The preponderance of oil pollution has a major environmental concern in many
countries and this has led to a concerted efforts geared towards providing various
possible methods of remediation. There are several remediation technologies
suited for cleanup of oil in polluted ecosystems. The most widely used procedures
are the physical and chemical methods. These methods are however not favourable
because of the introduction of poisonous substances into the environment (Davis
and Wilson, 2005). The use of biological methods of remediation which are
more environmental friendly, therefore becomes imperative. A number of factors,
such as site physicochemical conditions, inherent microbial population, as well
as nature and type of contaminant present, determine the suitability of a particular
bioremediation technology for a specific use. Some remediation technologies
involve either stimulation of resident microbial populations by nutrient modifications
or by the introduction of exogenous biodegrading microbes into a contaminated
site. In other to enhance the efficiency of a bioremediation technology, soil
is usually substrate-amended to increase microbial activities. A soil amendment
is any material that is added to a soil in order to improve its physicochemical
properties (Davis and Wilson, 2005).
Other remediation technologies include phytoremediation or the use of plants
to clean up the environment as well as Mycoremediation which employs fungi to
biodegrade contaminants (Cunningham et al., 1996).
Plant species required for current phytoremediation techniques are usually expected
to be found within and around the zone of contamination. However, a possible
drawback in the successful application of phytoremediation is the viability
of plants. A clear understanding of plants stress responses defines their
potential for tolerance of a wide range of contaminants.
Indispensable tools in soil remediation are plants. They are especially important
when the contaminant in its present concentration is not phytotoxic (Merckl
et al., 2004). This is even more importance given the activities
of diverse microbial populations in the rhizosphere. Frick
et al. (1999) reported that the activities of soil microorganisms
may be responsible for most of the biodegradation of petroleum hydrocarbons
in vegetated soils. The occurrence of plants under specific climatic conditions
(Banks et al., 2003) as well as tolerance of
inherent phytotoxic pollutants (Kirk et al., 2002)
are criteria for selection of suitable plants for phytoremediation of soils
contaminated with organic compounds. Other bases for plant selection include
the presence of organic compounds in the plants root exudates (Liste
and Alexander, 1999) or the plants capability to reduce the pollutant
concentration in soil. Aprill and Sims (1990), Anoliefo
et al. (2006) and Ikhajiagbe and Anoliefo (2010)
have employed grasses and legumes in the phytoremediation of oil contaminated
soils. The present study however employs the natural ability of the soil to
remediate itself through natural attenuation and through the help of resident
plant species in the soil seed bank. The researchers wish to investigate the
possibility for any change in the diversity of the resident weeds grown from
soil seed bank of oil polluted soil as influenced by substrate amendment as
well as intervals of exposure of soil to natural attenuation.
MATERIALS AND METHODS
The present study was conducted in April 2010, spanning through a period of
15 months. Soil used was collected from an area measuring 50x50 m marked on
a farmland on the main campus of the University of Benin, Benin City. Top soil
(0-10 cm) was collected and air-dried to constant weight. Prior to collection
of soil from the farmland, a list of all available weed species was made with
a view to determining the soil seeds bank (Table 1). Thereafter,
10 kg soil each was placed into 50 large perforated 60 cm-diameter bowls with
8 perforations made with 2 mm diameter nails per bowl. WEO was obtained as pooled
from an auto-mechanic workshop.
||Weeds identified in the area where soil was collected for
the present study
The soil in each bowl was then contaminated with WEO at 5 different levels
of pollution: 0, 1.0, 2.5, 5.0 and 10.0% w/w WEO according to the methods of
Ikhajiagbe and Anoliefo (2010).
The entire set up was left for 5 months, without mechanically disturbing the
soil. Soil was carefully irrigated twice every week with 200 mL of water. After
5 months, a list of weed species that sprouted in the bowls were identified
and counted. The entire set up was then divided into two. In the first set,
3 kg of soil was removed from each bowl and replaced with 3 kg air-dried sawdust
from Brachystegia nigerica. The bowls in the second set were left unamended
for the remaining period of the experiment. The setup was left for an additional
10 months, after which a second list of emerging weed species were identified
and counted. Weed biodiversity studies were therefore computed. Heavy metals
as well as total hydrocarbon contents of the soil was determined at both 5 and
15 months after pollution in the amended and the unamended soils, following
the methods of APHA (1985).
Computation of weed biodiversity studies: Biodiversity of weeds was computed using the formulas below. Only weeds that were >3 cm high were counted.
||total number of species
||total number of individuals
||number of individuals in the ith species
Species richness indices:
This index gives the level for which a plant population consists of several species in cohabitation.
The index varies between 0 and 1, where E = 1 gives the situation when all species are equally abundant.
Simpsons dominance indices:
The index varies between 0 and 1 and gives the probability that two individuals drawn at random from a population belong to the same species.
There were 19 different species of weeds found in the areas of soil collection for the experiment (Table 1). Prominent among the plant families were Asteraceae, Poaceae and Euphorbiaceae in that order.
Heavy metal composition of soil at 5 MAP as well as at 15 MAP which is 10 months after substrate amendment, is presented on Table 2. There was general reduction in heavy metal composition of soil during the two periods. The impact of substrate amendment enhanced further utilization of the heavy metals. In the oil polluted soil treatments, Total Hydrocarbon Content (THC) of soil ranged from 3028.42-8521.12 mg L-1 at 5 MAP compared to 786.18-1223.23 mg L-1 at 15 MAP in the unamended soil and 283.5- 926.23 mg L-1 at 15 MAP in the substrate amended soil.
After 5 months of exposing the polluted soil (Table 3) to natural environmental conditions, the following weeds were identified on the 5 month old polluted soil; Brachiaria deflexa, Eragrostis tenella, Euphorbia heterophylla, Panicum maximum and a few unidentified (<5 cm tall) plants were present in the 5 month old unpolluted soil (control). These plants must have developed from the soils seed bank. Most of these were of the family Poaceae, apart from E. heterophylla which is from the Euphorbiaceae family. E. heterophylla and a couple of unidentified plants were found at the lowest pollution level (SP1) whereas, 4 unidentified plant species were also found in SP2.5.
Table 4 provides computation for diversity and dominance indices for weeds at 5 months after pollution. Table 5 however describes diversity and dominance indices for weeds identified at 5 MAP. In the control (SP0) the species richness was 1.276 as against 0.514 in SP1. Shannon-Wiener index was 0.609 in SP0 and 0.260 SP1. Evenness index was given as 0.886 and 0.864 in SP0 and SP1 respectively. Dominance index was given as 0.274 and 0.592 in SP0 and SP1 respectively.
Weed distributions at 14 MAP is presented in Table 6. The following weeds were present in the A conyzoides, A. hispidum, E. hirta and E. heterophylla.
||Heavy metal composition and total hydrocarbon content of waste
engine soil-polluted Soil at 5 and 15 months after pollution with or without
substrate amendments, respectively
|nd: Not determine
Those of 2.5% polluted soil include: P. amarus, S. anthelmia, T. procumbens and P. polystachyum.
Computation for diversity and dominance indices for weeds at 15 months after pollution, with or without substrate amendment is provided in Table 7. Species richness decreased according to intensity of pollution, from 2.552 in SP0 to 1.365 in SP2.5 in the soil remediated by natural attenuation (SP) at 15 MAP (Table 8).
Weeds in the soil remediated by soil amendment (SSP) had species richness ranging
from 0.869-2.760. Shannon-Wiener index in the unamended soil treatment ranged
from 0.569-0.993 in decreasing index according to increased pollution intensity.
||Weed distribution in a 5-month old naturally attenuated remediated
waste engine oil-polluted soil
SSP-treatment gave Shannon-Wiener index ranges of 0.301-0.897 (Table
8). By comparing the results of corresponding levels, Shannon-Wiener indices
for SP- levels were comparatively higher relative to corresponding levels of
SSP, though no indices were determined for SP5.0 and SP10.0.
evenness indices for the unamended soil treatment levels ranged from 0.933-0.974
and 0.895-0.999 for the substrate-amended soil treatment levels.
||Computation for diversity and dominance indices for weeds
at 5 months after pollution
|fi: No. of each weed species present per treatment, N: Total
No. of individuals, ni = No. of individuals in the ith species, pi = ni/N
||Diversity and dominance indices for weeds present in the naturally
attenuated WEO-polluted soil at 5 months after pollution
|NA: Not available
|| Weeds distribution of waste engine oil-polluted soil at 15
months after pollution, with or without substrate amendment
||Computation for diversity and dominance indices for weeds
at 15 months after pollution, with or without substrate amendment
|SP: Unamended, SSP: Substrate-amended, fi: No. of each weed
species present per treatment, N: Total number of individuals, ni: No. of
individuals in the ith species, pi: ni/N
Comparatively, the unamended soil treatment levels gave higher evenness indices compared to the amended ones. Generally, the order of decreasing E was as follows; SSP10 > SP1.0 > SP2.5 >SP0 > SSP5.0 >SSP0
Dominance indices ranged from 0.137-0.284 in the unamended soil treatments and 0.160-0.500 in the substrate-amended soil treatment levels. Generally, dominance indices in each method of remediation increased with increasing levels of pollution. Orders of decreasing dominance indices include SSP10.0>SSP2.5 >SSP5.0 >
> SP0. dominance indices were not determined for SP5.0 and SP10.0. Simpsons index was highest in SSP2.5 (0.356) and lowest in SP0 (0.112).
The presence of heavy metals in soil raises a lot of environmental concerns.
This is particularly important in their relationships with plant growth. They
render the land unsuitable for plant growth thereby destroying the biodiversity.
The availability of metals and metalloids in soil to plants, to a large extent,
depends on the selective absorption from soil solution by plant roots. These
metals and metalloids may be bound to exterior exchange sites on the root and
not actually taken up. Efroymson et al. (1997)
however observes that these metals may enter the root passively in organic or
inorganic complexes or actively by metabolically controlled membrane transport
systems. Although several regulatory steps have been implemented to reduce the
release of pollutants in the soil, they are not sufficient for checking the
contamination. However, plant resistance to these pollutants has been demonstrated
in previous studies by Wong and Chu (1985) in Cynodon
dactylon, Anoliefo and Edegbai (2000) in Solanum
melongena and S. incanum, Dede et al.
(2003) in Celosia argentea and Ikhajiagbe and
Anoliefo (2010, 2011) in Vigna unguiculata.
Because of plants differential response to pollutants, such unhealthy environmental
practices as improper WEO disposal would affect the distribution of plants species
over time and space in affected areas.
In the present study, although reduction in metal concentration in soil occurred
even before the emergence of weeds on soil surface, remediation might have taken
place by a number of factors other than by green plants. One of such is the
possibility for dissolution of water soluble heavy metal compounds by soil water
and consequent translocation into ground water. Another way is by chelation.
In spite of this, Phytoremediation however lends greater benefit for heavy metal
remediation. Identification of plants species that are prevalent in pollutant-contaminated
sites is prerequisite to finding out their inherent capacity for tolerance and
potential for phytoremediation. At 5 MAP, Brachiaria deflexa, Eragrostis
tenella, Euphorbia heterophylla and unidentified weeds (<5 cm tall) were
present in the control (SP0) and Euphorbia heterophylla in
SP1. At 15 MAP, Euphorbia heterophylla was the most dominant
weed (present in both SP and SSP), with 15.11% occurrence, followed by Phyllanthus
amarus and Tridax procumbens (8.63%) and Panicum maximum (7.91%).
|| Diversity and dominance indices for weeds in waste engine
oil-polluted soil at 15 months after pollution, with or without substrate
|NA: Not available, SP: Unamended, SSP: Substrate-amended
Comparing species richness and diversity of weeds present in the soil treatments
at 15 MAP, species richness was better in the controls. Plant diversity was
higher in the unamended soil than in the substrate-amended soil treatments.
However, species dominance was higher in unamended soil treatments. There are
several plant characteristics that exclude species for possible use in phytoremediation
which need consideration. For example, T. procumbens is an undesirable
invasive weed that can replace natural pastures, a characteristic that could
cause conflicts with farmers in surrounding of remediation site and additional
costs for weed control (Merckl et al., 2004).
Anoliefo et al. (2006) identified a number a
plants in an oil-polluted auto-mechanic workshop, suggesting therefore that
these weeds could have a tolerance for oil. These weeds included Tridax procumbens,
Acanthospermum hispidum, Euphorbia heterophylla, Eragrostis tenella,
Panicum maximum and Fleurya aestuans. The capability for Talinum
triangulare, Celosia trigyna, Corchorus olitorius, Vernonia amygdalina and
Telfairia occidentalis as well as grasses like Eleusine indica, Cynodon
dactylon, Panicum maximum, Euphorbia hirta, Chromolaena odorata for recovery
of heavy metals from soil has also been reported (Wong and
Chu, 1985; Wong and Lau, 1985; Anoliefo
and Vwioko, 2001). A number of these weeds have been identified in the present
study (Table 3).
Current rate of urban development has informed the decision for a number of contaminated sites being converted to parks and other low-intensity public uses. This usually would include landscaping the sites with grasses or other weeds of aesthetic value. These sites, particularly with their greater flexibility in the timing and design of cleanup, frequently offer significant ecological opportunities. Business corporations traditionally landscape their premises with grasses for aesthetic reasons and storm run-off control. These functions may be combined with phytoremediation to offer significant opportunities. When properly designed and located, such landscaping could also provide long-term management and enhanced ecological habitats. A site owner may be willing to significantly expand the land committed to phytoremediative landscaping if that commitment would reduce overall cleanup costs and allow quicker site redevelopment. A phased approach, with intensive short-term treatment by one plant species followed by permanent plantings with more beneficial vegetation, may maximize ecosystem benefits.
The present study shows that amending the soil offers greater opportunities for increased weed diversity, thereby enhancing phytoremediation. A soil amendment improves the soils physical properties, such as water retention, permeability, water infiltration, drainage, aeration and structure. It also makes for better microbial action on the oil pollutant. Specifically, however, to assess the appropriateness of any phytoremediation technology, site-specific and contaminant-specific field data must be obtained in other to show the rate and extent of degradation or accumulation. Currently there is no industry or research consensus on which parameters are crucial to measure for most phytoremediation projects. Specific data are therefore needed on more plants, including plant presence and tolerance mechanisms in pollutant-contaminated soils, nature and mode of action of contaminants and climate conditions to enhance the current knowledge base. This must also include the standardization of monitoring systems.
The authors are thankful to Raw Materials Research and Development Council, Abuja, for the research grant.