Pesticides Xenobiotics in Soil Ecosystem and Their Remediation Approaches

Abstract

Globally, the rapid rise in the human population has increased the crop production, resulting in increased pesticide xenobiotics. Despite the fact that pesticide xenobiotics toxify the soil environment and ecosystem, synthetic pesticides have increased agricultural yields and reduced disease vectors. Pesticide use has increased, resulting in an increase in environmental pollution. Various methods of controlling and eliminating these contaminants have been proposed to address this issue. Pesticide impurity in the climate presents a genuine danger to individuals and other oceanic and earthly life. If not controlled, the pollution can prompt difficult issues for the climate. Some viable and cost-effective alternative approaches are needed to maintain this emission level at a low level. Phytoremediation and microbial remediation are effective methods for removing acaricide scrapings from the atmosphere using plants and organisms. This review gives an overview of different types of xenobiotics, how they get into the environment, and how the remediation of pesticides has progressed. It focuses on simple procedures that can be used in many countries. In addition, we have talked about the benefits and drawbacks of natural remediation methods.

1. Introduction

Chemical substances, including several pesticides, are extensively used in the agricultural ecosystem to reduce the insect-pest attack to minimize the losses in crop production and to control human diseases induced by vector insects [1,2,3,4]. The uncontrolled and continuous application of pesticides has emerged with their persistence in the environment in places such as water reservoirs, agricultural soils, and food crops implanting toxic impacts on human health [5,6,7,8,9]. In the early days, the increase in crop yield was a major aim, and the application of insecticides was high in order to reduce insect attacks in the agricultural farming system. The induction of residues of pesticides in the food chain, and their drastic influence on human health, were not thought to be a major concern in those decades [10]. Many chemicals were applied as insecticides without determining their persistence in ecology and their toxic effects on the upcoming era [11]. In addition, these chemicals started to accumulate in the environment, such as being the part of soils and sediments, water bodies, and groundwater resources. By accompanying the basic reservoirs of the food chain, the residues started their addition in the food web [5,12,13,14].

Persistent organic pollutants (POPs) are identified as hydrophobic in nature, which stimulates their accumulation in adipose tissues of living organisms, supporting them to bio-magnify easily and cause health problems over time [15,16,17]. Physico-chemical processes (composting, burning, chemical amendments, and landfilling) are effective approaches to remediate soils contaminated with POPs [5,18,19]. These methods are classified as ex situ methods and are of high cost due to their demanding excavation and transportation. During their use, the soil is removed from one place and transported to the other place, so it is also a destructive system. To overcome the losses induced by ex situ methods, the use of in situ remediation techniques is highly recommended to detoxify pesticide contaminated soils. In situ methods are cheaper, invasive, solar-driven, and require fewer management techniques [8].

Bioremediation technology was identified as an effective technique to decontaminate pesticide-contaminated soil and is mainly associated with plants (called phytoremediation) and microbes (bioremediation). While utilizing these technologies, only those naturally grown microbes and plants that have the capacity to adopt different mechanisms are used, including storage, use in metabolic activities, and sometimes for co-metabolism [20,21]. It is a complicated and difficult task to discover the microbial mechanisms and their effective use against each contaminant. The use of remediation technologies can be altered depending on the nature of the contaminant and its physico-chemical properties [22]. So, the selection of remediation techniques against a contaminant varies either by microbes or by plants, and mainly focuses on growth conditions in contaminated soils. Sometimes, the symbiotic relationship between plants and microbial species is found to be suitable for better decontamination. This method has increased the remediation efficiency significantly against POPs and other toxic substances [23].

The aim of the current review is to discuss remediation techniques associated with plants and microbes having the best output against pesticide contamination in the soil environment. Some important remediation species have been identified in the current review. Moreover, the problems to be faced in the future and their perspectives have also been discussed below.

2. Types of Pesticide Xenobiotics

The chemicals that are used to affect the growth and metabolism of living organisms are collectively called pesticides, including three main classes: fungicides, herbicides, and insecticides [24] (Figure 1). The pesticides are further divided into sub-groups, i.e., insecticides contain organo-chlorines, organophosphates, carbamate esters, pyrethroids, acetamides, triazoles, triazines and neonicotinoids [25]. These sub-groups depend on their hydrophobic properties, molecular structure, ionic or nonionic properties, and mode of action [26]. While discussing contamination induced by pesticides, organochlorines have a higher tendency to generate contamination and are a matter of concern for environmental scientists. DDE (dichlorodiphenyldichloroethylene), DDTs (dichlorodiphenyltrichloroethane), HCHs (hexacyclochlorohexanes), chlordane, and DDD (dichlorodiphenyldichloroethane) are common pesticide classes [7].

A group of pesticides was named the “Dirty Dozen” depending upon their higher persistence in soil environments, and among them, eight are organochlorines. During the convention, they were classified as highly persistent toxins and their use was banned with the recommendation to identify best remediation techniques to reduce their residues. The use of DDT was banned a long time ago, but still, its residual concentrations are available in low or high concentrations in soils [27]. The contaminants are highly hydrophobic in nature, and a significant change in their weathering was observed after many decades. Their log KOW (n-octanol/water partition coefficient) value varies from 5.5 to 6.9 [28]. The concentration of DDT was high, mainly in adipose tissues due to its lipophilic and hydrophobic nature. In these tissues, it can magnify easily and become part of the food chain, inducing toxic impacts on endocrine disruptors in mammals and eggshell thinning in birds [29].

Atrazine, commonly known as herbicide, has a notable contribution to environmental pollution [7]. It works as an inhibiting agent for the photosynthetic process and is applied to control annual weeds (broad-leaved weeds) that favour its higher use, resulting in environmental pollution [30]. The residual contents of atrazine were identified in groundwater resources and nearby water channels due to seepage and drainage. Atrazine is classified as an endocrine disruptor and its contamination induces disturbances in the development of amphibians [31,32]. Fungicides hexachlorobenzene (HCB) and pentachlorophenol (PCP) are major compounds with toxicological properties because HCB is hydrophobic and can easily bio-accumulate in soil, water, fish, birds, and human milk [33]. The production of HCB was stopped and banned in the 1970s, but it is being released as a by-product of simazine [34]. In contrast to the above discussion, less knowledge is available regarding the key role of fungicides as contaminating agents. To identify this, more studies are needed.

3. Impacts of Pesticides on Ecosystem

The activities of toxic elements, including pesticides, are dependent on the natural occurring reactions in the environment. A complex series of reactions, including chemical, biological, and physical, takes place when a pesticide particle interacts with a soil particle, water molecule, or a living organism [26]. Major processes involved in pesticide transformation in the soil environment include the transport of pesticide from one point to the other point. During this transportation process, the pesticide particles move from their original area to the nearby environment, i.e., watercourses and groundwater bodies, while the transfer of pesticide through a certain environment, such as biota, sediments, water, and atmosphere, is considered a transfer process. In the transformation process, the pesticides are changed into fractions and forms depending on their binding capacity, release behavior, and biodegradability in the soil environment. These processes depend on soil properties, contaminant properties, and environmental factors. Soil contaminant uptake and degradation rates are influenced by a variety of factors (Figure 2). Soil moisture, organic and mineral content, pH, temperature, physical and chemical properties of the soil are among these factors. The presence of soil microbes is an important factor in the decomposition process. On the other hand, these soil microbes depend on the environmental conditions of the soil to grow and destroy pesticides in the soil [35].

The study [36] describe bioremediation as a complicated and sensitive process that requires consideration of soil factors and characteristics. For instance, a decrease in soil pH increases the uptake of ionizing pesticides (such as picloram and atrazine) [37]. Important mechanisms involved in pesticides abundance in soil ecosystem are as:

• Pesticide transportation: The movement of pesticides from their point of origin to other parts of the environment is considered transportation [26]. It has been observed that most pesticides are applied through spraying, which can become part of the air through partial evaporation. Similarly, these contaminants can evaporate from the soil particles and plant body surface. After the application of pesticides and their reaction with colloidal particles, some of the contaminants do not get fixed with soil particles and become free to move to nearby watercourses and to the underground water sources, causing contamination in those bodies of water.
• Pesticide diffusion: The diffusion mechanism of pesticides between water, biota, soil, and atmosphere is also important regarding their movement in the environment [26]. Pesticides are mainly applied in solid and liquid formulations, but there are possibilities for their volatilization. Applied solid and liquid-phase pesticides are converted into a gaseous phase that will be part of the air [38,39]. With the help of air, these contaminants can move too far into areas from their point of application and can cause environmental pollution in large areas. Pesticide residues can get dissolved in water, which will move from soil to water bodies and will cause an accumulation of contaminants in watercourses. Residues can also be dissolved in rainwater and tend to leach into dissolved formulations. The leaching process is mainly dependent on the chemical properties of applied pesticides and the geological conditions of the area. Pesticide residues that are part of water, soil, and the atmosphere can easily be transferred to human beings and other living organisms. More information about the mechanisms of contaminant transfer in plant bodies and to microbial communities will be discussed further below.
• Pesticide conversion: Conversion is also an important mechanism that can influence the presence of pesticides in the environment [26]. Conversion of pesticides is mainly referred to as degradation of pesticides with the oxidation-reduction processes that can reduce their toxicity level. Pesticides applied to the environment can easily go through other conversion processes such as degradation. Upon application, the pesticide molecules encounter enzymes and microbes that break down their structure with the help of some chemicals and enzymatic activities. In addition to living organisms’ role in breaking down toxic substances, abiotic factors can also degrade pesticides. Photo-degradation can also take place upon exposure of these contaminants to sunlight. Microbial conversion is a highly influential remediation technique for the degradation of pesticide molecules. Further details will be discussed below as well.

4. Remediation of Pesticides Contaminated Soils

The soil and water environment are also contaminated by many pesticides (

Table 1. Classification of pesticides.

Pesticides	Groups
DDT, HCH, Cyhalothrin, Cypermethrin, Chlorpyrifos, Fenpropathrin, Carbaryl, Ethion, Cyanophos, Thiamethoxam, Bifenthrin, Methylparathion, Monocrotophos, Phorate, DDE, Fenamiphos, Naphtalene, Coumaphos, Diazonin, Parathion, Aldrin, Dieldrin, Heptachlor, Heptachlor epoxide, Fenitrothion, Permethrin, Fenvalerate, Dimethoate, Malathion, Endosulfan, Quinalphos, Profenos, Triazophos, Monocrotophos, Chlordane, Methoxychlor, Profenofos, Pentachlorophenol, Azinphosmethyl, Phosmet, Terbufos, Aldicarb, Phoxim, Endosulfan sulphate, DDD, Endrin, Gamma-cyhalothrin	Insecticides
Atrazine, Metolachlor, Butachlor, Metribuzin, 2,4-Dichlorophenoxyacetic acid, Napropamide, Trifluralin, Sulfentrazone, Alachlor, Acetochlor, Tertbutryn Propisochlor, Glyphosate, Simazine, Linuron, Tribufos, Clofibric acid, Diclofop-methyl, Pendimethalin, Terbuthylazine, Flazasulfuron, Picloram, Isoproturon, Cycloxidim, Napropamide	Herbicides
Acibenzolar-S-methyl, Propamocarb hydrochloride, Azoxystrobin, Difenoconazole, Copper sulphate, Dimethomorph, Dodemorph, Tridemorph, Hexachlorbenzene, Metalaxyl, Pyrimethanil	Fungicides

). The remediation technologies for detoxification of these systems mainly depend on physical, chemical, and biological approaches. These approaches can be used singularly, as well as in their combinations [7,40,41]. Contaminated soils are treated by excavation of contaminated areas and their transportation to specialized prepared landfills. Decontamination processes can be carried out sufficiently by on-site and off-site remediation techniques [20]. These techniques help in better detoxification of contaminated areas; but still, there are limitations to their best use. Excavation and transportation of contaminated soils are laborious, time-consuming, and costly as well. When these techniques are applied to the decontamination process, ecosystem disruption takes place and requires years of recovery from this disruption. That is a major limitation as well [42].

The limitation may also occur for the large area because these techniques require a moment of soil, so they are only suitable for a small area of land but not suitable for the large area. So, these are totally unfeasible for larger systems with medium contamination due to higher cost and difficulties in movement [43]. Important techniques that are effective as their alternate techniques involve bioremediation and phytoremediation. These innovative techniques are good for the significant removal of pesticide contaminants from water and soil ecosystems [44]. An overview of these technologies is given below.

4.1. Bioremediation

Microbial use includes bioremediation, which is the partial or complete removal of toxic substances by microorganisms with their conversion into their elemental constituents [21,45,46,47,48]. Bioremediation mainly depends on the availability of pesticide residues and the ability of the detoxifying agent to break down these residues [47,49]. The retention of residues with colloidal particles can significantly make them unavailable to microbes that will affect their degradation [8]. It is an estimation that more than 1 million bacterial cells are present in 1 g of bulk soil, and these cells are from 5000–7000 different species. Similarly, within microbial cells, more than 10,000 fungal colonies are present as well [50]. These communities, especially microbial communities, are highly involved in the detoxification of pesticide contamination in the ecosystem [51]. During the writing of the current review, the studied articles related to bioremediation show that many organisms are involved in the degradation of pesticide residues (Figure 3).

From the bacterial communities, Proteobacteria were identified as the most active organisms against contamination. It was recently noted that 35 bacterial species are involved in the remediation of pesticide contamination, and of these, 21 species belong to the Proteobacteria (11 Gammaproteobacteria, 6 Alphaproteobacteria, and 4 Betaproteobacteria) (Figure 3). Similarly, when discussing pesticide residues at the species level, Pseudomonas sp. was identified as the most active species in the literature. Through reading articles, it is known that these species are involved in the remediation of 25 kinds of pesticides, and their metabolites are significantly high. It was also observed that species extracted from plant bodies, rhizosphere, and bulk soil prove their omnipresence and high adaptation level in any environment. Scientists have paid more attention to bacteria as a source of decontaminant than to fungi. The reason for this big gap could be that fungi are less attributed to pesticide decontamination as compared to bacteria. In the case of fungi, very little data has been published for their use in pesticide residue decontamination (Figure 3). Similarly, scientists are unable to work on this approach due to a lack of research into fungi as a source of remediating agents. Fungus species used in bioremediation techniques are obtained from bulk soil and decaying wood. From the articles, it was found that 13 fungal species are involved in a decontamination process that includes Ascomycota (7 species), Basidiomycota (4 species) and Glomeromycota (1 species) (Figure 3).

From the current review, we have selected broad term acting pesticides (Table 1) that include common analytes, i.e., chlorpyrifos is an organophosphorus in nature with a higher persistence capacity in the soil environment, but many fungal and bacterial organisms have been identified involved in its breakdown [52,53]. Exposure of chlorpyrifos against Serratia sp. showed complete degradability of 100 mg/L in 18 h only [54], while Stenotrophomonas sp. showed similar results in 28 h [55]. Alam Gilani et al. [52] identified 14 strains of Pseudomonas that tend to degrade chlorpyrifos efficiently. Many soils where pesticide application is arried out for crop production are contaminated with pollutants of variable nature [56,57]. Application of bacterial strains to these contaminated soils could be helpful to remediate these areas. Contamination scenarios were remediated with different bacterial consortium strains for degrading deisopropylatrazine and atrazine [58]. It is also common for one bacterial strain to have the tendency to degrade more than one pesticide or group of pesticides, such as [59], who used Bacillus strain for the degrading of propamocarb hydrochloride, acidobenzolar-S-methyl, thiamethoxam, metribuzin, and napropamide. The efficiency of a remediation process depends on environmental factors including, i.e., temperature, moisture content, organic matter content, redox status, and pH [60]. Soil moisture is the soil water content that can influence water availability to degrading organisms as well as change the redox potential that can affect the biochemical degradation reactions significantly [61,62]. Similarly, excessive water content can lead to anoxic conditions that can affect microbial work as well. The results showed that anoxic conditions have an increased degradation process of pesticides as determined for HCH and DDT [63]. Temperature and pH are also important factors influencing the remediation or degradation process [9]. The degradation reactions, like other reactions, are temperature and pH range dependent for ideal productivity. In the case of fenitrothion and fenamiphos, the ideal temperature was kept between 15 °C and 40 °C [60].

The enzyme activities depend on pH as well, and most of the enzymes work elegantly at a pH range of 6.5–7.5 [44]. In addition to enzyme activity, soil pH can change the adsorption and desorption phenomena of abiotic substances. A decrease in pH results in the desorption of pesticide residues adsorbed on colloidal surfaces [64], which can affect bioavailability and remediation rate significantly. Adsorption and desorption reactions are also affected by the concentration of organic matter in the soil environment. While there is higher organic matter content in the soil environment, it can induce strong bonding of pesticide residues and thus make them unavailable to the microbial community. Furthermore, an abundance of organic matter can release many nutrients for microbial food, which will increase their working efficiency for higher degradation of contaminants [65]. In a comparison between inhibition processes and stimulation processes by the addition of organic matter content, results revealed that contamination degradation was increased significantly in benzonitrile by the addition of wheat-derived biochar [66]. Nevertheless, the addition of organic matter content resulted in the formation of hydrophobic compounds that were preferably fixed with a solid phase of organic matter [67]. To increase the availability of pesticide residues fixed with colloidal particles, soil microorganisms release some chemicals that can increase desorption and degradation potential [9,41]. Similarly, release of these chemicals may also induce extra-cellular degradation of contaminants [44]. This part of bioremediation technology is recommended for further research and will also increase the remediation processs efficiency as well because it does not require energy to transport the contaminants in the microbial body. Some researchers have reported successful bioremediation with the use of free enzymes [68,69,70]. Still, there is a need for research to identify field conditions suitability and site-specificity.

4.2. Phytoremediation

Degradation or decomposition of pesticide residues by plants is called phytoremediation. This mechanism is highly associated with the breakdown of residues either in the plant body or in the rhizosphere. It is noteworthy that phytoremediation is also an innovative, ecologically beneficial, and cost-effective technique for the degradation of residues [44]. Like bioremediation, phytoremediation also has a series of processes involved in the breakdown of residual contaminants that may include phytovolatilization, phytotransformation, phytodegradation, and rhizoremediation (Figure 4). By studying these processes, it has been identified that the first three require the entry of residues into the plant body for their degradation, while the fourth process can take place outside the plant body at the rhizosphere or soil-root interface. So, it is a fact that these processes are influenced by plant-associated organisms that can play a key role in the degradation of pesticide molecules [9,23,71].

4.2.1. Phytoremediation Ability of Various Plants

Pesticide accumulation is the base requirement for phytotransformation, phytodegradation, and phytovolatilization. The present knowledge reports that many plants have tendency to accumulate the pesticides efficiently (Figure 4). Zea mays and Cucurbita pepo are among the plant species most commonly used to address the phytoremediation of pesticides. The frequent use of these species is due to their significance in agriculture and gardening, high number of cultivars and good accumulation potential of variety of organic contaminants [72]. In addition, many soil and plant characteristics determined that Ricinus communis also has a potential to uptake and accumulate a huge number of various contaminants [44]. Moreover, pesticide uptake is also influenced by different factors including soil moisture, pH, temperature, organic matter content and the pesticide residues in soils whereas time-dependent decreases in availability can be associated with ageing or weathering of the residues [28]. Plant species are the major factor to influence the potential role of any plant in residual uptake [9,73,74,75,76]. It is important to create optimum conditions in combination of endophytes, soil, and plants to optimize the phytoremediation of contaminated field.

Bouldin [77] tested two plants i.e., Juncus effsus and Ludwigia peploides for their pesticides uptake potential (atrazine and lambda-cyhalothrin) and reported that Juncus effsus uptake higher atrazine and was efficiently translocated to the plant shoots, while the latter one accumulated more lambda-cyhalothrin and showed 98.2% retention in roots. The difference in pesticide uptake and translocation efficiency highly depends upon the type of pesticide and plant characteristics. In addition, the pesticide bioavailability and translocation in plants is highly affected by The log K_OW, or octanol-water partitioning coefficient [78]. Turgut [79] investigated the uptake of atrazine, trifluralin, cycloxidim, and terbutryn in Myriophyllum aquaticum, in which a direct relation of submerged shoot and root concentration factor with log K_OW was observed. The higher polarity containing compounds (hydrophilic) are difficult for the analyte to cross biological membranes, resulting in lower uptake compared to lipophilic compounds [80,81].

Significant interactions between the contaminants may occur in environments contaminated with several pollutants. The interaction among atrazine and Cd²⁺ minimizes contaminants the toxicity on Oryza sativa seedlings and enhance the atrazine translocation to plant tissues [82]. Soil-to-plant interaction plays vital role for the bioavailability of pesticides in plants, furthermore, the deposition from the air was observed in some cases via gaseous phase or contaminants where sorbed particles are subsequently deposited. In [83], study reports chlordane profiles have significant role in its uptake when it is taken up from air or soil. In the case of soil-to-plant uptake, pesticide accumulation potential can be influenced by various plant characteristics including root depth or structure and water uptake potential. Initially, plant root tissues retain the pesticides and then can be either immobilized in the roots or translocated to the aerial parts of the plants where the analytes can be stored, metabolized, or volatilized (Figure 4).

Generally, for the proper rededication the pesticide accumulation in roots is not much effective, whereas, in case of aquatic plant-based remediation systems, the removal of contaminants using plant roots can be significant. Eichhornia crassipes (water hyacinth) has a great potential to accumulate the insecticide ethion its root system as compared to its shoot system. Given that the roots make up over 50% of the plant mass and that the total plant, including roots, can easily be harvested, this system can be efficiently used for the phytoremediation of ethion-contaminated waters [84]. Pesticide molecules once accumulated in plant roots are mostly transported via xylem cells with the help of transpiration stream. The studies have indicated that plant growth mechanisms including cultivability play notable part in the distribution of pesticides within crops and in their species [85]. Various studies [72,86] have mentioned that the uptake of DDE by different C. pepo cultivars and reported shoot bioconcentration factors of up to 23.7 for the Raven cultivar.

Using trees, especially poplar and willow, using this mechanism for phytoremediation and phyto-pumping [87,88]. The harvesting of plant shoot is required after the uptake period when phytoremediation using solely phytoaccumulation is adopted. Furthermore, plant shoot tissues can subsequently be burned, composted, or disposed of by other means [7]. The volatile pesticides such as triflutrin, can be transported to the shoot system and subsequently be volatilized into the atmosphere (Figure 4). This process does not fully solve the unwanted side effect of the pesticides because the result is merely a relocation of the contamination from soil to air; however capturing the plant evaporation could be a solution [7].

Generally, the main objective of effective phytoremediation is not limited to phytoaccumulation but the metabolic breakdown of the contaminant within the plant tissues is also important and it can be done with or without the help of endophytic bacteria. The pesticides are metabolized by the plants into more polar molecular structures that can either be bound as residues in the cell walls or stored in vacuoles [89]. In Brassica rapa, atrazine residues were shown to be incorporated into plant cell walls as hydroxyatrazine (HO-A) [90]. Plant metabolic processes plays an important role in complete degradation of some compounds but most pesticides contain more than one aromatic cycles that are difficult to break naturally. In this scenario, the roles of fungi-or bacteria-enhanced phytoremediation become significant. Various bacteria, fungi, and endophytic microorganisms have pesticide-degrading capacities in soils (Figure 3). Moreover, endophytic bacteria played an efficient role in treating polluted water that is a mixture of fenpropathrin, chlorpyrifos, bifenthrin, and naphthalene. These bacteria reside in the aquatic plants, Potamogeton crispus, Najas marina, Nymphaea tetragona, and Phragmites communis [91]. Spirodela polyrhiza promotes the growth of endophytes that symbiotically contributes in degradation of fenpropathrin in its tissues [92].

Endophytic bacteria has dominated the community due to its beneficial property of pesticide-degrading capacity as compared to the non-degrading competitors [46]. Using endophytes in the phytoremediation process not only has an advantage when it comes to pesticide degradation, but they also often possess plant growth promoting properties. This plant growth promotion is shown through enhanced cycling of nutrients such as nitrogen and phosphate [93]. Endophytes have also been shown to be capable of phosphate solubilization [94,95], indole-3-acetic acid production [96], iron-binding siderophores production [97], and ACC-deaminase production [22]. Furthermore, the plants are also protected by the bacteria indirectly. Symbiotic plant endophytes also outcompete the pathogen within the microbial community and contributes in preventing or reducing the deleterious effects of certain pathogens [98].

4.2.2. Rhizoremediation Process

Pesticide degradation is greater in plant-associated soil or rhizosphere soil as compared to bulk soil, and this phenomenon can be written off as a “rhizosphere effect” [44]. Rhizosphere soil is nearly attached to plant roots and movement of roots, as well as the release of plant-associated chemicals that have influenced rhizosphere soil. The influenced rhizosphere soil converts into a complex environment that includes metabolic reactions that induce microbial communities and the volume of bulk soil. These microbial communities in rhizosphere soil can be 10–100 times more diverse than those in bulk soil or uncultivated land [7,23]. Sismilarly, it was identified that the concentration of these microbial cells was increased up to 1012 cells/g soil [99]. It can be concluded that the abundance of this rhizospheric community can influence the bulk soil nearby the rhizospheric region considerably and can enhance the microbial community and degradation of pesticides in the surrounding areas [100]. A microbial community present in the rhizosphere can produce a beneficial compound called 1-aminocyclopropane-1-carboxylate (ACC) deaminase [22], which is a precursor of the stress hormone, ethylene, that is produced in contaminated soils by plants. The action of ACC-deaminase causes the breakdown of ACC, which results in a lower ethylene level and the plant facing less stress against contaminants. This will help the plant with better growth, development, phytoremediation potential, and a reduction in plant toxicity.

The above discussion shows that microbes are highly specialized to facilitate nutrient availability for plant growth as well as their development [101]. They protect plants from pathogen attack by generating competition with pathogens. They are also helpful for plants as they reduce the toxicity of contaminants before they influence the plant’s body [102]. Rhizoremediation is a naturally occurring process but was improved by planting specific plant species and by the addition of suitable pesticide-degrading bacteria. Plants tend to release some allele-chemicals that are involved directly in degradation [102] and provide photosynthetic products to microbes as their food. In previous studies, many scientists have inoculated rhizospheric bacteria that have the capacity for the breakdown of contaminants (Figure 3). Higher degradation was observed when Holcus lanatus and Cytisus striatus were inoculated [103]. Similarly, a 50% increase in the remediation of chlorpyrifos was observed when the plant roots were inoculated with Bacillus pumilus C2A1. Successful degradation of phoxim was identified when carrots (Daucus carota) and green onions (Allium fistulosum) were inoculated with the arbuscular mycorrhizal fungi Glomus intraradices and Glomus mosseae [104]. In 7 days, the fungus Didemnum ligulum degraded phoxim (50 mg L⁻¹) in a liquid medium [105]. Out of 17 tested species, 16 white rot fungi were found to be capable of degrading pesticides including terbufos, parathion, tribufos, phosmet, and azinphos-methyl [106]. Biodegradation is highly dependent on pH, organic matter content, and temperature at the site. Rhizosphere properties are suitable for degradation due to plant-released hormones. So, degradation in the rhizosphere is more efficient than in bulk soil [44]. Pesticide runoff movement is non-negligible, but plants and microbes can interact and stop the movement to nearby watercourses [7].

4.3. Use of Bioaugmentation

Bioaugmentation is classified as green technology and is known as an improvement in the degradation capability of microbial communities by applying them in specific contaminated areas. Specific kinds of microbial consortiums can enhance breakdown significantly. This technology is mainly adopted in areas where native microorganisms do not have significant degradation of toxic complexes or insufficient degradation occurs [104,105,106]. It mainly depends on the enhancement of the catabolic potential of the microbial community for reclamation of contaminating agents. To achieve this goal, inoculation of desired microbial or fungal species can be accomplished. In addition to these, genetically modified and engineered organisms or micro-organisms (GMOs, GEMs) are highly suitable for degradation and enhanced bio-augmentation processing (Figure 3 and Figure 4). Appropriate strain selection is among the vital factors that influence bioaugmentation, so while planning for its use, this should also be taken under consideration [107,108,109,110,111]. To select a strain, the following factors should be noted: Strains should have a high potential for degradation of available contaminants, and the growth pattern of selected strains should be fast. The cultivation of strains has a higher ability to survive under higher contaminant concentrations. Moreover, the strain can easily survive under a wide range of harsh and suitable environmental conditions.

The effective use of microbial communities in bioaugmentation for pesticidal contamination has been studied several times [9,51,60,107] (Figure 3). Significant results obtained from these studies have encouraged scientists to identify new microbial species for better and faster degradation of pesticidal contamination in the soil environment [112,113]. To isolate the best strains for bioaugmentation, a collection of micro-organisms from contaminated soils is considered a good source. For a more efficient role in bio-augmentation of a microbial consortium, it is extremely important to identify their detoxification potential for pesticides under liquid media conditions.

5. Natural Remediation Technologies: Associated Benefits and Risks

Natural remediation technologies, including bioremediation, have shown many advantages that induce a preference to use them in the remediation process. The use of these technologies promotes the utilization of plant and microbial metabolic mechanisms that encourage them to utilize organic substances and their conversion to non-toxic substances like water and carbon dioxide. These technologies are suitable and highly attractive as they have no chemical application and restore the environmental substances with natural ability. The major advantage of such techniques is that they allow in situ use without inducing disturbance in an environment that reduces excavation and transportation. This ultimately tends towards lower costs for transportation and labor needs. Combining Bio and Phyto remediation techniques has advantages for nutrient supply and protection enhancement for each other. Establishing a combined growth pattern for microbes and plants is easy due to the above-mentioned correlation that provides a sufficient advantage over the sole growth of microbes where they have to compete for food with other microbial and fungal species that abrupt their role for remediation. So, it could be concluded that the development of remediation mechanisms in plant-grown soil is much easier as compared to bulk soil.

Moreover, the plants grown during phytoremediation have a potential to be used for green energy purposes as well as it provides stabilization to the soil. Such technologies have shown dominant advantages, but some drawbacks are also observed. All contaminants are not susceptible to being treated for biodegradation. The pollutants to which remediation techniques need to be applied could have harmful impacts on plants and microbial species that can deteriorate remediation processes. In addition to the above, sometimes partial degradation of the contaminant may occur that could generate compounds with a higher toxicity level than the original compound. Although bioremediation techniques are cheap as compared to classical techniques, there is still a need for monitoring of microbial development and efforts are required to know the site-specific behavior and remediation speed of each contaminant. There is no degradation of a contaminant in the plant body during phytoremediation, so the toxic compound can be moved by evapotranspiration and recycled with dying tissues of plants that can be released easily back into the environment and natural bodies.

The time used for remediation techniques is also included as de-merit because there is a need to monitor the soil for a long time. In these cases, it can be said that remediation can be changed at any time due to changes in the soil environment. In the traditional way of thinking, scientists concluded bio and phytoremediation were slow processes [102], but some studies contradict this conclusion. A case study was carried out to remediate BTEX plume contamination by using poplar trees, in which the cost-effectiveness of phytoremediation was compared with classical remediation strategies such as pump-and-treat. It was concluded that both phytoremediation and pump-and-treat achieved the remediation goal within 1 year, and the cost for phytoremediation was also low [114]. So, it can be concluded that with changing scenarios, bio and phytoremediation techniques become cost-effective.

Impact of pesticide residues on environment and human health has been is a worldwide regulated mainly for human exposure via ingestion, inhalation, and dermal contact, and regulatory acts with standard values for pesticides in air, drinking water, agricultural produce have been released since many years. Until now, for soils only some countries have residual limits for urban soils, whereas pesticide regulatory values for agricultural soils are scattered, even for the currently banned pesticides (e.g., DDT, HCH, atrazine, dieldrin). Therefore, their impact on soil are estimated by their theoretical persistence, based on worst case scenarios, considering the reactivity of active ingredients and application rates, crops types and crop management, soil main properties and soil management, soil hydrology and climatic conditions. This study emphasizes how maximum residues limits in soils are set to high values for human health but most of the data are hardly accessible due to varying influences [115,116]. Though setting legislation threshold limits is hard for legislation [115], determination of maximum residues limits for pesticides appear as a key need for improving the protection of soil, environment and human health in the future. For example, soil contamination on the base of DDT concentration is classified as: (i) negligible contamination, (<50 μg kg⁻¹ DDT), (ii) low contamination (50–500 μg kg⁻¹), (iii) medium contamination (500–1000 μg kg⁻¹), and (iv) high contamination (>1000 μg kg⁻¹) [117]. Such kinds of classification should be done in future researches for other pesticides groups. Hence the defined levels for pesticide residual limits cannot be specified based on aforesaid factors [118]. More than 80% tested soils were found contaminated with pesticide residues in total 166 pesticide combinations and uncontrolled application of pesticides results in land degradation [118,119]. The pesticides concentration in soils of different regions is lacking, so, it is recommended to consider the above mentioned factors. Because pesticides are persistent xenobiotics, they can have transboundary impacts, and therefore legislation is required either at regional and continental, but also global level to for all major groups’ pesticides.

6. Conclusions and Outlook

In an agricultural ecosystem, the use of pesticides has a vital role in controlling insects and pests. The application of pesticides tends to contaminate the soil environment up to a harmful level as well, which has drastic effects on human health. To remediate these soils, bioremediation technologies are efficient, but in major areas of the world, physic-chemical techniques are also being used in very large areas and very frequently. In the agriculture ecosystem, soil and plants have different relations with microbes that provide food for microbes and have the capacity for the breakdown of toxic elements into their non-toxic precursors. Recent advances in plant biotechnology and microbiology have made the use of this mechanism more efficient, such as next-generation sequencing, which can identify and utilize total microbial communities for degradation of contaminants. Results discussed in the current review showed that phyto- and bioremediation processes are widely being used to obtain successful results for the breakdown of toxic compounds, and these technologies should have greater application in the remediation of pesticide contamination in field soils. To increase the scale and efficiency of bio and phytoremediation, researchers must focus on identifying and elucidating bio-, rhizo-, and phytoremediation mechanisms in relevant biota. By using new technologies like next-generation sequencing, we can play an important role in investigating the composition of the total microbial community to identify more active degrading agents in soil and plant ecosystems and to introduce new microorganisms to the community. However, a specific research plan is needed to be executed in specific contaminated areas for better calculation of the site-specific working of microbes and degradation potential. Moreover, there is a need for research to expand the knowledge on how to convert efficient lab trials into efficient field trials and application as well.