Pedosphere 26(5): 717–724, 2016 doi:10.1016/S1002-0160(15)60082-0 ISSN 1002-0160/CN 32-1315/P c 2016 Soil Science Society of China ⃝ Published by Elsevier B.V. and Science Press
Eﬀects of Double Harvesting on Heavy Metal Uptake by Six Forage Species and the Potential for Phytoextraction in Field LI Ningyu1 , GUO Bin1 , LI Hua1 , FU Qinglin1 , FENG Renwei2 and DING Yongzhen2,∗ 1 Institute
of Soil and Fertilizer, Zhejiang Academy of Agricultural Sciences, Hangzhou 310021 (China) ProtectionInstitute, Ministry of Agriculture, Tianjin 300191 (China)
(Received January 25, 2015; revised June 17, 2016)
ABSTRACT The pollution of soils by heavy metals has dramatically increased in recent decades. Phytoextraction is a technology that extracts elements from polluted soils using hyperaccumulator plants. The selection of appropriate plant materials is an important factor for successful phytoextraction in ﬁeld. A ﬁeld study was conducted to compare the eﬃciency of six high-biomass forage species in their phytoextraction of heavy metals (Cd, Pb, and Zn) from contaminated soil under two harvesting strategies (double harvesting or single harvesting). Among the tested plants, amaranth accumulated the greatest amounts of Cd and Zn, whereas Rumex K-1 had the highest amount of Pb in the shoot under both double and single harvesting. Furthermore, double harvesting signiﬁcantly increased the shoot biomass of amaranth, sweet sorghum and sudangrass and resulted in higher heavy metal contents in the shoot. Under double harvesting, the total amounts of extracted Cd, Pb and Zn (i.e., in the ﬁrst plus second crops) for amaranth were 945, 2 650 and 12 400 g ha−1 , respectively, the highest recorded among the six plant species. The present results indicate that amaranth has great potential for the phytoextraction of Cd from contaminated soils. In addition, the double harvesting method is likely to increase phytoextraction eﬃciency in practice. Key Words:
amaranth, harvesting strategy, high-biomass, hyperaccumulator plant, soil pollution
Citation: Li N Y, Guo B, Li H, Fu Q L, Feng R W, Ding Y Z. 2016. Eﬀects of double harvesting on heavy metal uptake by six forage species and the potential for phytoextraction in ﬁeld. Pedosphere. 26(5): 717–724.
INTRODUCTION The pollution of soils by heavy metals has dramatically increased in recent decades due to the discharge of waste and wastewater from anthropogenic sources (Pacwa-Plociniczak et al., 2011). Phytoextraction is a technology that extracts elements from polluted or mineralized soils by using hyperaccumulator plants, which accumulate pollutants/contaminants in harvestable organs and tissues that can then be removed from the ﬁeld (Rascio and Navari-Izzo, 2011; Andreazza et al., 2013). This technique oﬀers the beneﬁts of being in situ, low cost, and environmentally sustainable (Sharma et al., 2007). However, ﬁeld trials or commercial operations have successfully demonstrated that the phytoremediation of metals are limited (Maxted et al., 2007). Only Alyssum species, which are hyperaccumulators of Ni, have been developed into a commercial-scale phytoremediation technology (Chaney et al., 2007). The main limiting factor in the application of this technology is the low remediation eﬃciency of hyperaccumulators due to limited element ∗ Corresponding
author. E-mail: [email protected]
accumulation in the shoot and restricted biomass production (Sarma, 2011). The selection of appropriate plant materials is an important factor for successful phytoextraction in the ﬁeld. Forage species may be good candidates under ﬁeld conditions because of their potentially higher biomass, adaptability to speciﬁc environmental conditions, abundant seed production, deep-rooting habit, ease of cultivation and suitability for repeated cropping, which make them superior to many of the other currently known hyperaccumulators (Zhang et al., 2010). Extensive research has been done on the heavy metal uptake of high-biomass forage species, such as alfalfa (Medicago sativa), vetiver (Vetiveria zizanioides) and tall fescue (Festuca arundinacea) (Chen et al., 2004; Begonia et al., 2005; Zaefarian et al., 2013). Furthermore, their extraction capacity may be enhanced if suitable strategies, based on good agronomic practices and management, are adopted (McGrath et al., 2006). To enhance the eﬃciency of phytoextraction, chelates and organic acids, such as ethylenediaminetetra-
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acetic acid (EDTA), ethylenediaminedisuccinates (EDDs) and citric acid, have been added to contaminated soils to accelerate the uptake of heavy metals by plants (Leˇstan et al., 2008; Almaroai et al., 2012). However, the excessive addition of chelating agents in the ﬁeld may result in secondary pollution of soils, and the leaching of chelating agents may cause groundwater contamination and increase the cost of phytoextraction (Almaroai et al., 2012). Additionally, in studies involving Salix, Thlaspi and Arabidopsis, chemical treatments have not signiﬁcantly increased the uptake of Cd, and the application of EDTA has decreased both biomass yield and shoot Cd concentration (McGrath et al., 2006). The use of cultural cropping techniques may be an ideal method for strengthening phytoextraction by increasing plant biomass (Gonzaga et al., 2008; Wei et al., 2008). Repeated harvests are the most common method used to promote plant growth and obtain higher biomass (Hamlin and Barker, 2006). Ji et al. (2011) showed that double harvesting signiﬁcantly enhanced shoot biomass and Cd extraction by Solanum nigrum in Cd-contaminated agricultural soils. In this study, six forage species were used: chicory (Cichorium intybus), amaranth (Amaranthus hypochondriacus), Rumex K-1 (Rumex patientia × R. tianschanicus), alfalfa (M. sativa), sweet sorghum (Sofrghum bicolor) and sudangrass (Sorghum sudanese). The main objective of this research was to compare the eﬃciency of the six forage species in their phytoextraction of Pb, Cd and Zn from contaminated soil under diﬀerent harvesting strategies (double harvesting or single harvesting). MATERIALS AND METHODS Site description A ﬁeld experiment was performed in Danshan Township, Shangyu City, Zhejiang Province, China (29◦ 59′ 54.08′′ N, 120◦ 46′ 48.47′′ E). The climate at the site is subtropical monsoon with an average annual temperature of 15.0–18.0 ◦ C and annual rainfall of 1 400 mm. The soil contamination has been caused by longterm irrigation with local heavy metal-contaminated water. Soil physicochemical properties were measured
at the beginning of the experiment (before planting), and selected soil properties and metal concentrations are shown in Table I. The Zn content is 1.42 times the corresponding background content in the soils of Zhejiang Province, and the contents of Cd and Pb are signiﬁcantly higher than the maximum heavy metal amounts (Cd: 0.4 mg kg−1 ; Pb: 50 mg kg−1 ) allowed under the China Environmental Quality Standard for Soils (GB15618-2008, Grade II). Experimental design The experiment used a six (forage species) × two (harvesting strategies) factorial design arranged in a randomized complete block, split plot design with four replicates. The main plot and subplot factors were forage species and cultivation method, respectively. The six forage species were chicory, amaranth, Rumex K-1, alfalfa, sweet sorghum and sudangrass, which are commonly grown for forage in southern China. The cultivation methods used were double harvesting, in which the ﬁrst crop was harvested 60 d after sowing and the second crop was harvested 120 d after sowing, and single harvesting, in which harvesting occurred 120 d after sowing. The seeds of all crops were sterilized in 2% (volume:volume) hydrogen peroxide for 10 min, washed several times with tap water and soaked in water overnight. The soaked seeds were sown directly in the designated ﬁeld plots on May 8, 2011. The main plot areas were trimmed to 2.0 m × 2.0 m, and seeding density was 25 plants m−2 . To prevent emergence failure, more seeds were sown in each hole than the number of plants required for the experiment, and 10 d after sowing, the seedlings were thinned to one plant per hole. Prior to sowing, the soil was fertilized with N:P:K (1:1:1) fertilizer, which was applied at the rate of 75 kg N ha−1 . During the experimental period, the ﬁeld plots were watered, and weeds were removed manually when necessary; no pesticides were used. Under double harvesting, 50 plants (2 m2 ) were mowed by hand in the ﬁrst crop to a stubble height of 1.0 m for the three tall crops (sweet sorghum, sudangrass and amaranth) and to 0.25 m for the three lowgrowing crops (Rumex K-1, chicory and alfalfa) 60 d
TABLE I Selected chemical properties of the soil in the study ﬁeld pH
Total Organic matter
Extractable N g kg−1 1.26
mg kg−1 96.74 1.02
DOUBLE HARVEST AND PHYTOEXTRACTION OF HEAVY METAL
after sowing. For the second crop, the whole plants of the same 50 plants (2 m2 ) were harvested by hand 120 d after sowing. Under single harvesting, the 50 whole plants were harvested by hand (2 m2 ) 120 d after sowing. Sample pretreatment and chemical analysis Plants from each subplot were rinsed with tap water to remove surface dirt, carefully washed with deionized water, and separated into roots and shoots. The fresh plant materials were weighed and oven-dried at 105 ◦ C for 30 min and then at 75 ◦ C to a constant weight. For each subplot, the ratio of the sample fresh weight to dry weight was determined, and the dried samples were homogenized in preparation for Cd, Pb, and Zn analyses. Soil samples were acid-digested with a microwave oven (CEM, MARSXpress) according to EPA Method 3051 (USEPA, 1994). After mineralization, the samples were ﬁltered and diluted. The total contents of Cd, Pb and Zn in the soil were determined with an atomic absorption spectrophotometer (AAS-800, PerkinElmer, USA). The extractable Cd, Pb and Zn in the soil were extracted with diethylenetriaminepenta acetic acid (DTPA). The quality of the plants under single harvesting was assessed by routinely analyzing plant standard reference materials and including blanks in digestion batches. Data are presented as the means of four replicates together with the standard errors. Phytoextraction eﬃciency
heavy metal concentration in soil; and Msoil is the soil mass. Statistical analysis Statistical analysis was performed using SPSS 13.0 statistical software (SPSS Inc., Chicago, USA). Data were analyzed by one-way analysis of variance with Fisher’s least signiﬁcant diﬀerence (LSD) test to determine whether any diﬀerences between treatments were signiﬁcant at P < 0.05, and a one-tailed t-test was used to assess the signiﬁcance of the diﬀerences between double and single harvesting. RESULTS AND DISCUSSION Biomass production During the entire experimental period, the six forage species grew well and showed no visible symptoms of heavy metal toxicity. Signiﬁcant diﬀerences in shoot biomass between single and double harvesting were observed for each of the six species (Fig. 1). Shoot biomass was signiﬁcantly (P < 0.05) higher under double harvesting compared with single harvesting. The highest average total biomass under single harvesting was observed in amaranth followed by sweet sorghum and sudangrass (46.2, 37.6 and 36.1 t ha−1 ). Double harvesting enhanced the biomass yields of all the tested plants. For example, at the time of ﬁrst harvest, the shoot biomass was 20.0, 14.1 and 12.0 t ha−1 for amaranth, sweet sorghum and sudangrass, respectively, whereas at the ﬁnal harvest, an additional 51.4, 41.0
The bioconcentration factor (BF) was deﬁned as the concentration of heavy metal element in shoot divided by the concentration in soil and can be used to evaluate the element-accumulating capacity of plants (Audet and Charest, 2007). The phytoextraction rate per year (PR, %) and phytoremediation time (PT, year) of the plants were calculated with the following equations. It was assumed that the forage species may be cultivated three times each year and that metal pollution only occurs in the active rooting zone, namely the uppermost 20 cm of the soil layer, giving a total soil mass of 2 600 t ha−1 (assuming a soil bulk density of 1.3 t m−3 ). PR = PI =
Cshoot × Mshoot × 3 × 100 Csoil × Msoil Csoil × Msoil Cshoot × Mshoot × 3
where Cshoot is the heavy metal concentration in plant shoot; Mshoot is the plant shoot biomass; Csoil is the
Fig. 1 Total shoot biomass (dry weight) of six forage species grown in the contaminated ﬁeld for 120 d under two harvesting strategies (double harvesting and single harvesting). Vertical bars indicate standard errors of the means (n = 4). The asterisk (*) indicates signiﬁcant diﬀerence between double and single harvesting for each species at P < 0.05 by one-tailed t-test.
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and 37.9 t ha−1 of the three species were produced. The total shoot biomass obtained with double harvesting for amaranth, sweet sorghum, sudangrass, chicory, rumex K-1 and alfalfa was 71.4, 55.1, 49.9, 14.5, 4.1 and 6.6 t ha−1 , respectively. The obtained shoot biomass of the six species was 54.5%, 46.5%, 38.2%, 39.7%, 27.9% and 20.6% higher, respectively, with double harvesting compared with single harvesting.
mg kg−1 at the ﬁrst and second harvests, respectively, under double harvesting and 123 mg kg−1 under single harvesting, which were signiﬁcantly higher (P < 0.05) than those of the other ﬁve species (Table II). The shoot Pb concentrations in sweet sorghum were similar to those of sudangrass and ranged from 40 to 52 mg kg−1 in the two species. The root Pb concentration in the two species ranged between 241 and 260 mg kg−1 . Shoot Zn concentrations ranged between 37 and 176 mg kg−1 , with the lowest concentrations observed in sudangrass and the highest in amaranth. The shoot Zn concentrations in amaranth at the ﬁrst and second harvests were 170 and 176 mg kg−1 , respectively, under double harvesting (Table II), which was not signiﬁcantly diﬀerent from that recorded under single harvesting (171 mg kg−1 ).
Metal accumulation in plant tissues The concentrations of Cd, Pb, and Zn in shoots and roots of the six forage species are presented in Table II. In general, amaranth contained the highest Cd and Zn concentrations, and Rumex K-1 accumulated the highest Pb concentration among the tested species. The Cd concentrations in shoot ranged from 0.2 to 13.1 mg kg−1 and were the highest in amaranth and the lowest in sweet sorghum. The shoot Cd concentrations in amaranth in the ﬁrst and second harvests were 13.1 and 13.0 mg kg−1 , respectively, under double harvesting and 11.9 mg kg−1 under single harvesting. The second highest Cd concentrations in shoot were recorded in chicory, being 2.7 and 2.8 mg kg−1 in the ﬁrst and second harvests, respectively, under double harvesting and 2.1 mg kg−1 under single harvesting. The highest Pb concentrations in shoot were observed in Rumex K-1, with the values of 111 and 124
Removal of heavy metals Among the six forage species, amaranth showed the highest total uptake of Cd and Zn from the metalcontaminated soil (Fig. 2). Under double harvesting, total uptake of heavy metals was 0.95 (for Cd), 2.66 (for Pd) and 12.40 kg ha−1 (for Zn) by amaranth and was 0.02 (for Cd), 2.63 (for Pd) and 2.07 kg ha−1 (for Zn) by sweet sorghum (Fig. 2). Compared with single harvesting, double harvesting signiﬁcantly (P < 0.05) increased Cd, Pb and Zn removal by amaranth, sudan-
TABLE II Concentrations of Cd, Pb and Zn in diﬀerent tissues of six forage species grown in the contaminated ﬁeld for 120 d under two harvesting strategies (double harvesting and single harvesting) Species
a) Means b) Means
Single harvesting 1st harvest 2nd harvest Single harvesting 1st harvest 2nd harvest Single harvesting 1st harvest 2nd harvest Single harvesting 1st harvest 2nd harvest Single harvesting 1st harvest 2nd harvest Single harvesting 1st harvest 2nd harvest
2.1 2.7 2.8 11.9 13.1 13.0 0.9 0.8 1.5 0.6 0.8 0.4 0.3 0.2 0.5 0.8 1.0 1.2
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
Root 0.3a) bb) 0.5a 0.1a 1.2a 1.5a 0.9a 0.05b 0.06b 0.13a 0.08b 0.05a 0.04c 0.02ab 0.01b 0.03a 0.05b 0.09ab 0.07a
1.6 – 2.8 2.6 – 3.8 1.1 – 1.8 1.3 – 1.1 3.4 – 3.9 2.2 – 1.1
± 0.5 ± 0.1 ± 0.1 ± 0.2 ± 0.2 ± 0.04 ± 0.06 ± 0.04 ± 0.04 ± 0.8 ± 0.6 ± 0.5
Shoot 80 88 83 36 41 36 123 111 124 56 64 46 43 42 50 46 52 40
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
Root mg 5.3a 4.8a 3.2a 2.4a 5.5a 1.9a 8.1a 7.8b 9.9a 3.5a 3.8a 3.4a 3.6a 8.1a 4.2a 1.2a 3.7a 2.8a
kg−1 156 – 163 311 – 224 60 – 76 114 – 132 260 – 241 245 – 259
± 2.4 ± 7.2 ± 11.7 ± 14.2 ± 3.2 ± 2.5 ± 4.8 ± 8.7 ± 11.1 ± 10.5 ± 17.1 ± 10.8
70 72 73 171 170 176 74 74 81 46 47 43 37 40 37 38 40 39
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
1.2a 3.1a 2.8a 4.9a 7.9a 9.4a 5.3b 5.1b 4.8a 1.2a 2.0a 5.2a 1.3a 1.7a 0.9a 0.7a 1.5a 1.6a
± standard errors (n = 4). followed by the same letter(s) within each column for a given species are not signiﬁcantly diﬀerent at P < 0.05.
56 – 48 68 – 88 59 – 70 41 – 50 77 – 72 101 – 84
± 11.2 ± 10.6 ± 3.9 ± 5.1 ± 5.4 ± 4.9 ± 3.8 ± 5.1 ± 7.1 ± 5.9 ± 7.8 ± 6.9
DOUBLE HARVEST AND PHYTOEXTRACTION OF HEAVY METAL
Fig. 2 Uptake of Cd, Pb and Zn in the shoots of six forage species grown in the contaminated ﬁeld for 120 d under two harvesting strategies (double harvesting and single harvesting). Vertical bars indicate standard errors of the means (n = 4). The asterisk (*) indicates signiﬁcant diﬀerence between double and single harvesting for each species at P < 0.05 by one-tailed t-test.
grass and sweet sorghum. For example, total Cd, Pb and Zn uptake by amaranth was increased by 0.8, 0.5 and 0.6 times, respectively, under double harvesting compared with that under single harvesting. The ability of a plant to absorb and accumulate heavy metals is not only aﬀected by its genotype but also impacted by rhizospheric microﬂora, soil physical and chemical properties, and bioavailability of heavy metals in soil (Sharma et al., 2007; Saier and Trevors, 2010). Eﬀective phytoextraction requires that plants accumulate large amounts of contaminants in their aboveground biomass; hyperaccumulators possess this
capability, but such species typically produce so little biomass that their extraction eﬃciency is limited (F¨assler et al., 2010). The use of high-yielding crop species may overcome the limitation of low biomass of hyperaccumulators. A signiﬁcant increase in plant biomass can result in increased accumulation of heavy metals so that ecologically signiﬁcant quantities of metals can be removed from soil. Compared with agricultural crops, such as maize (L´opez-Chuken et al., 2012), sunﬂower (Madej´on et al., 2003) and barley (Soriano et al., 2003), forage species extract Cd, Pb and Zn from contaminated soil with higher eﬃciency. This approach has been applied using Pennisetum americanum × P. purpureum (Zhang et al., 2010), A. hypochondriacus (Li et al., 2012) and S. bicolor (Zhuang et al., 2009). The six plant species tested in this study are highstalk species that are cultivated widely in China for forage. The heavy metal-accumulating ability of the six species was evaluated by determining the concentrations of heavy metals accumulated in plant tissues, the quantities of metals extracted from soil and the bioconcentration factor (BF). Based on these data (Table III), amaranth is considered as a more eﬃcient species for uptake of Cd and Zn, as it showed extraordinarily high BF values for Cd (2.56 under single harvesting and 2.98 under double harvesting) and for Zn (1.79 and 1.80, respectively). Chicory also showed relatively high BF values for Cd; although the extracted amounts were small due to low biomass (Fig. 1), it may be regarded as one candidate species. In contrast, sweet sorghum showed quite low BF values for Pb, whereas its shoot biomass was very high, so the total Pb accumulation in shoot was high. For both double and single harvesting, amaranth extracted the largest quantity of heavy metals among the six species tested (Fig. 2). The shoot Cd and Zn concentrations in amaranth did not meet the criteria for hyperaccumulation of Cd or Zn (> 100 mg kg−1 for Cd or 10 000 mg kg−1 for Zn) (Table II), but its high biomass compensated for the moderate heavy metal concentrations. Consequently, the amaranth shoot extracted 945 and 12 400 g ha−1 of Cd and Zn, respectively, signiﬁcantly higher than those extracted by the other ﬁve species (Fig. 2). These amounts were 4.6 times higher than those found for the Cd-hyperaccumulator Viola baoshanensis (Zhuang et al., 2007) and approximately equal to those for the Cd-hyperaccumulator S. nigrum (Ji et al., 2011). Variation in phytoextraction eﬃciency between cultivation methods For eﬃcient phytoextraction, plants must grow vi-
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TABLE III Bioconcentration factor, phytoextraction rate and phytoremediation time needed for six forage species grown in the contaminated ﬁeld for 120 d under two harvesting strategies (double harvesting and single harvesting) Species
Chicory Amaranth Rumex K-1 Alfalfa Sweet sorghum Sudangrass
Single harvesting Double harvesting Single harvesting Double harvesting Single harvesting Double harvesting Single harvesting Double harvesting Single harvesting Double harvesting Single harvesting Double harvesting
Phytoremediation time needed
0.55 1.01 13.65 24.22 0.09 0.17 0.08 0.08 0.30 0.59 0.79 1.29
% 0.01 0.02 0.03 0.04 0.01 0.01 0.00 0.01 0.03 0.04 0.03 0.03
0.46 0.58 2.56 2.98 0.19 0.25 0.12 0.13 0.07 0.08 0.19 0.22
Pb 0.11 0.12 0.05 0.05 0.17 0.16 0.08 0.08 0.06 0.06 0.06 0.06
Zn 0.73 0.76 1.79 1.80 0.79 0.81 0.48 0.46 0.39 0.40 0.40 0.41
gorously, which is diﬃcult in polluted soils. To address this problem, plants will have to be grown on polluted soil for several generations and be managed to maximize both biomass yield and heavy metal accumulation. For these reasons, adoption of advanced agricultural technologies may present a shortcut to application of phytoextraction at a large, commercial scale (Wei et al., 2008). To obtain the highest biomass, repeated harvesting is a common and eﬀective technique (Nakano et al., 2009). The present ﬁeld experiment indicated that double harvesting enhanced the phytoextraction eﬃciency of the six plant species by increasing total biomass yield, which is consistent with previous work on S. nigrum by Pei et al. (2007) and Ji et al. (2011). In our ﬁeld experiment, plant shoot biomass ranged from 4.1 to 71.4 t ha−1 (Fig. 1), and biomass production by amaranth, sweet sorghum and sudangrass was the highest among the six species tested. As illustrated in Fig. 1, the dry shoot biomass of these three species obtained with double harvesting was 25.2, 17.5 and 13.8, respectively, higher than that achieved with single harvesting. The ﬁeld experiment further suggested that the biomass production of non-accumulating forage species was notably higher than that of hyperaccumulators (V. baoshanensis and Sedum alfredii with 6.5 and 5.5 t ha−1 , respectively) (Zhuang et al., 2007) and much higher than that of agricultural crops, such as sunﬂower and maize (F¨assler et al., 2010; Meers et al., 2010). Signiﬁcantly higher plant biomass can compensate for relatively low capacity for heavy metal accumulation and result in plant accumulation and removal from the soil of large quantities of heavy metals. The results from the present study showed that
Zn 0.86 1.26 9.45 14.84 0.37 0.49 0.30 0.33 1.66 2.47 1.64 2.33
140.8 77.1 5.7 3.2 867.3 468.1 1 002.8 982.2 258.4 131.9 84.4 60.4
Pb years 647.6 440.5 319.1 202.7 1 060.1 856.7 1 744.3 1 648.7 332.9 205.0 321.4 249.1
no signiﬁcant diﬀerences were observed in heavy metal concentrations in most plants between the ﬁrst and second harvests under double harvesting, and the concentrations were slightly higher than those recorded under single harvesting (Table II). Based on the total biomass obtained and the heavy metal concentrations in the shoot, the total Cd extracted from the soil under double harvesting was signiﬁcantly higher (P < 0.05) than that under single harvesting. For example, the total Cd extracted by amaranth was 0.95 kg ha−1 under double harvesting and 0.53 g ha−1 under single harvesting (Fig. 2). Therefore, double harvesting allows plants to increase their biomass yield to enhance the removal of pollutants, which is consistent with the ﬁndings of Aravindhakshan et al. (2011). Thus, double harvesting allows for improved plant regrowth and enhanced biomass and thereby improves phytoextraction eﬃciency. Potential for phytoremediation of contaminated soil The eﬃciency of phytoremediation depends on four variables: plant biomass, plant metal concentration, soil metal concentration, and soil mass in the root zone (Andreazza et al., 2013). The phytoremediation potential of the six forage species was evaluated by determining the phytoextraction rate (PR) and phytoremediation time (PT). On the basis of the present experimental data (Table III), amaranth showed the highest Cd uptake eﬃciency with extraordinarily high PR values (13.65% under single harvesting and 24.22% under double harvesting). Cultivation of amaranth for 3.2 years by double harvesting and for 5.7 years by single harvesting can reduce soil Cd level from the initial concentration of 4.5 to 1.0 mg kg−1 . In addition, there
DOUBLE HARVEST AND PHYTOEXTRACTION OF HEAVY METAL
are many reports that show the great potential for phytoextraction of Cd from contaminated soil using amaranth. In a monoculture pot experiment, A. hypochondriacus accumulated more than 100 mg kg−1 Cd in shoots from the soil containing 5 mg kg−1 Cd (Li et al., 2012). In another experiment, when intercropped with maize, A. hypochondriacus accumulated over 50 mg kg−1 Cd in shoots and over 90 mg kg−1 Cd in roots from the soil containing 3 mg kg−1 Cd (Li et al., 2009). Chunilall et al. (2005) and Fan and Zhou (2009) reported that many cultivated amaranthaceous plants are potentially useful candidates for the phytoextraction of Cd from contaminated soil. To decrease soil Pb level from an initial concentration of 721 to 100 mg kg−1 , the six test species would need to be cultivated for more than 250 years and are thus unsuitable for the remediation of Pb-polluted soil. CONCLUSIONS Double harvesting enhanced the phytoextraction eﬃciency of all forage species tested by increasing their total biomass yield. Amaranth was shown to be useful for phytoextraction and achieved the highest BF and PR values for Cd and Zn under both double and single harvesting. Chicory showed the high BF values for Cd; although the amounts of metals extracted and the PR value were small due to low biomass, chicory may be regarded as a candidate species. None of the species showed the capability for eﬀective phytoextraction of Pb from contaminated soil. ACKNOWLEDGEMENT The study was supported by the National Natural Science Foundation of China (No. 41501340) and the Zhejiang Provincial Natural Science Foundation of China (No. LQ14D010002). REFERENCES Almaroai Y A, Usman A R A, Ahmad M, Kim K R, Moon D H, Lee S S, Ok Y S. 2012. Eﬀects of synthetic chelators and lowmolecular-weight organic acids on chromium, copper, and arsenic uptake and translocation in Maize (Zea mays L.). Soil Sci. 177: 655–663. Andreazza R, Bortolon L, Pieniz S, Camargo F A O. 2013. Use of high-yielding bioenergy plant castor bean (Ricinuscommunis L.) as a potential phytoremediator for copper-contaminated soils. Pedosphere. 23: 651–661. Aravindhakshan S C, Epplin F M, Taliaferro C M. 2011. Switchgrass, bermudagrass, ﬂaccidgrass, and lovegrass biomass yield response to nitrogen for single and double harvest. Biomass Bioenerg. 35: 308–319. Audet P, Charest C. 2007. Heavy metal phytoremediation from a meta-analytical perspective. Environ Pollut. 147: 231–237.
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