared. The treat-ments and the following assayswere in triplicate. Each 5mL ofsolution was dispensed over the surface of soil or BRS-amended soils as evenly as possible; the bags were sealedand the contentswere shaken thoroughly for 1min to disperseclomazone in the soil or BRS-amended soils. The bags werethen re-opened, placed in a hood with ventilation for 48 h toallowmethanol to evaporate from the bags. The dried soil andBRS-amended soil treatments were then transferred to codedpotsmade from 275-mL disposable plastic drinking cups cut to7 cm in height. This size of cup was sufficient to hold thetreatments with a 1-cm rim remaining.
BYG seeds were collected from plants grown on the ricefield specified earlier. Twenty-five grams of seeds were placedinto a 250-mL Erlenmyer flask and acid-scarified in 200 mL of1 N nitric acid for 1 h at room temperature with constantagitation on a magnetic stir-plate. The treated seeds werethoroughly washed with deionized water for 5 min, spreadevenly on filter paper in Petri dishes, covered with anotherfilter paper, watered with 3 mL of deionized water, andincubated in the dark at 30 °C for 72 h for germination. Forplanting, a small quantity of soil or a BRS-amended soiltreatment (~20 g) was first removed from each cup. Fifteenseeds showing healthy germination with extended radiclesand hypocotyls were placed evenly on the soil surface andthen covered with the previously removed soil. Followingplanting, cups were placed in a completely randomized blockdesign in a greenhouse, watered with 40 mL of deionizedwater, and maintained moist throughout the experiment. Thegreenhouse was maintained at 34/20 °C day/night tempera-tures with a 14-h lighting cycle. BYG seedling injury wasvisually rated 9 days after planting as percentage of injurybetween 0 and 100, with 0% representing no injury and 100%the complete injury. The injury rating was performed inde-pendently by three individuals. The bioassay was repeatedthree times. Injury ratings from all the three assays (total 9sets of data) were pooled and averaged. BYG injury wasidentified as bleaching of the emerged hypocotyls and firstleaves. Heavily injured BYG plants eventually shriveled anddied.
Following the first bioassay, three replicate samples of eachof the selected soils were thoroughly mixed and analyzed forresidual clomazone. The following 9 soils were selected basedon BRS amount, clomazone rate and BYG injury rating: S0–0,S0–0.15, S0–0.30, S0.01–0.15, S0.01–0.30, S0.5–0.30, S1–0.30, S1–0.60, and S1–3.0, where the first and second numbers of thesuffix to S represent the BRS amount and soil concentration ofclomazone, respectively. Soils (10 g) were mixed with 10 mL ofdeionized water and 10 mL of water-saturated ethyl acetate in50-mL glass tubes. The tubes were shaken vigorously for 2 h.The ethyl acetate phases were then transferred to 25-mL glasscentrifuge tubes, centrifuged to remove remaining suspendedsolids, and dried with anhydrous sodium sulfate. The cloma-zone concentrations in the ethyl acetate phases were ana-lyzed by Gas Chromatography-Mass Spectroscopy (GC-MS).The measured recovery by the above procedure was 97.4%.The presence of 1% BRS in soil did not affect the clomazonerecovery. The clomazone concentrations were adjusted for therecovery.
2.4. Data treatment and statistical analyses
Sorption data were fit to the following Freundlich equation:
Q ¼ Kf � CNe
where Q and Ce are the amount of clomazone sorbed and theequilibrium concentration, respectively, Kf and N are con-stants. The curve-fitting was performed using the SigmaPlotversion 10 program (Systat Software, Inc., San Jose, USA). Thestandard errors and R2 values were obtained.
The BYG injury ratings were submitted to two-wayanalyses of variance (ANOVA) to statistically test the influ-ences of the application rate of clomazone and BRS amountrepresenting the two independent variables. Subsequently, amulti-comparison test of means (Tukey test) for different BRSamounts was performed. A one-way ANOVA was used todetermine the significant differences in BYG injury ratings foreach application rate of clomazone between a specific BRSamount and the corresponding control (i.e., no BRS). Thesoftware package used for the statistical analysis was theOrigin 7.0 program (Northampton, MA, USA).
3. Results and discussion
The sorption of clomazone by soil, BRS and BRS-amended soilsis presented in Fig. 1, in which the isotherms with error barsare plotted as the amount of clomazone sorbed by thesorbents (mg kg−1) against the equilibrium concentration(mg L−1) in water. BRS displayed a much higher sorptivity forclomazone than did soil. Using the data in Fig. 1, calculationsshow that, over the equilibrium concentration range from 0 to1 mg L−1, BRS was ~1000–20,000 times more effective than soilin sorbing clomazone fromwater. A burned wheat (T. aestivumL.) straw and burned paspalum (Paspalum dilatatum L.) werefound also highly effective in sorbing diuron (Toth andMilham, 1975; Yang and Sheng, 2003a). The results of curve-fitting of sorption data to the Freundlich equation arepresented in Table 1. In all the cases, the standard errors are<6% of their respective Kf or N values and the R2 values are
Fig. 2 –Effect of BRSonclomazoneavailability tobarnyardgrass9 days after planting.
Fig. 3 –Percent injury of barnyardgrass 9 days after plantingas a function of BRS amount in soil and clomazoneapplication rate. An asterisk represents the BYG injury ratingat a specific BRS amount and given application rate ofclomazone significantly different when compared with thecorresponding control without BRS at p<0.001 level.
Table 1 – Kf and N values from the curve-fitting ofclomazone sorption data to the Freundlich equation
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≥0.988, indicating excellent fitting. The curve-fitting resultedin N values of 0.824 and 0.228 for soil and BRS, respectively. Asthe parameter N is an indicator of the isotherm nonlinearity,the smaller N value for BRS suggests that BRS may sorbclomazone more strongly in a fashion similar to that ofactivated carbon as a surface adsorbent.
Much higher sorption of clomazone by BRS than by soilsuggests that the presence of BRS in soil may substantiallyenhance the sorptivity of the soil. As illustrated in Fig. 1, the0.1% BRS-amended soil showed an enhanced sorptivity forclomazone. Assuming the BRS amendment did not change thesorptivity of soil itself, the isotherms show that 0.1% BRS insoil enhanced clomazone sorption by ~20–40%. Higher BRSamount in the 0.5% BRS-amended soil resulted in an evenhigher sorptivity of the soil and 0.5% BRS contributed N60% tothe total clomazone sorption. These results indicate theimportance of burned crop residues in influencing pesticidesorption in agricultural soils. The curve-fitting to the Freun-dlich equation resulted in the N values of 0.712 and 0.691 forthe 0.1% and 0.5% BRS-amended soils, respectively. A decreas-ing order of the Freundlich N values with increasing BRSamount in soil indicates a progressively increasing degree ofisotherm nonlinearity, additional evidence of BRS contribu-tions to clomazone sorption by soil. Enhanced clomazonesorption by soil in the presence of BRSmay be of great concernas, at recommended application rates specifically designed forclomazone in rice production, it reduces the concentration ofclomazone in the soil solution available for BYG control andincreases the environmental persistence of the herbicide. Inaddition, slow release of sorbed clomazone over time mayimpact the growth of subsequent crops.
The BYG response to clomazone in soil treatments in theabsence and presence of BRS 9 days after planting from thefirst bioassay is presented in Fig. 2, in which cups were placedde-randomized to aid in visual observation. The solventcontrols showed that use of methanol to introduce clomazoneinto soils did not have any effect on BYG growth. Elementalanalysis indicated that BRS contained 8.5% (w/w) K, 312 mgkg−1 Fe, 4336 mg kg−1 Mn, 89.9 mg kg−1 Zn, 23.0 mg kg−1 Cu,13.1 mg kg−1 B, and other trace nutrients. In the absence ofclomazone, the BYG growth at all the BRS amounts did notshow an observable stimulation or inhibition (Fig. 2). By 9 daysafter planting, the BYG injury increased with increasingclomazone application rate in the absence of BRS. In thepresence of BRS, the availability of clomazone was reduced asBRS amount increased. While we were unable to establish aquantitative relationship between the availability of cloma-zone to BYG and BRS amount in soil, the observation is in
accordwith the fact that clomazone sorption by BRS-amendedsoils increased with increasing BRS amount in soil.
Fig. 3 shows the average BYG injury ratings from threeseparate bioassays at various clomazone application rates as afunction of % BRS in soil. As expected, the effect of BRS onclomazone availability depended on the clomazone applica-tion rate. An application rate of 0.25X, in the absence of BRS,showed poor BYG inhibition with a 24% injury rating. Injuryrating declined to <10% with 0.02 and 0.05% BRS in soil.Clomazone was completely unavailable with 0.1% and higheramounts of BRS. Similarly, a half rate (0.5X) of clomazoneshowed only 60% BYG injury for control samples and a declineat BRS amounts of 0.02% and higher. Clomazone wascompletely unavailable at BRS amounts of 0.5% and higher.
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A full rate (1X) of clomazone showed complete BYG injurywhen BRS was not present. Injury rating declined by 5–19%with BRS amounts up to 0.05%. Injury rating declined to 64%with 0.1% BRS in soil. There was no injury when BRS amountwas 0.5% or higher. Compared to the full rate, doubling theclomazone concentration (2X rate) provided better inhibitionof BYG. Although there was a 100% BYG injury with the 2X rateas compared to 64% injury with the 1X rate in the 0.1% BRS-amended soil treatment, the injury rating with the 2X raterapidly declined to approximately 30% when BRS amountincreased from 0.1% to 0.5%. Clomazone was almost comple-tely unavailable at 1% BRS. While a very high rate (10X) ofclomazone eliminated the BRS effect with a 100%BYG injury atmost of the tested BRS amounts, addition of 1% BRS into soilresulted in the injury rating to decline to 70% of the controlvalue. The Two-Way ANOVA indicated that at the probability(p)<0.001 the BYG injury ratings were influenced by both theapplication rate of clomazone and the BRS amount, as well asthe interaction of the two factors (the F values for theapplication rate of clomazone, BRS amount, and their inter-action were 26.33, 8.61, and 345.64, respectively). The multi-comparison tests of means further indicated that the BRSamounts of 0.5% and 1.0% posed the most significantinfluence on BYG injury. The One-Way ANOVA showed thatin most cases at a given application rate of clomazone thedifferences in BYG injury rating in the presence of variousamounts of BRS, as compared to no BRS, were statisticallysignificant. More specifically, the BYG injury was significantlylower for ≥0.02% BRSwith clomazone rates of 1X and lower, for0.5% BRS and higher with 2X of clomazone, and for 1.0% BRSwith 10X of clomazone.
The results suggested that BRS in soil can reduce cloma-zone availability to BYG. This is due presumably to effectiveadsorption of clomazone to BRS (Fig. 1). While the adsorptionto BRSmay not be the only factor on clomazone availability, itsprimary role is demonstrated by measuring residual cloma-zone in selected soils. Table 2 shows themeasured clomazoneconcentrations in 9 selected soil treatments from the firstbioassay. The original soil treatment (S0–0) did not contain ameasurable quantity of clomazone. All the soil treatmentsthat received clomazone application prior to BYG plantingcontained residual clomazone yet 9 days after planting. Theconcentrations of clomazone in all soil treatments werereduced to 23–39% of their respective applied rates. Samples
Table 2 – Residual concentrations of clomazone inselected soil and BRS-amended soil treatments 9 daysafter planting of barnyardgrass from the first bioassay
S0–0.15 and S0.01–0.15 with no and low BRS amounts,respectively, and a half rate of clomazone showed similarBYG injury ratings and had virtually identical concentrationsof residual clomazone (0.058 and 0.056 µg g−1, respectively).Four soil treatments (S0–0.3, S0.01–0.3, S0.5–0.3, and S1–0.3)with a full rate of clomazone had residual clomazoneconcentrations higher than those in soil treatments (S0–0.15and S0.01–0.15) with a half rate of clomazone. The residualconcentrations of clomazone in these treatments fell into twogroups, 0.112–0.114 µg g−1 for those samples (S0–0.3 and S0.01–0.3) containing no or 0.01% BRS and 0.069–0.072 µg g−1 for those(S0.5–0.3 and S1–0.3) containing 0.5% and 1% BRS, the formerbeing associated with near 100% BYG injuries and the latterwith no injuries. Lower residual concentrations of clomazonein soil treatments (S0.5–0.3 and S1–0.3) where BYG grew wellmay reflect the facilitated uptake or decomposition of theherbicide in BYG root zones. These phenomena were furtherobserved in soil treatment (S1–0.6) with a double rate ofclomazone and a normal BYG growth. This soil treatment (S1–0.6) had a residual clomazone concentration twice those inS0.5–0.3 and S1–0.3. The residual concentration of clomazone(0.784 µg g−1) in soil treatment (S1–3) with a 10X rate remainedhigh and was equivalent to a 2.5X rate, yet the injury rating of55% was comparable to those for treatments S0–0.15 andS0.01–0.15 that received 20-fold less clomazone and no or lowBRS with much lower residual concentrations. The fact thathigher degree of BYG injury in soil treatments with lower BRSamounts and lower residual concentrations of clomazonethan in those with higher BRS amounts and higher residualconcentrations of clomazone (e.g., S0–0.15 vs S1–0.60) illus-trates the capability of BRS in reducing clomazone availabilityto BYG.
These experiments show that BRS can significantly affectclomazone availability to BYG at typical application rates. Theresults suggest that burning rice stubble prior to fieldpreparation and agrochemical treatment may be a factorresponsible for lower availability of pesticides. The degree ofreduction in pesticide availability is related to the amount ofburned crop residues in soils and the applied concentration ofpesticides. At typical application rate of clomazone (1X=0.3 µgg−1), as low as 0.02% BRS in soil may cause an appreciablereduction in the availability of clomazone to BYG (Fig. 3). It hasbeen reported that burning rice straw under natural condi-tions produced ~16% (by weight) BRS (Yang and Sheng, 2003a).Using the average production of rice straw of 12,000 kg ha−1, asingle burning would result in ~1,920 kg ha−1 BRS and 0.091%(w/w) in soil assuming BRS is mixed with a furrow slice of soil(i.e., 0.15-m deep) with a density of 1.4 g cm−3. Burned cropresidues are expected to accumulate in soils due to repeatedburning and remain to be effective in adsorbing pesticidesover an extended period of time (Yang and Sheng, 2003b).Consequently, in a burned field situation, there is a highpotential for reduced availability of pesticides.
Burned crop residues are often left on soil surface for anextended period of time before agrochemical applicationbegins. Environmental factors such as wind or rain runoffcan cause burned crop residues to form pockets in low areas inthe field. Such pockets can easily raise the localized concen-tration of burned crop residues several-fold higher. In such asituation, theremay be patches of the field that show very low
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availability of pesticides. Although only BRS was tested for itsimpact on clomazone bioavailability, this study may havegreater implication than for burned rice fields. For example,vegetation and other crop residues such as wheat straw andsugarcane trash are commonly burned, and pesticides otherthan clomazone are expected to also be adsorbed. Propermanagement of crop residues alternative to field burning maythus reduce the environmental impact of pesticides inagricultural soils.
Acknowledgements
This work was supported by the National Natural Sciencefoundation of China Distinguished Young Scholars Program(No. 20688702) and the Program for Changjiang Scholars andInnovative Research Teams in Universities (No. IRT0653).
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