| Heavy metal contamination in soils has become one of the most serious global environmental issues in decades due to the continuous intensification of industrial production and other anthropogenic activities.With the implement of the Western Development Policy in China,an increasing number of factories were introduced into the Northwest arid and semi-arid area.However,due to the climate and rainfall factors,the habitat in the Northwest region is relatively vulnerable and susceptive to the massive input of heavy metal.Large metal accumulation in the soil of industrial parks and surrounding areas not only harms the local organisms,but also poses a great threat to the middle reaches of the Yellow River,given that heavy metal is very likely to transport into river through the surface runoff and groundwater circulation.Halophytes in Northwest China are able to adapt to multiple-stressed environments such as salinity and heavy metals.Different group of halophytes possess different strategies for salt tolerance and adapt to variable natural habitats.Therefore,it is of great significance to reveal the mechanisms of phytoremediation and resistance to heavy metals(e.g.,Cd)in halophytes with different salt-tolerances,which would contribute to exploring the remediation capability and application prospect of halophytes for the soil remediation in northwest industrial zone subjected to multiple pollutants.In the present study,Huinong Industrial District was selected as a typical Northwest saline industrial zone,and the pollution pattern of As,Cd,Cr,Cu,Pb,and Zn in soils and the potential for remediation of native halophytes were thoroughly investigated.Based on the survey results,Cd,the most polluted element,was chosen as a typical element and several soil pot experiments were conducted to examine the effects of salinity on the Cd distribution characteristics,tolerance mechanism and restoration effect in different types of native halophytes,namely the euhalophyte Suaeda glauca and the recretohalophyte Limonium aureum.The relationship between salt tolerance strategies and Cd tolerance characteristics of different species was also studied.The main results were as follows:(1)Results of the pollution pattern of As,Cd,Cr,Cu,Pb,and Zn in the soil of Huinong Industrial Park showed that the average Nemerow Pollution Index was 2.96,and the Potential Ecological Risk Index was 176.18,showing a moderate pollution level.The main pollution areas were north and southwest of the study area.Among six metals,Cd pollution was the most serious.The average soil Cd content exceeded the national standard value of class Ⅱ(pH > 7.5)by 3.92 times,indicating a high potential ecological risk.As was at the moderate pollution level,and the potential ecological risk was medium.The content of As exceeded the standard value by 2.77 times.Cr,Cu,Pb and Zn were lower than the respective standard values,among which Cr,Pb and Zn were slightly higher than the soil background values in Ningxia.(2)The spatial distribution of Cd and As exhibited a certain similarity,and both were found in high concentrations in the north and southwest parts of the study area,which was closely related to industrial production activities in the Huinong District.Most Zn was enriched in the southern part,which originated from agricultural activities such as the wide application of fertilizers and pesticides.Cr and Cu were mainly derived from natural parent materials,whereas Pb was mainly associated with mixed factors such as transportation and weathering.Human activities and land use types had significant effects on the accumulation and distribution of heavy metals in the soil.Among all types of land use,As and Cd had the highest content in the industrial land,which were 105.03 and 3.63 mg/kg,respectively.Pb had the highest content in unutilized land(97.86 mg/kg),followed by the industrial land(90.13 mg/kg).Except for Zn,the contents of other five metals in the industrial land were generally higher,while the heavy metals in the forest land were generally lower.Moreover,the contents of metals in the industrial land and water were relatively higher than those in other land use types,indicating a considerable risk of metal migration and enrichment to rivers and the groundwater.(3)The vegetation type in the study area was desert steppe vegetation,dominated by herbaceous species.Shrub and woody species were relatively rare.The main dominant families were Amaranthaceae,Asteraceae,and Poaceae.Cd bioaccumulation capacity of 13 native halophytes differed greatly.Cd absorption and root-to-shoot translocation by Suaeda glauca was the highest among all studied species.The average Cd content in roots and shoots of S.glauca was 0.40 and 0.30 mg/kg,respectively,with the bioconcentration factor(BCF)being 0.23,exhibiting great potential as a candidate species for subsequent remediation research.Except for S.glauca,the BCFs of the other species were between 0.05-0.13.The enrichment ability of native halophyte species to As,Pb,and Zn was lower.Most species had the BCFs below 0.1.As content in the roots of Chenopodium glaucum was the highest,followed by Phragmites communis and Salsola collina.Pb content in Helianthus annuus and S.collina was higher than in other species.The bioenrichment capacity of Cr and Cu in all studied native halophyte species was relatively high,ranging from 0.2-0.8 of BCFs.The BCFs of S.glauca,P.communis,and H.annuus were higher in all native halophyte species.(4)The results of Cd accumulation characteristics in different native halophytes showed that Cd significantly inhibited the growth and biomass accumulation of all species,and salt promoted the root and shoot biomass of the euhalophyte Suaeda glauca plants under low Cd level,which was 1.65 and 1.31 times of the corresponding value in LCd group(P < 0.05).The biomass of other three species was significantly inhibited by salinity.Salt tolerance of different types of native halophytes was in the order of: euhalophyte > recretohalophyte > pseudohalophyte.During the treatment period,the pseudohalophyte Sophora alopecuroides and the glycophyte Medicago sativa died after 48 days of salt addition,indicating that these two types are not suitable for the phytoremediation of Cdcontaminated saline soils in the Northwest China.NaCl addition significantly increased the Cd content of all studied species,whilst the total Cd accumulation in the shoot of S.alopecuroides and M.sativa was less affected by the treatments,and it tended to decrease under saline conditions.Under low and high Cd levels,salt-imposed increase of BCF in Suaeda glauca was 4.65 and 4.13 times of the corresponding value in non-saline treatment group.In addition,the translocation factor(TF)of Cd in S.glauca and Limonium aureum was significantly improved by salinity.Compared to S.alopecuroides and M.sativa,the total shoot Cd accumulation of S.glauca and L.aureum reached up to 32.9 μg and 6.6 μg under salt conditions.The shoot Cd accumulation of S.glauca at both Cd levels and that of L.aureum at high Cd level were greatly promoted by NaCl.(5)Both S.glauca and L.aureum plants exhibited certain tolerance to Cd,but NaCl had differing effects on their physiological response under Cd stress.For the euhalophyte S.glauca,Cd toxicity was profoundly mitigated by 0.3% NaCl through the alleviation of photosynthesis and stimulation of antioxidant enzymes.Except that the CAT activity was not substantially increased in the HCd + S group,NaCl dramatically promoted the enzyme activity of S.glauca leaves at both Cd levels.In contrast,NaCl further inhibited the growth and photosynthesis of L.aureum under Cd stress,showing an additive effect of Cd and NaCl stress.Despite this,NaCl stimulated antioxidant enzyme activity and osmolytes(such as proline and soluble protein)accumulation in leaves of L.aureum.At each Cd level,NaCl-induced proline content was 7.86,4.28 and 5.23 times higher than that in the absence of salt.Moreover,NaCl-induced Na accumulation in leaves of the two species might be used as a cheap osmolyte for rapid response and regulation of cell osmotic pressure.The results of this study indicate that salt-imposed distinct strategies are employed by these two halophytes to cope with Cd exposure.(6)There were significant differences in the distribution and accumulation of Cd in S.glauca and L.aureum.Salinity significantly increased the uptake and transport of Cd in the two species by reducing the pH and by increasing the bioavailable Cd of the rhizosphere soil.At high Cd levels,the bioavailable Cd in the rhizosphere of S.glauca and L.aureum was significantly increased by NaCl,which was 1.25 and 1.22 times than that of the HCd group,respectively.In addition,the root Cd uptake efficiency of the two species was greatly increased by NaCl,and the Cd uptake efficiency of S.glauca was much higher than that of L.aureum.The salt adaptative strategies of two halophytes(succulence or excretion)contributed enormously to Cd detoxification and were boosted by NaCl.In both halophytes,a majority of Cd was distributed in the cell wall fraction(36–69%),followed by the soluble(23–59%)and organelle fractions(3–17%).Higher Cd concentrations in all fractions were found in roots and leaves of S.glauca than in that of L.aureum.For the tissues of both species,the NaCl extractable Cd(39–74%)was predominant,followed by the HAc extractable(11–31%)and water-soluble Cd(8–21%).Moreover,as compared with L.aureum,S.glauca plants possessed higher proportions of Cd form extracted by 2% HAc and 0.6 M HCl(P = 0.034)and lower proportions of Cd form extracted by 80% ethanol and deionized water(P < 0.001).Therefore,the euhalophyte,S.glauca,is proved more capable of Cd enrichment.In conclusion,for the most severely polluted element,Cd,the euhalophyte and recretohalophyte are both fairly promising candidates for phytoremediation.By contrast,the euhalophyte is more applicable for the phytoremediation of saline soil in the industrial zone of Northwest China due to its impressive tolerance to and the accumulation capacity of Cd. |
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