Assessment of compost and three biochars associated with Ailanthus altissima(Miller)Swingle for lead and arsenic stabilization in a post-mining Technosol

Ibrahim ALIDOU-ARZIKAManhattan LEBRUNFlorie MIARDRomain NANDILLONGülriz BAYÇUSylvain BOURGERIE and Domenico MORABITO

1University of Orléans,INRA USC1328,LBLGC EA1207,Orléans 45067(France)

2Division of Botany,Department of Biology,Faculty of Science,Istanbul University,Istanbul 34134(Turkey)

3Department of Bioscience and Territory,University of Molise,Pesche 86090(Italy)

(Received July 18,2019;revised December 27,2019)

ABSTRACT The elevated presence of metal(loid)s in the environment significantly impacts ecosystems and human health and is generally largely due to industrial and mining activities.Thus,in the current study,we investigated and proposed an environmentally friendly method(phytomanagement)aimed at reducing the negative impacts associated with metal(loid)pollution through the use of soil amendments(biochar and compost)to permit Ailanthus altissima growth on a highly contaminated mining Technosol,with arsenic(As)and lead(Pb)contents of 539.06 and 11 453 mg kg-1,respectively.The objective was to examine the impacts of three biochars and compost on i)the physicochemical characteristics of soil,ii)metal(loid)immobilization in soil,and iii)A.altissima growth.We revealed that the application of biochar as a soil amendment improved soil conditions by increasing soil electrical conductivity,pH,and water-holding capacity.Moreover,concomitantly,we observed a large reduction(99%)in Pb mobility and availability following application of the hardwood biochar in combination with compost(HBCP).Thus,this combined soil amendment was most effective in promoting A.altissima growth.In addition,the HBCP treatment prevented As translocation in the upper parts of plants,although soil pore water As concentration was not diminished by amendment application.

Key Words: amendment,metal(loid),mining soil,phytomanagement,soil pore water

*Corresponding author.E-mail:domenico.morabito@univ-orleans.fr.

INTRODUCTION

Metal(loid)s,including arsenic(As),cadmium(Cd),lead(Pb),mercury,nickel,and zinc(Zn)(Duruibeet al.,2007;Panagoset al.,2013),are natural constituents of the earth’s crust;they are found in the form of arsenic sulphide,lead sulphide, aluminium oxide, or manganese oxide. Despite their natural occurrence,metal(loid)s sometimes become a severe environmental pollutant due to several anthropogenic activities and are often found around mining areas(Duruibeet al.,2007).After mining processes,these metal pollutants are left behind as tailings or transported longer distances by water and wind erosion(Kabiret al.,2012).More than 10 million sites are contaminated worldwide(Heet al.,2015),of which 50% are contaminated by metal(loid)s (Khalidet al.,2017).Contamination of soil with such elements poses significant risks to both the environment and human health.For example, among these elements, As is a highly toxic element to all organisms and is a known carcinogen(Ahmadet al.,2015;Singhet al.,2015).Moreover,as an analogue of phosphate,this element can interfere with essential cellular processes in humans,as well as the chloroplast structure and photosynthesis process in plants(Aliet al.,2013;Mustafa and Komatsu, 2016). Additionally, Pb can be absorbed by the human body through respiration or dermal contact and subsequently passes into the blood stream,disrupting cellular metabolic pathways. Lead further interferes with enzyme activities and displaces essential metal ions from metalloenzymes(Sharma and Agrawal,2005).

Therefore,it is essential to remediate unvegetated soils contaminated with these pollutants by controlling metal(loid)fluxes in the environment,improving soil fertility,and stabilizing metal(loid)s in soil.To achieve this goal,phytomanagement,which involves improving soil characteristics in order to allow selected plants to grow in unfavourable conditions(Gerhardtet al.,2017),has been proposed as a viable solution(Aliet al.,2013).For instance,plants tolerant to soil pollution and having a fast growth with deep root systems and high biomass production have been proposed as efficient tools to reduce environmental risks(Tack and Meers,2010;Evangelouet al.,2015).However,in achieving remediation of soils contaminated with metal(loid)s, employing such a strategy must involve metal(loid) tolerant plants. Baker(1981)reported that certain plants developed diverse mechanisms to tolerate high metal(loid) concentrations in their environment,including:i)exclusion by restricting plant invasion by metal(loid)s;ii)inclusion by maintaining metal(loid)accumulation in shoots at a concentration near to that in the surrounding soil,or iii)bioaccumulation by concentrating metal(loid)s in shoots and roots at high levels(Bayçu,1995;Chibuike and Obiora,2014).Thus,barren areas revegetated by resistant and fast-growing plant species prevent the migration of contaminated soil particlesviawind or water erosion(Barceló and Poschenrieder,2003)and minimize the transfer of pollutants into the food chain(Collinset al.,2006).However, soil conditioning may be required for effective colonization of these plants in contaminated areas(Barceló and Poschenrieder,2003).

Therefore,soil amendments play an important role in the overall functioning of polluted soil as they improve its physical,biological,and chemical components,which are critical to soil fertility.Among the various remediation options for mining soils,the use of soil amendments is an environmentally friendly and cost-effectivein situchemical stabilization process,which can reduce the bioavailability of metal(loid)s and thus enable ecological restoration(Prasadet al.,2018).This strategy has been classified as “assisted phytostabilization”and is especially important for remediating soils in abandoned mine areas (Galendeet al., 2014). Further,the application of soil amendments significantly improves soil physico-chemical characteristics,nutrient supply,plant establishment,soil biological activity,and immobilization of excess metals(Galendeet al.,2014;Wanget al.,2017).

Biochar is produced by pyrolysis of organic materials(e.g.,forestry and agricultural residues,manures)and has emerged as an effective tool for reducing soil metal(loid)bioavailability(Luet al.,2017).According to Lianget al.(2016), biochar characteristics depend on several factors including feedstock particle size and type,as well as pyrolysis conditions.Additionally,high organic carbon(C)content is among the most important properties of biochar(Barrow,2012;Oket al.,2015),which may vary between 50 to 900 g kg-1depending on the feedstock and the manufacturing methods used(Allaire and Lange,2013).Further,most biochars have a micro-porous structure,large surface area,alkaline pH,active organic functional groups,high cation exchange capacity,and strong adsorption affinity(Paz-Ferreiroet al.,2014;Luet al.,2017;Tanet al.,2017).

Application of biochar as an amendment in soil has many benefits on soil properties:i)increasing soil pH,electrical conductivity(EC),dissolved organic carbon(DOC),organic matter(OM),and water-holding capacity(WHC)(Hossainet al.,2010;Masuliliet al.,2010;Jianget al.,2012;Nigussieet al.,2012;Markset al.,2014;Molnáret al.,2016);ii)reducing metal mobility and availability as well as concentration in soil pore water(SPW)(Jianget al.,2012;Houbenet al.,2013;Paz-Ferreiroet al.,2014;Houben and Sonnet,2015;Reeset al., 2015; Lomaglioet al., 2017; Luet al., 2017;Egeneet al.,2018);iii)increasing the diversity and activity of soil micro-organisms(Lehmannet al.,2011;Paz-Ferreiroet al.,2014);and iv)increasing C sequestration,which in turn further reduces greenhouse gas emissions(Verheijenet al.,2010;Paz-Ferreiroet al.,2014).In addition,biochar application can increase plant growth and reduce metal(loid)concentrations in plants(Liuet al.,2013;Gascó.,2019).

In addition to biochar,compost,a product of microbial degradation of organic waste,has proven to be an effective soil amendment as it is rich in humus, microorganisms,and both organic and inorganic components,thus providing nutrients for plants and improving soil fertility(Huanget al.,2016).Therefore,compost significantly benefits various soil properties by increasing nutrient content,soil organic carbon(SOC)content, WHC,and soil microbial activity(Brown and Cotton, 2011); immobilizing metals in contaminated soils through absorption reactions(Parket al.,2011b);and enhancing plant colonization and development (Madejónet al.,2006;Gondeket al.,2018).

Moreover,researchers have successfully combined soil amendments to further enhance soil properties and thus effectively remediate contaminated soils.For instance,Cárdenas-Aguiaret al.(2017)applied biochar and/or compost amendments to soil artificially contaminated with copper(Cu)and examined Cu immobilization, germination and biomass production of various plant species, and soil microbial biomass.They revealed that the addition of biochar,alone or combined with compost, successfully immobilized Cu and increased plant germination and biomass production.Moreover,these researchers recommended the combined application of biochar and compost as it was the only treatment that enhanced soil microbial biomass.

Plant species selection is an important parameter for remediation success of phytomanagement/phytostabilzation.For example, pioneer plants have evolved various adaptations to tolerate a multitude of stresses, inclusive of a tendency to accumulate low concentrations of pollutants in their aboveground tissues;thus,these plants have proven to be effective mediators in areas polluted with mine tailings due to their tolerance to toxic metal(loid)s. In particular,Ailanthus altissima(Miller)Swingle is an ornamental tree of the Simaroubaceae family,which easily grows on non-fertile soils and under poor soil conditions(Sladonjaet al.,2015).Specifically,this species can tolerate acidic,alkaline,saline,and arid soil conditions(Kowarik and Säumel,2007;Filippouet al.,2014).Thus,this pioneer plant species has been used previously for reclamation of degraded soils,especially for erosion prevention in arid regions given its tolerance to drought stress(Kowarik and Säumel,2007;Ranieri and Gikas, 2014). Moreover, it has a discernible tolerance to high levels of metal pollution(e.g.,Cd and Pb)in both soil and air(Bayçu and Önal,1993;Bayçu,1995;Gatti,2008;Huet al.,2014;Ranieri and Gikas,2014;Samuilovet al.,2014).

The aims of this study were to investigate the effects of biochar and/or compost amendments on:i)the physicochemical properties and metal(loid)mobility of soil from a former mine site and ii)A.altissimagrowth and accumulation of As and Pb.Thus,this study suggests the most effective applications of soil amendments towards developing an efficient phytomanagement strategy usingA.altissima.

MATERIALS AND METHODS

Site description

Technosol samples were collected from 0—20 cm depth at a disused(since 1897)silver-Pb mine extraction site located in Pontgibaud,France(45°49′59′′N,2°51′04′′E);this site was among the largest mining and metallurgical districts in Europe during the nineteenth century.The Technosol used in this study was a sandy acidic soil mainly contaminated by As and Pb,with average pseudo-total concentrations of 539.06 and 11 453 mg kg-1, respectively (Lebrunet al.,2017).These concentrations severely exceed the maximum permissible As and Pb limits in soil(20 and 300 mg kg-1,respectively)(Ashrafet al.,2019).

Biochars and compost used

Three biochars(0.2—0.4 mm granulometry)and a compost were used as soil amendments in the present study.The biochars were obtained from a private company (La Carbonerie,France)and corresponded to three different wood feedstocks,including hardwood(Quercus,Fagus,andCarpinusbiomass), lightwood(Betulabiomass), and pinewood(Pinusbiomass),which were slowly pyrolyzed at 500°C(3 h residence time and 2.5°C min-1heating rate).The compost was obtained from a commercial company(Scotts France SAS, France). Main characteristics of this compost were:OM,220 g kg-1;dry matter,380 g kg-1;total phosphorus,4 g P2O5kg-1; total potassium,5 g K2O kg-1; and total nitrogen(N),7 g kg-1(5 g kg-1organic N;C/N=15).

Following agitation in distilled water(1:7,solid:liquid)for 4 h, pH and EC of the biochars and compost were measured with a pH meter (FE20/EL20, Metler Toledo AG 2007,USA)and conductometer(CDM210,Radiometer analytical,France),respectively.Biochar and compost WHC was then determined according to Lebrunet al.(2018a)and calculated as follows:WHC(%)=(saturated weight-dried weight)/dried weight×100.

Biochar specific surface area, total pore volume, and mean pore diameters were determined by conducting Brunauer-Emmett-Teller(BET)measurements using a Belsorp Mini II(Micro Trace Bel,LMI,France).First,the samples were degassed under vacuum at 150°C for 4 h. Samples were subsequently heated from ambient temperature up to 100°C at 3°C min-1, maintained for 1 min at a 100°C isotherm, heated up to 150°C at 5°C min-1, and finally maintained for 4 h at a 150°C isotherm.The specific surface area was then calculated by the BET equation using the relative pressure(P/P0)interval of 0.05 ≤P/P0 ≤0.35 and a value of 16.2 Å2for the cross sectional area of molecular dinitrogen(N2).

Pot experiment

Soil from the former mining area was dried and sieved(2 mm). Eight experimental treatments were set up with 0.4-L pots (8.7 cm in diameter, 11.3 cm in height): nonamended Technosol (CK), Technosol amended with 5%(weight:weight)lightwood biochar(LB),Technosol amended with 5%lightwood biochar and 5%(weight:weight)compost(LBCP), Technosol amended with 5% hardwood biochar(HB),Technosol amended with 5%hardwood biochar and 5%compost(HBCP),Technosol amended with 5%pinewood biochar(PB),Technosol amended with 5%pinewood biochar and 5%compost(PBCP),and Technosol amended with 5%compost(CP).Doses were chosen based on a previous study(Lebrunet al., 2019). For each treatment, 13 pots were prepared:10 pots were vegetated with youngA.altissimaseedlings (46 d old), whereas the remaining three were unvegetated.Plants were watered daily to field capacity.

Sampling and analyses

pH, EC, and WHC of the solid phase in the different treatments at the beginning (T0) of the experiment were determined as described in a previous section. For pH,EC, and Pb and As concentrations of SPW, pore water samples were collected from six pots of each treatment(three vegetated and three unvegetated pots) both at the beginning and end(36 d later,T36)of the experiment using a soil moisture sampler(Rhizon®,model MOM,Rhizosphere Research Products,The Netherlands).pH and EC were determined as described in a previous section.Concentrations of Pb and As were measured using an inductively coupled plasma-optical emission spectrometer(ICP-OES)(ULTIMA 2,HORIBA,Labcompare,USA)after acidification according to Bartet al.(2016).

After 36-d growth,the plants were harvested.For dry weight determination, the upper parts (stems and leaves)were separated from the roots and dried in an oven at 60°C for 72 h. For determination of As and Pb concentrations,plant samples were ground using a shredder (Model A11 BS000, IKA, Germany), acid-digested at 180°C with a microwave oven (Multiwave ECO, Anton Paar, France).The concentrations of As and Pb were measured using an ICP-OES(ULTIMA 2,HORIBA,Labcompare,USA).

Statistical analyses

Data were statistically analyzed using R software(version 3.3.2)(R Development Core Team, 2009). Data normality and homogeneity of variances were calculated using Shapiro’s test and Bartlett’s test,respectively.Means were compared using Student’st-test for normal data and Wilcox test for non-normal data.For SPW physico-chemical parameters,time and plant effects were assessed using analysis of variance(ANOVA)followed by Tukey’s honestly significant difference(HSD)test(for normal data),or Kruskal-Wallis test followed by pairwise Wilcox test(for non-normal data)atP ＜0.05.

RESULTS

Properties of the amendments and Technosol

Biochars used in this experiment had an alkaline pH,and the measured values were significantly different among the three biochars(Table I).Similarly,EC and WHC significantly differed between the biochars,with pinewood biochar exhibiting the highest WHC(308%)and lightwood(194%)and hardwood(183%)biochars exhibiting the lowest.Among the soil amendments,compost had the lowest pH(7.4)and WHC(51%);moreover,the EC value of compost(801 μS cm-1) was lower than that of lightwood biochar (849 μS cm-1),but higher than those of pinewood(496 μS cm-1)and hardwood(432 μS cm-1)biochars.

The lightwood biochar had the highest specific surface area, whereas the hardwood and pinewood biochars had a specific area 5.7 and 11.8 times, respectively, less than that of lightwood biochar(Table I).Similarly,the total pore volume of lightwood biochar was 4.1-and 8.7-fold higher than those of hardwood and pinewood biochars, respectively.Conversely,the mean pore diameters of hardwood and pinewood biochars were higher than that of the lightwood biochar.

The initial pH of the Technosol was acidic with low EC and WHC of 24%(Table II).Biochar or compost application at 5%significantly and positively affected the physicochemical properties of the Technosol.Application of the lightwood,hardwood,and pinewood biochars and compost induced increases of 1.45,1.95,0.74 and 1.36,respectively,in pH unit.Moreover, the combined addition of 5%compost and 5%biochar to the Technosol induced a soil pH increase of 1.1 and 1.7 units for HBCP and PBCP,respectively,compared with treatments with only biochar added.

Soil EC in LB and HB was more than twice that in CK,whereas the EC in PB was not significantly different from that in CK (Table II). Compost alone (CP) did not affect EC,whereas the effects of combined application of compostand biochar on EC varied: a 18.0% decrease in LBCP as compared with LB, no effect in HBCP as compared with HB,and a 2.2-fold increase in PBCP as compared with PB.

TABLE I Physicochemical properties of the hardwood biochar,lightwood biochar,pinewood biochar,and compost used as soil amendments in this study

TABLE II Soil pH,electrical conductivity(EC),and water-holding capacity(WHC)in the different treatments at the beginning of the pot experiment with a highly contaminated mining Technosol

In PB, soil WHC increased by 16.4%, which was the greatest change obtained when the amendments were added alone(Table II).Moreover,the addition of 5%compost to PB(PBCP treatment)induced a supplemental 9.0%increase in WHC.Compost alone only yielded a 2%increase in WHC,whereas HB and LB yielded a 8.0% and 10.7% increase in WHC,respectively.The WHC in LBCP and HBCP was increased by approximately 1.3%as compared with that in LB and HB,respectively.

pH and EC of SPW

The application of soil amendments resulted in a 2.5-unit increase in SPW pH on average at the beginning of the experiment(T0)(Table III).Additionally,biochar addition increased SPW EC by 2.0-,2.1-,and 1.8-fold in LB,HB,and PB,respectively.Moreover,the addition of 5%compost to biochar treatments further increased EC compared with treatments containing only biochar(2.1-fold higher for LBCP,2.2-fold higher for PBCP,and 1.3-fold higher for HBCP).

According to results obtained at the end of the experiment(T36)for unvegetated pots(T36-V),only PB demonstrated a significant increase in SPW pH(0.83 units),whereas all treatments demonstrated significant increases in SPW EC(Table III). Regarding the plant effect (T36+V) on SPW properties,the pH of LB and HB was significantly affected(approximately 0.2-unit decrease).For CK and LBCP,although there was no difference in pH when compared with T36-V,there was a 1.3-unit increase and a 0.4-unit decrease in pH,respectively,compared with T0.Similarly,the EC in the vegetated pots of LB,LBCP,HB,PB,and CP decreased significantly compared with that in unvegetated pots at T36.

Concentrations of Pb and As in SPW

Concentration of Pb in SPW was significantly different between the treatments(Table IV).At T0,Pb concentration was highest in CK(48 mg L-1).Application of amendments alone or in combination decreased Pb concentration in SPW by 84%—99%.

The Pb concentration in SPW of CK decreased by approximately 70%in both the vegetated and unvegetated pots at T36 compared with that at T0(Table IV).Moreover,at T36 for the unvegetated pots,only PB demonstrated a significant decrease in SPW Pb concentration(0.64 mg L-1)as compared with T0,which approximately corresponded to the values obtained for the other two biochar alone treatments at T0.

At T0,biochar or compost,alone or in combination,did not significantly affect SPW As concentration(Table IV).At T36,however,all unvegetated treatments showed a significant decrease in As concentration as compared with T0,except for LBCP.Finally,As concentrations in SPW of the vegetated soils(T36+V)were significantly lower than those measured at T0 except for PBCP and CP.

Plant dry weight

In the treatments with application of biochar and/or compost,the dry weights of plant roots,stems,and leaves were significantly higher compared with those in CK(Fig.1).Biochar amendments increased the dry weight of leaves by 1.5-,2.4-,and 1.9-fold and that of roots by 3.9-,6.1-,and 4.7-fold for LB,HB,and PB,respectively,compared with CK. Among all treatments, compost alone (CP) resulted in the highest dry weight for roots and leaves. Moreover,combined application of biochar and compost showed no significant effect on root and stem dry weights compared withapplication of biochar alone. In contrast, the dry weights of leaves significantly increased in LBCP and HBCP; in these treatments,the dry weight of leaves was twice those in treatments without compost.Furthermore,it is important to note that the dry weight results of CP were close to those of HBCP.

TABLE III pH and electrical conductivity(EC)of the soil pore water in the different treatments at the beginning(T0)and end(T36)of the pot experiment with a highly contaminated mining Technosol,in which 10 pots were vegetated with Alianthus altissima(+V)and three pots were unvegetated(-V)for each treatment

TABLE IV Concentrations of Pb and As in soil pore water from different treatments at the beginning(T0)and end(T36)of the pot experiment with a highly contaminated mining Technosol,in which 10 pots were vegetated with Alianthus altissima(+V)and three pots were unvegetated(-V)for each treatment

Fig.1 Dry weights of the roots,stems,and leaves of Ailanthus altissima after 36 d of growth in the different treatments of the pot experiment with a highly contaminated mining Technosol.CK=control with no biochar or compost applied;LB=lightwood biochar was applied;LBCP=lightwood biochar and compost were applied;HB=hardwood biochar was applied;HBCP=hardwood biochar and compost were applied;PB=pinewood biochar was applied;PBCP=pinewood biochar and compost were applied;CP=compost was applied.Error bars are standard errors(n=5).Different letters for a plant part indicate significant differences between treatments(P ＜0.05).

Plant Pb and As concentrations

The highest Pb concentrations were always found in roots compared with stems and leaves(Fig.2).Moreover,all biochars(alone or in combination with compost)decreased Pb concentration in the roots and stems.Specifically,regard-

Fig.2 Concentrations of Pb in the roots,stems,and leaves of Ailanthus altissima after 36 d of growth in the different treatments of the pot experiment with a highly contaminated mining Technosol. CK = control with no biochar or compost applied;LB=lightwood biochar was applied;LBCP=lightwood biochar and compost were applied;HB=hardwood biochar was applied;HBCP=hardwood biochar and compost were applied;PB=pinewood biochar was applied;PBCP=pinewood biochar and compost were applied;CP=compost was applied.Error bars are standard errors(n =5).Different letters for a plant part indicate significant differences between treatments(P ＜0.05).

less of biochar type,root Pb concentration was decreased by approximately 78%compared with that in CK.In addition,when biochar was combined with compost,a 91%decrease occurred.Similarly,biochar alone or combined with compost decreased Pb concentration in stems. In contrast, biochar alone resulted in a 4.8-fold increase in leaf Pb concentration,whereas decreases in Pb occurred in the treatments with combined application of compost and biochar, but values were not significantly different compared with those in CK.Concentration of Pb in CP was 87%and 93%lower in the roots and stems,respectively,than that in CK and 3.1-time higher in leaves than that in CK.

The highest As concentrations were recorded in the roots compared with those recorded in stems and leaves(Fig.3).Compared with CK,only PBCP yielded a significant increase(2.9-fold)in root As concentration,whereas biochar or compost alone did not significantly affect root As concentration.Compared with CK,stem As concentration was 2.8,2.9,and 2.5 times lower in LB, LBCP, and HB, respectively. Further,stem As concentration was markedly reduced in HBCP(100 times lower than that in CK).Stem As concentration was 21.5-and 3.9-fold lower in PB and PBCP,respectively,than that in CK.Leaf As concentration in LB and HB was not significantly different from that in CK.However,leaf As concentration declined in LBCP and HBCP.Moreover,leaf As concentration was 5 times lower in PB and PBCP than in CK.Similarly,leaf As concentration was significantly lower in CP than in CK.

Fig. 3 Arsenic (As) concentrations in the roots, stems, and leaves of Ailanthus altissima after 36 d of growth in the different treatments of the pot experiment with a highly contaminated mining Technosol.CK=control with no biochar or compost applied;LB=lightwood biochar was applied; LBCP = lightwood biochar and compost were applied; HB =hardwood biochar was applied;HBCP=hardwood biochar and compost were applied; PB =pinewood biochar was applied; PBCP = pinewood biochar and compost were applied; CP = compost was applied. Error bars are standard errors(n=5).Different letters for a plant part indicate significant differences between treatments(P ＜0.05).

DISCUSSION

In this study,the addition of the different amendments(biochars and compost)significantly increased pH and EC in soil and SPW.Similar results were obtained following application of wood biochars to former mine soils contaminated with As and Pb(Lebrunet al.,2018a,b),as well as after addition of greenwaste compost to a Pb-and Cu-contaminated soil(Karamiet al.,2011).In the present study,pH increase was likely due to the alkaline nature of the utilized amendments,which permits the release of basic cations into the soil.Accordingly, it was previously reported that both biochar and compost can release base cations,and biochar can also release alkali salts and soluble carbonates which neutralize soil acidity(Lebrunet al.,2019).The variability in pH rise depends on the type of biochar and compost used(Mooreet al.,2018).Moreover,as demonstrated by Beesleyet al.(2010)and Liuet al.(2012),the application of both biochar and compost permits a higher increase in pH than when applied individually, likely due to an increased release of acidity-neutralizing compounds.

Coupled with pH rise,there was an increase in soil EC following biochar and compost application.Indeed,it is well known that pH plays a crucial role in the mobility of ions(Remon,2006).Higher soil pH,which was observed under the amended conditions,increases the dissolution of anion salts. Moreover, biochar can increase soil EC through the increase in the leaching of nutrients,leading to more ions in the solution and thus a higher EC(Lebrunet al.,2019).In addition,this increase in EC may be attributed to the high EC of the amendments(432—849 μS cm-1).

The SPW Pb concentration was effectively decreased by 83%—99%following the application of the amendments,likely due to the ability of biochar in reducing Pb bioavailability and mobility(Beesleyet al.,2011;Paz-Ferreiroet al.,2014).Further,this reduction in Pb bioavailability/mobility may be due to biochar’s ability to fix Pb through sorption on its surface(Beesleyet al.,2011).Effective immobilization of heavy metals in soil depends upon the type of biochar used.For example,Parket al.(2011a)applied two biochars to a Cd-,Cu-,and Pb-spiked soil and found that the chicken manure biochar was more effective than the greenwaste biochar in reducing the extractable fractions of metal(loid)s. This was further supported by Uchimiyaet al.(2012)and Karamiet al. (2011). In accordance with the results from these previous studies,we demonstrated that hardwood and lightwood biochars were more effective than pinewood biochar in immobilizing Pb. In addition, Karamiet al. (2011) attributed the ability of compost to sorb metal cations to its high levels of organic matter.Moreover,in the present study,the combined application of soil amendments yielded higher Pb immobilization than when amendments were applied individually.These results were likely associated with the pH increase, which was higher in the combined application treatments. Indeed, this phenomenon was previously described by Lebrunet al. (2018b); using four hardwood biochars on the same soil,these researchers reported that an increase in SPW pH was associated with a decrease in SPW Pb concentration.

Biochar or compost amendments did not immediately affect As concentration in SPW.However,regardless of treatment,As concentration decreased slightly with time.These results contradicted those of Beesleyet al.(2010)in that the application of hardwood biochar to a soil contaminated by As,Cd,Cu,Pb,and Zn increased As concentration in SPW.Moreover,although biochar is not particularly known for its ability to sorb As,it is possible that sorption of As on the surface of biochars occurrs through ion exchange and surface complexation,as demonstrated by Niaziet al.(2018).

In the present study, totalA. altissimadry weight increased by 2.2-, 1.8-, and 2.9-fold in PB, LB, and HB,respectively.Biochar enhancement of biomass production was greater when biochar was combined with compost.However,compost alone yielded the largest increase in dry biomass due to the supply of nutrients.When combined with biochar,the positive effect of compost was reduced except for combination with hardwood biochar.These differences are likely attributable to the nutrient sorption capacity of biochars(Yaoet al.,2012).Nonetheless,regardless of treatment,A. altissimabiomass was always enhanced. These results are in accordance with compost and biochar capacity of improving soil properties(e.g.,pH,EC,and WHC)and enhancing the activity of soil microorganisms,which play a role in the bioavailability of nutrients for plants(Brown and Cotton,2011;Agegnehuet al.,2016;de Sousa Limaet al.,2018).

Diminution of metal(loid)concentrations(As and Pb)inA. altissimaorgans was correlated with the reduction of associated ions in the SPW, which was induced by the application of soil amendments.Indeed,the present study revealed that the reduction in SPW Pb concentration was associated with a reduction of Pb concentration in stems and roots.Moreover,lower Pb concentrations in plants grown on the amended soil may be attributable to dilution effect given that the dry weights of plants grown on treated soils were higher than those of plants grown on non-amended soils.

CONCLUSIONS

Our results revealed many potential benefits of biochar or compost application on a Technosol.These soil amendments immobilized Pb,improved soil physicochemical properties by increasing pH and EC,and enhancedA.altissimabiomass production in a non-fertile soil contaminated with metal(loid)s. In contrast to previous studies, there was no mobilization of As,whereas Pb concentration was diminished in both SPW and the plant. Furthermore, according to our results, hardwood biochar combined with compost(HBCP)was the most effective soil amendment in promoting metal(loid)stabilization in soil and reducing the leaching of metal(loid)s into groundwater.Therefore,this soil amendment would generate favorable soil conditions and promote successful establishment ofA.altissima,which in turn would likely prevent wind erosion and thus reduce the negative environmental impacts associated with metal(loid)pollution.However,subsequent studies are required to further evaluate the positive effects associated with the application of this soil amendment,specifically regarding its ability to reduce wind erosion and water leaching.

ACKNOWLEDGEMENT

The authors thank the Bureau of Geological and Mining Research(BRGM),France for providing access to the Technosol, La Carbonerie (Léger J. C.) for providing the biochars,and the Turkish Scholarship Organization(YTB)and Erasmus Program for supporting IAA.