**Restoration of Cadmium (Cd) Pollution Soils by Use of Weeds**

Masaru Ogasawara

*Weed Science Center (WSC), Utsunomiya University, 350 Mine-machi, Utsunomiya Japan* 

#### **1. Introduction**

216 Soil Health and Land Use Management

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Soil contamination by heavy metals such as cadmium (Cd), copper and mercury has become a big concern particularly in metal plating plants, mining sites and surrounding areas as well as residential area and farmlands in the river downstream region neighboring these facilities. In some cases, heavy metals in soils leach into river water and then diffuse onto farmlands with irrigation, resulting in relatively low levels of heavy metals being spread into wider areas rather than being localized in high concentrations (Asami 1972). Therefore, when civil engineering methods including addition of topsoil and removal of contaminated soils are used for remediation of heavy metal contaminated soils, a large amount of uncontaminated fresh soil and large disposal areas are required, which creates a bottle neck for the remediation. Thus, the development of a new remediation technology to replace the conventional civil engineering technology is needed. The remediation technology for soils contaminated with harmful substances using plants is called phyto-remediation, and its potential has demonstrated by many researchers (Elizabeth 2005). However, this new remediation technology is in the initial stage even at present, except for some cases, e. g., the remediation of oil-spilled soil using Italian ryegrass (Kaimi *et al*. 2006) and that of Cdcontaminated paddy soil using rice plants (Honma *et al*. 2009) are currently being conducted, because no appropriate remediation plant and no remediation systems have been found and developed. Thus, weed species, possessing high adaptability to environment, have been pointed out as a suitable plant for soil remediation. Although research on phyto-remediation using weeds has just begun and there are many issues which need to be resolved, this remediation technique is expected to become a valuable technology for the alleviation of heavy metal contaminated soils in the near future.

This chapter focuses on the potential of weed for remediation of Cd-contaminated soils. In order to better understand phyto-remediation by weeds, the rationale of using weeds for Cd remediation and the biological characteristics of weeds are explained. Herbaceous plants are classified into several groups such as crops, grasses, weeds and wild plants. Crops are plants that require artificial protection such as pest control, fertilization, watering and etc., ; on the other hand, weeds can thrive under severe growth conditions. For example, asiatic plantain (*Plantago asiatica*) in highly compacted soil areas, crabgrass (*Digitaria cilliari*) in dry regions, annual bluegrass (*Poa annua*) in cool wet regions, field horsetail (*Equisetum arvense*) in acidic soils, saltbush (*Atriplex subcordata*) in salt-accumulation areas, and broomsedge

Restoration of Cadmium (Cd) Pollution Soils by Use of Weeds 219

*Phaseolus angularis* (cv. Tanba Dinagon)

\* Susuceptible weed species having ≤ 2 of I50 value (mg Cd kg-1): the concentrations of Cd causing a 50% reduction in fresh weight of shoot to that of the untreated plant grown under sand culture conditions

About 6,000 plant species are accounted as a weed in the world. Cd sensitivities weeds vary depending on species (Fig. 1 and Fig. 2). In this section, Cd sensitivity and Cd content of

It is reported that *Arabidopsis halleri* and *Thlaspi caerulescens* are highly tolerant to Cd (Bert et al. 2003; Brown et al. 1995); however, *Hibiscus cannabinus*, *Portulaca oleracea*, *Xanthium strumarium*, *Amsinckia barbata*, *Anthoxathium odoratum*, *Arthoraxon hipidus*, *Digitaria ciliaris*, *Echinochloa crus-galli var. praticola*, *Lolium multiflorum*, *Panicum bisulcatum*, *Paspalum dilatatum*, *Poa pratensis and Setaria faberi* are have also been reported to have a high tolerance to Cd (I50: > 30 mg Cd Kg-1) (Abe et al. 2006) (Table-3). When the weed habitat is considered, *E. crus-galli var. praticola*, *D. ciliaris*, *S. faberi* and *P. dilatatum*, *A. odoratum*, *L. multiflorum* and *P. pratensis* are the most suitable for saturated, semi-arid, warm and cold conditions, correspondingly. When the weed biomass is considered, *X. strumarium* growing up 1–2 m is the most suitable for Cd extraction. When the life-span of weed is considered, *A. odoratum*, *P. bisulcatum*, *P. dilatatum*, *P. thunbergii*, and *P. pratensis* are perennials, and particularly *P. pratensis* is a long-lived grass that can survive more than a few decades; therefore, phytoremediation will be proceeded continuously once perennial weed species such as *P. pratensis*

On the other hand, there are great variations on Cd tolerance among weed species. However, the tolerance can be predicted when they belong to the same family. Abe *et al*. (2006) reported that *Gramineae* and *Compositae* weed species were torelant, while *Leguminoseae* and *Cruciferae* weed species were sensitive according to the pot tests conducted

50 value (mg Cd kg-1)

1.5 1.3 2.0 0.8 1.7 2.3 1.1 1.6 1.9 1.4 1.5 1.8 \*

Family name Scientific name I

*Stellaria alsine* var. *undulata*

*Epilobium angustifolium Geranium carolinianum*

*Phaseolus aureus Trifolium fragiferum Rorippa cantoniensis Arenaria serpyllifolia* 

*Stellaria graminea Antenoron filiforme Eclipta prostrata Sonchus asper* 

*Onagraceae Geraniaceae Leguminosae* 

*Cruciferae* 

*Caryophyllaceae* 

*Polygonaceae Compositae* 

weeds are referred.

Table 2. Susceptible weed species to Cd

is introduced into the Cd pollution area.

**2. Sensitivities of weeds to Cd** 

(*Andropogon virginicu)* in phosphate-deficient soils can grow vigorously under these conditions (Takematsu and Ichizen 1987, 1993, 1997). For plants, not only Cd contaminated soil but also various environmental factors such as low temperature, aridity, low sunlight (shade), nutrient deficiency (infertile soil), poorly drained soil, competitions between plants for water, nutrition, and light, and allelochemicals generated from plants are regarded as adverse growth conditions. Therefore, remediation plant (plant using for restoration of Cd pollution soils) must be able to grow under these adverse weather and soil conditions. On the other hand, several methods are considered in phyto-remediation, and the capability of remediation plants are depending on the approach considered. Unlike organic compounds, Cd cannot be degraded; therefore, absorption (phyto-extraction) and fixation (phytostabilization) are the most effective methods proposed for Cd-remediation. Phyto-extraction is the chemical removal method of Cd by absorption through the roots and accumulation in shoots, followed by plant harvesting. Phyto-stabilization is a method of retaining Cd on the adjacent surface of plant roots. Mulching, which prevents the run-off of Cd contaminated soil into the surrounding non-polluted area by the root system extending in soils, can also be considered for phyto-remediation technologies. Particularly in slopes, mulching with plants may be prior to Cd extraction from the contaminated soils.

Among several screening studies on the remediation plants conducted, it is demonstrated that *Athyrium yokoscense* is highly tolerant to heavy metals (*Nishizono et al.* 1987); however, its biomass is extremely small to be valuable as a remediation plant. The plant species best suited for phyto-extraction require the ability to accumulate large amounts of Cd in their shoots, extension of their roots into soil, rapid growth and a long growing period. The plant species suitable for mulching require the ability to extend their roots into soil, a high LAI (leaf area index) value; plants with a high LAI value can reduce the physical strength of rainfall to scour soil, large biomass, rapid growth and long growing period. Furthermore, it is important that whether seeds and vegetative reproductive organs such as rhizomes and tubers can be inexpensively supplied in large quantities for the remediation plant. As compared with crops, weeds are generally superior in several points such as environment adaptability and Cd tolerance and accumulation, however, are inferior in seed supply (Table-1). Thus, in case when seeds of remediation plants (weeds) are difficult to obtain, top soil (seed bank) of non-pollution areas where weeds are densely grown, are available. As well as Cd tolerant weed species, Cd sensitive weed species are also useful for Cd remediation (Table-2); e. g., results of phyto-remediation at the pollution area can be evaluated by distribution and biomass of the Cd sensitive weed species such as *Arenaria serphylliforia*, *Geranium carolinianum* and *Phseolus aureas*.


\*: drought, cool, shade, wet, salinity and infertile land tolerance

Table 1. Comparison of weeds and crops as a Cd remediation plants

(*Andropogon virginicu)* in phosphate-deficient soils can grow vigorously under these conditions (Takematsu and Ichizen 1987, 1993, 1997). For plants, not only Cd contaminated soil but also various environmental factors such as low temperature, aridity, low sunlight (shade), nutrient deficiency (infertile soil), poorly drained soil, competitions between plants for water, nutrition, and light, and allelochemicals generated from plants are regarded as adverse growth conditions. Therefore, remediation plant (plant using for restoration of Cd pollution soils) must be able to grow under these adverse weather and soil conditions. On the other hand, several methods are considered in phyto-remediation, and the capability of remediation plants are depending on the approach considered. Unlike organic compounds, Cd cannot be degraded; therefore, absorption (phyto-extraction) and fixation (phytostabilization) are the most effective methods proposed for Cd-remediation. Phyto-extraction is the chemical removal method of Cd by absorption through the roots and accumulation in shoots, followed by plant harvesting. Phyto-stabilization is a method of retaining Cd on the adjacent surface of plant roots. Mulching, which prevents the run-off of Cd contaminated soil into the surrounding non-polluted area by the root system extending in soils, can also be considered for phyto-remediation technologies. Particularly in slopes, mulching with plants

Among several screening studies on the remediation plants conducted, it is demonstrated that *Athyrium yokoscense* is highly tolerant to heavy metals (*Nishizono et al.* 1987); however, its biomass is extremely small to be valuable as a remediation plant. The plant species best suited for phyto-extraction require the ability to accumulate large amounts of Cd in their shoots, extension of their roots into soil, rapid growth and a long growing period. The plant species suitable for mulching require the ability to extend their roots into soil, a high LAI (leaf area index) value; plants with a high LAI value can reduce the physical strength of rainfall to scour soil, large biomass, rapid growth and long growing period. Furthermore, it is important that whether seeds and vegetative reproductive organs such as rhizomes and tubers can be inexpensively supplied in large quantities for the remediation plant. As compared with crops, weeds are generally superior in several points such as environment adaptability and Cd tolerance and accumulation, however, are inferior in seed supply (Table-1). Thus, in case when seeds of remediation plants (weeds) are difficult to obtain, top soil (seed bank) of non-pollution areas where weeds are densely grown, are available. As well as Cd tolerant weed species, Cd sensitive weed species are also useful for Cd remediation (Table-2); e. g., results of phyto-remediation at the pollution area can be evaluated by distribution and biomass of the Cd sensitive weed species such as *Arenaria* 

Factors weeds Crops

Superior Superior Superior Superior Inferior Superior Inferior Inferior Inferior Inferior Superior Inferior

may be prior to Cd extraction from the contaminated soils.

*serphylliforia*, *Geranium carolinianum* and *Phseolus aureas*.

Adaptability to environment \* Pest tolerance (disease, insect) Seed or seedling supply

Table 1. Comparison of weeds and crops as a Cd remediation plants

Cd tolerance Cd accumulation

Growth speed

\*: drought, cool, shade, wet, salinity and infertile land tolerance


\* Susuceptible weed species having ≤ 2 of I50 value (mg Cd kg-1): the concentrations of Cd causing a 50% reduction in fresh weight of shoot to that of the untreated plant grown under sand culture conditions

Table 2. Susceptible weed species to Cd

## **2. Sensitivities of weeds to Cd**

About 6,000 plant species are accounted as a weed in the world. Cd sensitivities weeds vary depending on species (Fig. 1 and Fig. 2). In this section, Cd sensitivity and Cd content of weeds are referred.

It is reported that *Arabidopsis halleri* and *Thlaspi caerulescens* are highly tolerant to Cd (Bert et al. 2003; Brown et al. 1995); however, *Hibiscus cannabinus*, *Portulaca oleracea*, *Xanthium strumarium*, *Amsinckia barbata*, *Anthoxathium odoratum*, *Arthoraxon hipidus*, *Digitaria ciliaris*, *Echinochloa crus-galli var. praticola*, *Lolium multiflorum*, *Panicum bisulcatum*, *Paspalum dilatatum*, *Poa pratensis and Setaria faberi* are have also been reported to have a high tolerance to Cd (I50: > 30 mg Cd Kg-1) (Abe et al. 2006) (Table-3). When the weed habitat is considered, *E. crus-galli var. praticola*, *D. ciliaris*, *S. faberi* and *P. dilatatum*, *A. odoratum*, *L. multiflorum* and *P. pratensis* are the most suitable for saturated, semi-arid, warm and cold conditions, correspondingly. When the weed biomass is considered, *X. strumarium* growing up 1–2 m is the most suitable for Cd extraction. When the life-span of weed is considered, *A. odoratum*, *P. bisulcatum*, *P. dilatatum*, *P. thunbergii*, and *P. pratensis* are perennials, and particularly *P. pratensis* is a long-lived grass that can survive more than a few decades; therefore, phytoremediation will be proceeded continuously once perennial weed species such as *P. pratensis* is introduced into the Cd pollution area.

On the other hand, there are great variations on Cd tolerance among weed species. However, the tolerance can be predicted when they belong to the same family. Abe *et al*. (2006) reported that *Gramineae* and *Compositae* weed species were torelant, while *Leguminoseae* and *Cruciferae* weed species were sensitive according to the pot tests conducted

Restoration of Cadmium (Cd) Pollution Soils by Use of Weeds 221

*Hibiscus cannabinus Malva sylvestris Silene ameria Portulaca oleracea Phytolacca americana Xanthium atrumarium Amsinckia barbata Anthoxanthum odoratum Anthraxon hispidus Digitaria ciliaris* 

*Lolium multiflorum Panicum bisulcatum Pasplum dilatatum Poa pratensis Setaria faberi Agrostis stolonifera* Tolerant weed species having ≥ 30 of I50 value (mg Cd kg-1): the concentrations of Cd causing a 50% reduction in fresh weight of shoot to that of the untreated plant grown under

Fig. 3. Relation of susceptibility (I50 value) to Cd and Cd content in shoot of weeds

*Echinochloa crus-galli* var. *pracilola* 

Family name Scientific name

*Malvaceae* 

sand culture conditions

belonged to 4 families

Table 3. Tolerant weed species to Cd\*

*Caryophyllaceae Portulaceae Phytolaccaceae Compositae Boraginaceae Gramineae* 

Fig. 1. Susceptibilty of *Echinochloa crus-galli* var. *crus-galli* to Cd

Fig. 2. Susceptibilty of *Bidens frondo*sa to Cd

Fig. 1. Susceptibilty of *Echinochloa crus-galli* var. *crus-galli* to Cd

Fig. 2. Susceptibilty of *Bidens frondo*sa to Cd


Tolerant weed species having ≥ 30 of I50 value (mg Cd kg-1): the concentrations of Cd causing a 50% reduction in fresh weight of shoot to that of the untreated plant grown under sand culture conditions

Table 3. Tolerant weed species to Cd\*

Fig. 3. Relation of susceptibility (I50 value) to Cd and Cd content in shoot of weeds belonged to 4 families

Restoration of Cadmium (Cd) Pollution Soils by Use of Weeds 223

Family name Scientific name Cd conc. (mg kg-1 dry weight) Plant species with relatively high Cd conc.

Plant species with relatively low Cd conc.

Table 5. Weed species which accumulate relatively high (≥30) and low (≤3 ) concentrations

Family name Scientific name Cd conc. (mg kg-1 dry weight) Plant species with relatively high Cd conc.

Plant species with relatively low Cd conc.

77.0 64.4 56.2 48.3 48.2 48.2 41.0

> 0.8 1.0 1.2 1.4 1.5 1.8 1.9 2.1 2.1

171.9 122.6 122.2 117.5 116.6 115.4 113.0 111.9

> 7.7 9.4 11.4 12.2 14.2 15.9

*Cichorium intybus Matricaria chamomilla Polygonum thunbergii* 

*Sisymbrium orientale Sisymbrium altissimum Picris echinoides* 

*Cyperus brevifolius* var. *leiolepis* 

*Oryza sativa* (cv. Milyang 42) *Panicum dichotomiflorum Dactylis glomerata Plantago virginica Panicum bisulcatum Oenothera biennis Digitaria ciliaris Sesbannia exaltata* 

*Echinochloa crus-galli* var. *prachicola*

*Calystegia sepium* var. *1 americana* 

*Oryza sativa* (cv. Milyang 25)

*Rumex crispus* subsp. *japonicus* 

Table 6. Weed species which accumulate relatively high (≥110) and low (≤16)

*Oenothera biennis* 

*Cassia obtusifolia* 

*Festuca arundinaceae* 

*Salvia plebeia* 

*Silene dioica Sida rhombifolia* 

*Bidens pilosa Bidens frondosa Lactuca indica* 

*Sochus oleraceus Camelina sativa*

concentrations (mg kg-1 DW) of Cd in their roots

*Compositae Compositae Polygonaceae Cyperaceae Cruciferae Cruciferae Compositae* 

*Gramineae Gramineae Gramineae Plantagineaceae Gramineae Onagraceae Gramineae Leguminosae Gramineae*

*Onagraceae Convolvulaceae Leguminosae Gramineae Lubiatae Gramineae Caryophyllaceae Malvaceae* 

*Compositae Compositae Compositae Polygonaceae Compositae Cruciferae* 

(mg kg-1 DW) of Cd in their shoots

on about 200 weed species (Fig. 3 and Table-4). This result indicated that the Cd concentrations of soils in a certain areas could be predicted by analysis of weed vegetation. Among weed species belonging to *Gramineae* and *Leguminosae* species, appropriate Cd remediation plant and Cd indicator may be found.


Group 1, I50 value <10; Group 2, I50 value 10 ~ <20; Group 3, I50 value 20 ~ <30; Group 4, I50 value≧30

Table 4. Susceptibilities of *Caryophyllaceae*, *Cruciferae*, *Leguminosae*, *Compositae* and *Gramineae* weed species to Cd

## **3. Cd accumulation abilities of weeds**

High Cd accumulation ability as well as Cd tolerance is required for the remediation plants; however, there is no positive correlation between Cd accumulation and Cd sensitivity. Weed species with highly tolerant to Cd can be divided into two groups: weeds that absorb hardly Cd (Cd exclusion type) and weeds that absorb and detoxify Cd (Cd detoxaccumulation type). Abe *et al.* (2008a) revealed from a pot test that *Cichorium intybus* (77 mg kg-1 DW) and *Matricaria chamomilla* (64.4 mg kg-1 DW) can accumulate large amounts of Cd in their shoots; these accumulations are more than *Polygonum thunbergii* (56.2mg kg-1 DW) that is known as a hyper-Cd accumulator (Shinmachi *et al.* 2003) (Table-5). The mechanism involved in Cd accumulation by weeds are still not well understood; however, it can be predicted that the absorbed Cd is detoxified by forming chelates with phytochelatin, malic acid, and citric acid in plants. On the other hand, *Gramineae* plant, e. g., *Oryza sativa* (cv. Milyang) accumulate high concentrations of Cd (>100 mg kg-1 DW) in its roots but accumulate little Cd in their shoots, in contrary, *Compositae* plant, e. g., *Bidens frondosa* accumulate low concentration of Cd in the root but accumulate much Cd in its shoots (Table-6, Table-7, Table-8). The Cd content ratio between roots and shoots (SR ratio) of *C. intybus* (3.56; it means that shoots contain 3.56 times more Cd than roots) is higher than that of *O. sativa* (cv. Milyang; 0.02); therefore, the results indicate that the Cd is easily translocated from roots to shoots in dicoltyledons as compared to monocotyldons *(Gramineae)* (Table 6). Therefore, *Bidens frondosa*, *B. pilosa* and *Amaranthus viridis*, which accumulate more Cd in their shoots than in roots and possess a large biomass, may be useful for phyto-extraction.


on about 200 weed species (Fig. 3 and Table-4). This result indicated that the Cd concentrations of soils in a certain areas could be predicted by analysis of weed vegetation. Among weed species belonging to *Gramineae* and *Leguminosae* species, appropriate Cd

Number of plant species (%) Mean of I50

1 (5.9%) - - 2 (6.5%) 12 (26.7%)

1 (5.9%) - - 4 (12.9%) 18 (40.0%)

Group 1 Group 2 Group 3 Group 4

2 (11.8%) 5 (33.3%) - 12 (38.7%) 15 (33.3%)

Group 1, I50 value <10; Group 2, I50 value 10 ~ <20; Group 3, I50 value 20 ~ <30; Group 4, I50 value≧30 Table 4. Susceptibilities of *Caryophyllaceae*, *Cruciferae*, *Leguminosae*, *Compositae* and *Gramineae*

High Cd accumulation ability as well as Cd tolerance is required for the remediation plants; however, there is no positive correlation between Cd accumulation and Cd sensitivity. Weed species with highly tolerant to Cd can be divided into two groups: weeds that absorb hardly Cd (Cd exclusion type) and weeds that absorb and detoxify Cd (Cd detoxaccumulation type). Abe *et al.* (2008a) revealed from a pot test that *Cichorium intybus* (77 mg kg-1 DW) and *Matricaria chamomilla* (64.4 mg kg-1 DW) can accumulate large amounts of Cd in their shoots; these accumulations are more than *Polygonum thunbergii* (56.2mg kg-1 DW) that is known as a hyper-Cd accumulator (Shinmachi *et al.* 2003) (Table-5). The mechanism involved in Cd accumulation by weeds are still not well understood; however, it can be predicted that the absorbed Cd is detoxified by forming chelates with phytochelatin, malic acid, and citric acid in plants. On the other hand, *Gramineae* plant, e. g., *Oryza sativa* (cv. Milyang) accumulate high concentrations of Cd (>100 mg kg-1 DW) in its roots but accumulate little Cd in their shoots, in contrary, *Compositae* plant, e. g., *Bidens frondosa* accumulate low concentration of Cd in the root but accumulate much Cd in its shoots (Table-6, Table-7, Table-8). The Cd content ratio between roots and shoots (SR ratio) of *C. intybus* (3.56; it means that shoots contain 3.56 times more Cd than roots) is higher than that of *O. sativa* (cv. Milyang; 0.02); therefore, the results indicate that the Cd is easily translocated from roots to shoots in dicoltyledons as compared to monocotyldons *(Gramineae)* (Table 6). Therefore, *Bidens frondosa*, *B. pilosa* and *Amaranthus viridis*, which accumulate more Cd in their shoots than in roots and possess a large biomass, may be useful

values (Cd mg kg-1

> 8.8 7.5 4.6 13.2 22.6

)

remediation plant and Cd indicator may be found.

number of plant species tested

> 17 (100%) 15 (100%) 15 (100%) 31 (100%) 45 (100%)

**3. Cd accumulation abilities of weeds** 

13 (76.4%) 10 (66.7%) 15 (100%) 13 (41.9%) -

Family name Total

*Caryophyllaceae Cruciferae Leguminosae Compositae Gramineae*

weed species to Cd

for phyto-extraction.




Table 6. Weed species which accumulate relatively high (≥110) and low (≤16) concentrations (mg kg-1 DW) of Cd in their roots

Restoration of Cadmium (Cd) Pollution Soils by Use of Weeds 225

0.32 ± 0.08 to 0.53 ± 0.04 in roots. This result indicates that Cd tolerance of remediation plants can be further enhanced by HMI. (Fig. 4, Fig. 5, Fig. 6) HMI, plant growth regulator (PGR) that shows similar action to those of plant hormones, has been used as rooting agent for paddy rice (Ogawa and Ota 1976) and thus it is thought that HMI can be also easily applied for Cd

Fig. 4. Effect of HMI on Cd accumulation in shoot and root of *Echinochloa frumentacea*  The error bars represent the standard deviation of the mean (n=3).The means followed by the same letter within a column are not significantly different by Tukey's multiple range test

Fig. 5. Effect of HMI on *Echinochloa frumentacea* growth inhibited by Cd The error bars represent the standard deviation of the mean (n=3)

remediation with weeds, particular in the mulching.

(p<0.05)


Table 7. Weed species which accumulate relatively high (≥20 μg plant-1) content of Cd in shoots


\* shoot-root ratio : the ratio of Cd conc. in shoot and root in plant

Table 8. Weed species which have relatively high (≥2.0) and low (≤0.03) shoot-root ratios\*

#### **4. Utilization of PGRs to Cd remediation by use of weeds**

So far, various cultivation methods and materials have been used to control the Cd elution into soils (Turgut *et al*. 2004; Hattori *et al*. 2006); e. g., treatment of Calcium materials and deep flooding (rice paddy fields) have been used to diminish Cd elution into the free water in soils; on the other hand, drying of soil and treatment of chlorides and EDTA have been used to enhance the Cd elution. In addition to these materials, it is reported that certain plant hormones directly affect the growth of remediation plants. Abscisic acid suppresses a decrease in chlorophyll content caused by Cd in *Brassica napus*, while *28-homobrassinolide* mitigates the growth inhibition caused by Cd in *Cicer arietium*. According to a hydroponic test using white Japanese millet (*Echinochloa frumentacea*) conducted by Abe *et al*. (2011), it was shown that HMI (3-hydroxy-5-methylisoxazole) at 2.5 × 103 mmol/l significantly reduced the growth inhibition (dry weight; % of control) of roots and shoots caused by Cd at 4.4 × 10 mmol/l from 4.2% to 23.9% and from 48.5% to 82.6%, respectively, while increasing Cd content (μg plant-1) from

Family name Scientific name Shoot Cd content (μg plant-1)

Table 7. Weed species which accumulate relatively high (≥20 μg plant-1) content of Cd in

Family name Scientific name Cd conc. (mg kg-1 dry weight) Plant species with relatively high shoot-root ratio

Plant species with relatively low shoot-root ratio

Table 8. Weed species which have relatively high (≥2.0) and low (≤0.03) shoot-root ratios\*

So far, various cultivation methods and materials have been used to control the Cd elution into soils (Turgut *et al*. 2004; Hattori *et al*. 2006); e. g., treatment of Calcium materials and deep flooding (rice paddy fields) have been used to diminish Cd elution into the free water in soils; on the other hand, drying of soil and treatment of chlorides and EDTA have been used to enhance the Cd elution. In addition to these materials, it is reported that certain plant hormones directly affect the growth of remediation plants. Abscisic acid suppresses a decrease in chlorophyll content caused by Cd in *Brassica napus*, while *28-homobrassinolide* mitigates the growth inhibition caused by Cd in *Cicer arietium*. According to a hydroponic test using white Japanese millet (*Echinochloa frumentacea*) conducted by Abe *et al*. (2011), it was shown that HMI (3-hydroxy-5-methylisoxazole) at 2.5 × 103 mmol/l significantly reduced the growth inhibition (dry weight; % of control) of roots and shoots caused by Cd at 4.4 × 10 mmol/l from 4.2% to 23.9% and from 48.5% to 82.6%, respectively, while increasing Cd content (μg plant-1) from

50.0 49.7 40.5 38.8 30.3 24.1 22.5 21.4 21.4 21.4 21.1 20.7

3.56 3.30 2.27 2.21

0.01 0.02 0.02 0.03

*Cyperus brevifolius* var. leiolepis

*Polygonum thunbergii Bidens frondosa Cichorium intybus Cyperus globosus Bidens pilosa Amaranthus viridis Hibicus cannabinus* 

*Sisymbrium altissimum* 

*Sinapis alba*

*Silene notiflora Amaranthus hybridus* 

*Cichorium intybus Bidens frondosa Lactuca indica Bidens pilosa* 

*Oenothra biennis Plantago virginica* 

*Dactylis glomerata* 

**4. Utilization of PGRs to Cd remediation by use of weeds** 

\* shoot-root ratio : the ratio of Cd conc. in shoot and root in plant

*Oryza sativa* (cv. Milyyang 42)

*Cyperaceae Polygonaceae Compositae Compositae Cyperaceae Compositae Amaranthaceae Malvaceae Cruciferae Cruciferae Caryophylaceae Amaranthaceae* 

shoots

*Compositae Compositae Compositae Compositae* 

*Onagraceae Plantaginaceae Gramineae Gramineae* 

0.32 ± 0.08 to 0.53 ± 0.04 in roots. This result indicates that Cd tolerance of remediation plants can be further enhanced by HMI. (Fig. 4, Fig. 5, Fig. 6) HMI, plant growth regulator (PGR) that shows similar action to those of plant hormones, has been used as rooting agent for paddy rice (Ogawa and Ota 1976) and thus it is thought that HMI can be also easily applied for Cd remediation with weeds, particular in the mulching.

Fig. 4. Effect of HMI on Cd accumulation in shoot and root of *Echinochloa frumentacea*  The error bars represent the standard deviation of the mean (n=3).The means followed by the same letter within a column are not significantly different by Tukey's multiple range test (p<0.05)

Fig. 5. Effect of HMI on *Echinochloa frumentacea* growth inhibited by Cd The error bars represent the standard deviation of the mean (n=3)

Restoration of Cadmium (Cd) Pollution Soils by Use of Weeds 227

depends on the not only Cd contamination levels in soil but also environmental and geographical conditions of the pollution areas, e. g., when the area is located in slopes, prevention of run-off of Cd contaminated soil by mulching should be preceded to Cd extraction. For the Cd remediation using weeds, several goals are suggested as follows; 1) prevention of Cd elution into the surrounding non-polluted areas; vegetation is restored by mulching with weeds and followed by introduction of grasses, turf grasses and shrubs as needed, but Cd extract is not involved in this case, 2) extraction of Cd from the pollution soils; Cd extraction is conducted by use of the remediation plants, and thereafter when postmitigated area is reused as a farmland, introduced remediation plants (weeds) should be controlled completely by herbicides and practical methods before land reuses (Fig. 7). As mentioned above, Cd remediation approach varies with goals, however, preliminary survey of Cd concentrations in soil, weather conditions (temperature, sunlight and precipitation by month), physicochemical characteristics of soils, drainage, fertility and slope angle are needed regardless of the approach and the goal to decide the remediation plant (weed species) and the remediation methods (phyto-extraction, phyto-stabilization and/or malching). Moreover, in some cases, repeated introduction of the remediation plants into the Cd pollution areaa may be required, because seed production of the plant is possibly be inhibited by Cd even when Cd tolerant plants are used. For example, it is reported that the number of seeds / plant of *Portulaca oleracea* var. *sativa*, which is tolerant to Cd, is inhibited by 65% at 20 mgkg-1 of Cd (Abe *et al*. 2008b). Use of hormone and PGR may increase in the seed production and biomass of the remediation plants, and those seedlings habituated in the Cd contaminated soils before introducing them into the pollution areas, may be useful for the remediation.

Weeds are highly adaptive to adverse environmental conditions compared to crops, which is a crucial factor to consider weeds for developing soil remediation technologies. However, it might be considerably more difficult to ensure the necessary seed and seedling supply of weeds compare to crops, which can hamper the development of potential remediation technique. Thus when weeds are used for restoration of Cd pollution soils, to develop an effective seed and seeding production systems may become a crucial requirement. Besides, the proposed phyto-remediation with weeds might be envisioned as a long term approach due to the time requirements of the approach. In general, there is a great tendency that Cd extraction is much spotlighted than other goals, however, supplying the soils abundant in organic matter which absorb Cd and keep it in the soils by transplanting of herbaceous plants including weeds; resulting in the prevention of the run-off of Cd contaminated soil into surrounding nonpolluted areas, may be efficient in case that Cd pollution is distributed over the extensive area.

Abe T., Fukami M., Ichizen N., and Ogasawara M., 2006: Susceptibility of weed species to cadmium evaluated in a sand culture. *Weed Biology and Management*, 6, 107-114. Abe T., Fukami M., and Ogasawara M., 2008a: Cadmium accumulation in the shoots and

Abe T., SutoY., and Ogasawara M., 2008b: Effects of cadmium on the growth and the seed

production of *Portulaca oleracea* L. var.*sativa* (Haw.) DC. *J. Weed Sci. Tech*., 53, 1-7. (in

roots of 93 weed species. *Soil Sci. Plant Nutr*., 54, 566-573.

Japanese with English summary)

**6. Conclusion** 

**7. References** 

Fig. 6. Mitigation effect of hymexazole (HMI; 3-hydroxy-5-methylisoxazole) against *Echinochloa frumentacea* growth injury caused by Cd

## **5. Goal and approach for the restoration of Cd pollution soils by use of Weeds**

In general, Cd pollution of soils are distributed over wide ranges, and recovery of vegetation at the pollution area and Cd extraction from the soils require a long time; thus it is important to make a grand design on goals and approach of the remediation of Cd pollution soils using weeds based on long term foresight. The remediation approach

Fig. 7. Goal and approach of remediation of Cd pollution soils by use of weeds

depends on the not only Cd contamination levels in soil but also environmental and geographical conditions of the pollution areas, e. g., when the area is located in slopes, prevention of run-off of Cd contaminated soil by mulching should be preceded to Cd extraction. For the Cd remediation using weeds, several goals are suggested as follows; 1) prevention of Cd elution into the surrounding non-polluted areas; vegetation is restored by mulching with weeds and followed by introduction of grasses, turf grasses and shrubs as needed, but Cd extract is not involved in this case, 2) extraction of Cd from the pollution soils; Cd extraction is conducted by use of the remediation plants, and thereafter when postmitigated area is reused as a farmland, introduced remediation plants (weeds) should be controlled completely by herbicides and practical methods before land reuses (Fig. 7). As mentioned above, Cd remediation approach varies with goals, however, preliminary survey of Cd concentrations in soil, weather conditions (temperature, sunlight and precipitation by month), physicochemical characteristics of soils, drainage, fertility and slope angle are needed regardless of the approach and the goal to decide the remediation plant (weed species) and the remediation methods (phyto-extraction, phyto-stabilization and/or malching). Moreover, in some cases, repeated introduction of the remediation plants into the Cd pollution areaa may be required, because seed production of the plant is possibly be inhibited by Cd even when Cd tolerant plants are used. For example, it is reported that the number of seeds / plant of *Portulaca oleracea* var. *sativa*, which is tolerant to Cd, is inhibited by 65% at 20 mgkg-1 of Cd (Abe *et al*. 2008b). Use of hormone and PGR may increase in the seed production and biomass of the remediation plants, and those seedlings habituated in the Cd contaminated soils before introducing them into the pollution areas, may be useful for the remediation.

## **6. Conclusion**

226 Soil Health and Land Use Management

Fig. 6. Mitigation effect of hymexazole (HMI; 3-hydroxy-5-methylisoxazole) against

**5. Goal and approach for the restoration of Cd pollution soils by use of** 

Fig. 7. Goal and approach of remediation of Cd pollution soils by use of weeds

In general, Cd pollution of soils are distributed over wide ranges, and recovery of vegetation at the pollution area and Cd extraction from the soils require a long time; thus it is important to make a grand design on goals and approach of the remediation of Cd pollution soils using weeds based on long term foresight. The remediation approach

Cd + HM1

*Echinochloa frumentacea* growth injury caused by Cd

**Weeds** 

Weeds are highly adaptive to adverse environmental conditions compared to crops, which is a crucial factor to consider weeds for developing soil remediation technologies. However, it might be considerably more difficult to ensure the necessary seed and seedling supply of weeds compare to crops, which can hamper the development of potential remediation technique. Thus when weeds are used for restoration of Cd pollution soils, to develop an effective seed and seeding production systems may become a crucial requirement. Besides, the proposed phyto-remediation with weeds might be envisioned as a long term approach due to the time requirements of the approach. In general, there is a great tendency that Cd extraction is much spotlighted than other goals, however, supplying the soils abundant in organic matter which absorb Cd and keep it in the soils by transplanting of herbaceous plants including weeds; resulting in the prevention of the run-off of Cd contaminated soil into surrounding nonpolluted areas, may be efficient in case that Cd pollution is distributed over the extensive area.

#### **7. References**


**13** 

*USA* 

**Herbicide Off-Site Transport** 

Timothy J. Gish1, John H. Prueger2, William P. Kustas1, Jerry L. Hatfield2, Lynn G. McKee1 and Andrew Russ1

*1USDA-ARS, Hydrology and Remote Sensing Laboratory, Beltsville, Maryland, 2USDA-ARS, National Laboratory for Agriculture and the Environment, Ames, Iowa* 

Herbicides are an important part of modern agriculture as they control weeds that would otherwise reduce yields by competing for water and nutrients. The U.S. Environmental Protection Agency estimated that 226,000 metric tons of herbicides were used in the U.S, alone during 2007, which accounts for 25% of the globally usage in 2007 (USEPA, 2011). Additionally, during 2007, 89% of the herbicide usage in the U.S. was for agriculture (USEPA, 2011). Since it has been proposed that increasing agricultural food and fiber production will be necessary to maintain political and social stability in developing countries (Tilman et al., 2002), herbicide use will become increasingly important to meet these global needs, especially as marginal lands are converted to agriculture (Helling, 1993). Although critical to production, herbicides can be toxic to humans and other organisms, even at low concentrations (Jin-Clark et al., 2002; USEPA, 2008). To maintain productive and sustainable agricultural systems there is an immediate need to understand field-scale

During the past three decades several national surveys in the U.S. have shed light on the prevalence of herbicides in the environment. One of the first national surveys was conducted by the U.S. Environmental Protection Agency (USEPA, 1990) which determined that about 10% of community water system wells contained detectable amounts of at least one herbicide. From 1993 to 1995, the National Water-Quality Assessment program monitored 20 major basins in the U.S. and found herbicides in over 50% of the sites sampled (Koplin et al., 1998). Furthermore, the U.S. Geological Survey observed that 97 percent of all streams sampled from agricultural and urban areas contain detectable concentrations of at least one herbicide, while 65 percent of the streams in undeveloped areas contained observable levels of herbicide (Gillion et al., 2006). Clearly, herbicide occurrence in streams, groundwater aquifers, and community wells are well documented, but determining the relative importance of major off-site transports processes at the field and watershed scales is

As summarized in Figure 1, herbicide off-site transport occurs primarily through surface runoff (Wauchope, 1978; Shipitalo and Owens, 2006), groundwater leaching (Isensee et al., 1990; Gaynor et al., 2001; Hansen et al., 2001), and/or volatilization (Taylor and Spencer, 1990; Gish et al., 2011). Precipitation events are especially crucial for determining which loss pathways are most critical in governing herbicide off-site transport. For example, if a

processes governing herbicide use and off-site transport.

**1. Introduction** 

still in its infancy.

