Plants diversity and phytoaccumulators identification on the Akouedo landfill ( Abidjan , Côte d ’ Ivoire )

Phytoremediation is an emerging technology to remediate contaminated soils. Their implementation in developing countries may be limited by the plant species used. To overcome this situation, plants from the Akouedo landfill have been inventoried in order to identify indigenous plants adapted to soil and environmental conditions. Two sampling zones have been defined, based on the waste dumping activities or not. Four plots (50 × 50 m 2 ) have been defined. The plant species collected were used to constitute vouchers for identification. Plants frequency occurrence was used to evaluate their relative importance, while the Shannon and evenness indices were calculated to characterize the diversity. The plant families and species identified were compared to those described as heavy metals phytoaccumulators in the literature. A total of 130 taxa belonging to 39 families have been recorded. The most frequent families (36.9% of the total taxa) were Poaceae, Euphorbiaceae and Cyperaceae. The dominant taxa on the old waste dumpsite having an average density superior to 5 plants/m 2 and occurring less frequently on the control site were Alternanthera sessilis, Amaranthus spinosus, Cyperus rotundus, Cyperus iria, Eleusine indica, Euphorbia glomerifera, Ipomoea triloba, Portulaca oleracea and Trianthema portulacastrum. These plant species may be indigenous phytoaccumulators on the Akouedo landfill.


INTRODUCTION
Throughout the world, heavy metals pollute soils because of industrial and municipal wastes dumpsites or atmospheric deposition.This situation constitutes hazards to human and animal health (Maiti et al., 2004).Municipal solid waste is a major source of heavy metals contamination of soil (Miquel, 2001).The impacts of heavy metals in the environment differ according to the biosphere compartment (Thangavel and Subbhuraam, 2004) and depend on metals mobility, utilization by organisms or micro-organisms, adsorption, precipitation, etc.However, some plants growing on metals contaminated soils accumulate some at high levels (Xiong, 1998;Cobb et al., 2000;Benson and Ebong, 2005).These plants are considered to be phyto-*Corresponding author.E-mail: coulacina2003@yahoo.fr.Tel: (+225) 07-49-71-53.Fax: (+225) 20-30-43-01.accumulators.They may constitute serious health hazard if they are consumed, because of the toxicity of some metals to human being and animals (Knasmuller et al., 1998;Micieta and Murin, 1998;Baudouin et al., 2002;Ellis and Salt, 2003;Pillay et al., 2003).
Industrial development and demographic growth in the Abidjan district are responsible for potential waste generation.Waste production was evaluated to be 550 000 tons in 2000 (Kouadio et al., 2000).Now, it is around one million tons in that district.Approximately, 70% of these wastes have been disposed into the Akouedo landfill since four decades, causing soil pollution in this area.Kouamé et al. (2006) have characterized lead (Pb) and cadmium (Cd) in this landfill soil and their concentrations varied respectively between 10.3 to 1500 ppm, and 1 to 11.5 ppm.These metal-contaminated soils are used for peri-urban agriculture to produce edible crops (maize, okra, tomatoes and eggplants).Some of those plants may accumulate heavy metals and their consumption could contaminate the food chain.This in turn could constitute potential health implications to humans and animals consuming such crops (Ellis and Salt, 2003;Pillay et al., 2003).
Metals-contaminated soils are remediated by chemical, physical or biological techniques (McEldowney et al., 1993).The first two treatment methods have irreversible effects on soil biodiversity and properties that render them unusable for agriculture (Prabha and Loretta, 2007).Phytoremediation is one of the biological soil remediation technologies.It is a promising cost-effective and plantbased technology for heavy metals remediation (Raskin et al., 1994;Cunningham and Ow, 1996;Anoliefo et al., 2008).It may be attractive in tropical countries because of favourable climatic conditions for plants growth (Lombi et al., 2001).However, one of the limited factors affecting the implementation of this technology in Côte d'Ivoire is the difficulty associated with the growth of the plant species in use.The utilization of phytoaccumulation technology in Côte d'Ivoire needs identification of endogenous phytoaccumulators.In this context, the Akouedo landfill is a natural pressure area where phytoaccumulators could be detected.Therefore, this research was aimed at: (1) making the inventory of plant species on the Akouedo landfill site, (2) identifying potential phytoaccumulators and (3) analysing the plant ecology that constitutes the first steps in phytoremediation strategy.

Study area
The Akouedo landfill is situated between 5°20'30' ' and 5°21'37'' N and 3°55'52'' and 3°56'30'' W located in Abidjan District,near Akouedo village (Figure 1).It covers an area of about 153 ha.The climate of the zone, which is sub-equatorial, was characterized by four seasons: (1) a major dry season (December to March), (2) a major rainy season (April to July), (3) a minor dry season (August to September) and (4) a minor rainy season (October to November).The annual rainfall ranges from 1 040 to 2 079 mm, with an inter- Ni, As Rajakaruna and Bohm (2002); Mahmud et al. (2008) annual of 1 364 mm average.The maximum temperatures range from 25 to 33°C, while the minimum varies between 22 and 24°C.
The sunshine ranges from 117 h (August) and 224 h (November).
The average relative humidity varies between 78% (January) and 87% (August).Evaporation is very important in the major dry season with a peak in March (65 mm).In August, the evaporation decreased significantly and can reach 25.7 mm.Moreover, the soil of the study area was contaminated with inorganic pollutants.In the superficial stratum (less than 50 cm depth), the average concentrations of zinc (Zn), chromium (Cr), cadmium (Cd), lead (Pb), iron (Fe), and copper (Cu) were respectively of 250, 50, 5, 140, 1 400 and 80 ppm.The mean value of pH was 8.25 (Kouamé et al., 2006).

Inventory and identification of taxa
Two sampling zones have been defined according to their characteristics linked to waste dumping site or not on them.An old waste dumpsite and a control site that has never received waste were considered for the plants inventory.Plants recording were done in quadrats established on the study area.These quadrats were established after a visual visit of the site in order to record a maximum representative species.Eight plots of 50 m by side (2500 m 2 ) were established on the site.Each plot was divided into hundred quadrats of 25 m 2 .Ten of these quadrats were randomly selected for inventory.In each quadrat, plants were collected to constitute vouchers.These vouchers were dried in an oven at 30°C for 72 h, and then taken to the "Centre National de Floristique" for identification.Moreover, on each of the sampling quadrats, 5 squares (one in each corner (4) and one in the centre (1) of 1 m 2 were established to evaluate the taxa density.These taxa density were calculated considering quadrat's surface.

Phytoaccumulators identification
The phytoaccumulators were identified in the plant samples according to plants characterized as phytoaccumulators in the literature (Tables 1 and 2).

Data analysis
A data base was constituted with the plant taxa collected and identified.The taxonomic richness and diversity were analysed using frequency of occurrence (F), Shannon (H') and evenness (E) indices.Sorensen index (K) was also used to comprehend the flora community's similarity of the sampling zones.These different parameters were calculated according to the following formulas: Where, Fi is the total releve contained in the taxon i and Ft is the total number of releve.
Where, S is the total number of taxa and pi is the proportion of individual of the taxon i in relation to the total number of individuals.

' log ( )
Where, a is the total number of taxa on the old waste dumpsite; b   is the total number of taxa on the control site; c is the total number of common taxa.The relative importance of taxa was examined with the frequency of occurrence:tThe dominant taxa (F > 50%), secondary taxa (25% < F ≤ 50%) and rare taxa (F ≤ 25%) were defined.The Mann-Whitney test was used to assess differences in taxonomic richness and Shannon index at p= 0.05 of significance level.The statistical analysis was performed with the software package STATISTICA 7.1.

RESULTS
A total of 130 plant taxa belonging to 39 families were recorded (  indicates a difference between plant communities on the two zones investigated.Among the plant families identified, 20 of them were both present on the control and the waste dumping site.However, five families (Bombacaceae, Caesalpiniaceae, Ficoidaceae, Lamiaceae, Passifloraceae) were specific to the waste dumpsite, and the others 14 (Verbenaceae, Acanthaceae, Dioscoreaceae, Apocynaceae, Asclepiadaceae, Bignoniaceae, Combretaceae, Connaraceae, Flagellariaceae, Loganiaceae, Molluginaceae, Pedaliaceae, Periplocaceae, Ulmaceae) were only founded on the control site.Considering the taxa, 25 of them were common to the two zones investigated.The characteristic taxa of the waste dumpsite and the control site were respectively 43 and 62.On the waste dumpsite, 68 taxa distributed among 25 families were recorded.

DISCUSSION
The plant species inventoried on Akouedo landfill have permitted the identification of 130 taxa belonging to 39 families.These taxa were distributed as follow: 68 and 87 taxa were recorded on the old waste dumpsite and the control area respectively.The taxonomic richness and the diversity of the two zones investigated were not significantly different (Mann-Whitney U test: p> 0.05).The high diversity index values reveal the trend of a high floristic diversity.This result could be explained by cessation of waste spreading on the old waste dumpsite that has permitted the transformation of the waste into stabilised humus as on the control site.The relative low taxonomic richness encountered on the old waste dumpsite may be explained by the pressure on the surface: presence of pollutants and gas.According to Maurice et al. (1995), taxonomic diversity decreased with landfill age.Moreover, the Sorensen coefficient indicates a difference between the plant communities established on the two areas investigated.This difference could be due to the local conditions such as temperature, drainage, nature of soils and gas emissions (Nagendra et al., 2006).The presence of elevated number of exclusives plants families and taxa could also be due to these specific conditions prevailing over the sites.However, 20 families and 25 taxa were common to the two studied area.Plants presenting ruderary characters (Aké-Assi, 2001, 2002) may explain their presence on the two sites.
Relative to the plants capacities to contribute to metals extraction in polluted soils, more than 400 species covering over 45 families have been identified as hypercaccumulators (Salt et al., 1998;Reeves and Baker, 2000;Dushenkov, 2003).Among these families, were some collected on the Akouedo landfill (Amaranthaceae, Asteraceae, Commelinaceae, Convolvulaceae, Cyperaceae, Euphorbiaceae, Fabaceae, Lamiaceae, Malvaceae, Poaceae, Portulaceae, Rubiaceae, Solanaceae, Tiliaceae and Urticaceae).Moreover, over 60% of the families recorded on the old waste dumpsite were also classified as phytoaccumulators.The presence of these plant families on this site could be explained by the suitable climatic conditions.On the other hand, their presence on the control site could be due to the transportation of the plants grains on the study area.Ensley et al. (1997) indicated that the family of Euphorbiaceae was predominant in tropical area.Among these families, the Fabaceae was considered by Thangavel and Subbhuraam (2004) as the first hyperaccumlators.
According to Prasad and Freitas (2003), the dominant hyperaccumlators families included Asteraceae, Brassicaceae, Caryophylaceae, Cyperaceae, Cunouniaceae, Fabaceae, Flacourtiaceae, Lamiaceae, Poaceae, Violaceae and Euphorbiaceae.On the old waste dumpsite, six (Asteraceae, Cyperaceae, Fabaceae, Lamiaceae, Poaceae and Euphorbiaceae) of these dominant families were recorded.Most plant species grown on this site present capacity to tolerate heavy metals accumulation (Kouame et al., 2006).The high abundance of phytoaccumulators on this site demonstrates this capacity.In this group, Ni phytoaccumulators constitute more than 80%.The dominance of Ni phytoaccumulators could be explained by their proportion in the world.In fact, Thangavel and Subbhuraam (2004) noted that over two thirds of the species recorded are recognized as Ni hyperaccumulators.They represented approximately 300 species according to Prasad and Freitas (2003) and Prasad (2004).Regarding the taxa identified, several studies have confirmed their phytoaccumulation capacity (Prasad, 2001).These plants (A. sessilis, A. spinosus, A. viridis, Bidens pilosa, C. odorata, C. dactylon, C. rotundus, Echinochloa crus-galli, E. indica, P. maximum, P. oleraceae) were suitable for phytoextraction of Cd, Zn, Fe, Pb and Cu contaminated soils.

Conclusion
The Akouedo landfill presented high floristic diversity.In total, 130 taxa were distributed into 39 families have been listed.The most frequent families were Poaceae, Euphorbiaceae and Cyperaceae.The dominant taxa (F ≥ 50%) on the old waste dumping site with high average density (≥ 5 plants/m 2 ) and occurred less frequently (F ≤

Figure 1 .
Figure 1.Sampling plots location on the study area.

Figure 2 .
Figure 2. Taxonomic richness of families grown on the waste dumpsite (AD) and on the control site (SC).

Figure 3 .
Figure 3. Variation of the taxonomic richness (A) and Shannon diversity index (B).

Figure 4 .
Figure 4. Average density of dominant taxa on the waste dumpsite (AD) and rare on the control site (SC).

Table 1 .
Plant families identified with heavy metals phytoextraction capacity.

Table 2 .
Plant species identified with heavy metals phytoextraction capacity.

Table 3 .
Plants inventoried on the old wastes dumpsite (AD) and on the control site (SC).