Environmental Fate and Sustainable Management of Pesticides in Soils: A Critical Review Focusing on Sustainable Agriculture

Abstract

Pesticides are inevitable agrochemicals employed as plant protection agents and their application follows good agricultural practice (GAP). Although pesticides are primarily used for plant protection purposes, the residual pesticides may pose a threat to the next crops and/or off-target biota. Another important aspect of applied pesticides is the transformation into toxic metabolites. As a result, misuse or overuse of pesticides can lead to raised residual uncertainty, hidden risk of transformed metabolites, and potential risk to off-target biota. As per pesticide safety guidelines, regulations for the maximum limit of residual pesticides, addressing toxic metabolites derived from parent pesticides, and managing the potential risk of pesticides for off-targets are considered vital components. Despite the countable number of studies that have already been published on pesticide fate, residual risk, and metabolism in soils and plants, several vital research gaps remain untouched. In this study, the vital research gap of pesticide fate and transport is explored through vital keyword searches, followed by sorting of relevant articles using scholarly search engines. According to the study outcomes, residual uncertainty, secondary pollution, diversified fate and transport, and toxic metabolites, including their persistence, were detected as key research pitfalls. Thus, this paper critically addresses the current trends and research gaps and suggests specific recommendations for pesticide fate and potential risk studies.

1. Introduction

Pesticides from diversified chemical classes with specific modes of action are employed as essential agrochemicals in global agriculture [1,2]. The recommended dose of certified and registered pesticides as vital plant protection agents is prescribed [3]. In the global context, the major groups of pesticides include organophosphorus, carbamates, triazole, and newly formulated synthetic pesticides for the sustainable protection from pest and insect infestations on agricultural land. This followed a global ban on obsolete pesticide classes, including organochlorine and their derivatives due to their persistence at the application sites and effects on off-target biota [1]. According to statistics, commercial pesticide formulations with diversified active ingredients have increased over time, and currently, over 20,000 commercial products have been widely reported globally [4,5]. Intensive farming practices use hybrid crop cultivars that require more pesticides than conventional farming. As a result, residual pesticide may be deposited in the soils or at application sites in arable agricultural fields under intensive cultivation [3]. Despite regular monitoring of the current use of pesticides on farming land and routine laboratory studies having been performed, residual uncertainty is still a global problem due to the complex chemistry of pesticides and their metabolism at the application sites [6].

Pesticides can be applied on farming lands either by foliar application on the standing crops or soil application, depending on the purpose of application and crops grown therein [2,7]. The application process of pesticides is dependent on the formulation types and mode of action on the target pests. However, foliar pesticides are adsorbed by the foliage of standing crops, and a small fraction can penetrate the stomatal opening of the foliage [8]. On the contrary, soil-applied pesticides are primarily deposited on soil surfaces either by sorption or soil penetration processes [9,10]. The second step is plant uptake (if the root system is available in the vadose zone) or the leaching of the pesticides into deeper soil layers (if the pesticide is mobile). The remaining portion of the pesticide is either deposited as a residual pesticide or transformed into metabolites under the effect of various soil and climatic factors [5,11]. In particular, plant uptake of pesticides is governed by the root structure, root–soil–plant interactions, root concentration factor (RCF) ratio, bioconcentration factor (BCF), and half-life (t_1/2) of the applied pesticides [6,12]. On the other hand, the chemical nature of the pesticide, solubility, and soil adsorption potential together affect the parent pesticide behavior and metabolic transformation in soils [11].

Globally, diversified synthetic pesticides are applied for plant protection purposes and to boost crop sustainability [13]. However, only a fraction (approximately <5% of applied pesticides) is effective for real crop protection, while the rest of the pesticide is either deposited in the soils (e.g., residual pesticides and transformed into metabolites) or taken up by soil–root–plant interactions [14,15]. The residual pesticide, including toxic metabolites, is treated as a threat to the following crops and/or off-target biota and acts as the critical barrier to sustainable agriculture [16]. The residual uncertainty of applied pesticides is considered a global dilemma due to the uncertain release of pesticides from the application sites along with their toxic metabolites [5]. Thus, regular monitoring of global pesticides is suggested and practiced to establish safety guidelines for pesticide application to arable lands [6,9].

In the global context, environmental fate and residual monitoring of pesticides in different environmental samples, including plant, soil, and water, have been reported by previously published papers [9,12,17]. However, uncertainty remains, and further recommendations have been suggested for addressing the current research pitfalls regarding fate and persistence studies of pesticides in soils under varied climatic conditions [18,19]. Currently, the total pesticide residue definition, which includes parent pesticide and transformed metabolites, is partially employed by various developed countries and their respective regulatory bodies, including the United States and the European Union. The existing definition of pesticide residue, however, being revised [20]. However, the inclusion of residual pesticide is not widely accepted by the developing countries. In addition, the simultaneous calculation of parent pesticides and their metabolites is not practically possible due to a lack of analytical data or scarcity of sophisticated instruments across the globe. Thus, research uncertainty regarding pesticide metabolites is considered a critical gap for the comprehensive analysis of pesticides where resources are limited [3,11]. The reference laboratories of the EU and the Organization for Economic Co-operation and Development (OECD) have started to minimize this research gap to achieve a unified research protocol, but this research activity is still in the preliminary stage [20]. In addition, a wide variation in pesticide monitoring is evident due to soil and plant diversity throughout the world, and essential amendments to regulatory parameters are necessary to enable good agricultural practice (GAP) to be followed [2].

Considering the above circumstances, this review focuses on critical analysis of research insights into and practical challenges for global pesticide residue monitoring and fate behavior studies in soils and sediments. The specific objectives are (i) to comprehend the advances in pesticide monitoring and persistence studies across the world, (ii) to address the research uncertainty regarding hidden threats and unknown transformation products (TP) from parent pesticides, (iii) to synchronize global safety guidelines of pesticide application following good agricultural practices, and (iv) to decipher the research gap and recommend sustainable solutions for resolving the current knowledge gap in the fate and safety studies of pesticide application.

2. Review Motivation and Arrangement of Literature

Pesticide fate, dissipation, and metabolic transformation are critical issues for sustainable agriculture. In addition, pesticide degradation and subsequent reaction pathways are complex processes. Thus, the core concept of this review is filled with logical keywords to avoid ambiguity. The current review summarizes and scrutinizes vital information through a holistic approach, including web surfing databases and respective website-derived databases. An initial database was identified via major keyword-derived gross references accounting for approximately 3000 online reference materials (including both review and original research). However, the screening of the most relevant data archives was sorted using an inclusion/exclusion process but not strictly to any bibliometric methodology. Briefly, well-known databases, including Web of Science (WoS), Scopus, Springer, Elsevier, and Pubmed, were randomly searched using vital keywords. The vital keywords were, but were not limited to, “pesticide degradation”, “pesticide metabolite”, “half-life”, “research gap of pesticide”, “fate of pesticide”, “factors of pesticide dissipation”, “pesticide toxicity”, “risk of pesticide”, and “pesticide remediation”.

Although pesticide monitoring and safety guidelines are studied regularly, the research gap regarding the fate of pesticides, metabolic transformation, and policy regulations has not been comprehensively compiled [4,21]. The present study highlights the current trends in pesticide research and key shortcomings critically to offer a more vivid pesticide safety study of the handling and application of pesticides at the application sites. Although pesticide fate and residual monitoring have been explored in previous studies, studies regarding the transformation of pesticides into toxic metabolites are scarce. Thus, we assembled those research trends with vital knowledge gaps regarding pesticide fate and safety guidelines to ensure the judicious application of pesticides and their sustainable management following good agricultural practice guidelines. The process did not follow the specific toolkit during the screening and selection of the review topic, but a judicious selection of articles was performed. Although this review did not follow any specific review strategy, the screening and selection process included the initial sorting, skimming, and include/rejection strategies. The review and the include or exclude process for the referred papers are illustrated in Figure 1.

3. Pesticide Persistence and Sustainable Agriculture

As plant protection becomes essential for global agriculture, a large number of pesticides are applied to tackle devastating insects, plant diseases, and notorious weeds [22]. Due to persistence and toxicity, the group of organochlorine pesticides was banned and classified as a persistent organic pollutants (POPs) group [13]. After the POPs, new groups of chemical pesticides emerged as alternative plant protection agents with moderate persistence and lower toxicity to off-target biota. In particular, the global ban on the dirty dozen (i.e., obsolete POPs) addressed the deep concerns of regulatory authorities [16,23]. The statistics of global pesticide use have dramatically changed due to the persistence of old pesticides and the emergence of new pesticides [24]. The persistence of pesticides is evaluated through the calculation of the dissipation half-life (t_1/2) of pesticides. Although a global ban on obsolete pesticides restricted the use of those POPs as current pesticides, their persistence remains a global environmental problem due to decades-long dissipation [3,23]. For instance, traces of obsolete pesticides, including their parent forms and structurally similar toxic metabolites, have been detected even 20 years after the global ban [4]. The detection of DDT and DDE (a toxic metabolite derived from DDT) in farming land and surface water samples has increased the uncertainty of the obsolete pesticides, thus posing a critical obstacle for pesticide safety guidelines, including MRL setting and safe PHI prediction [25,26].

The non-degradability of the parent pesticide and slow transformation into the metabolites is the root cause of pesticide persistence and a global burden for sustainable agriculture [3]. This may happen due to the stubborn chemical structure of organochlorine compounds or related pesticide groups [23]. In general, a pesticide is degraded over time and converted into structurally similar metabolites [5]. However, the organochlorine pesticides and related derivatives show non-degradability in soils and application sites. In addition, the transformed metabolites also showed longer persistence and higher toxicity than the parent pesticide [27]. As a result, banned pesticide traces can be found in agricultural soils even after a long period [25,28]. It is noted that the transformation of parent pesticides into metabolites is not a permanent solution for sustainable pesticide management, as the transformed metabolites have already been reported as an emerging concern due to higher toxicity and persistence [29,30].

Additionally, soil-bound pesticide is considered a hidden threat to sustainable agriculture and the development of pesticide safety guidelines [31]. Soil-bound pesticides are a non-extractable fraction of pesticides that can be chemically bound to clay minerals and cannot be found in the soil solution [32]. Further, the slow release of those soil-bound pesticides into the soil solution can pose a hidden risk during MRL and pesticide-safe guideline preparations [32,33]. On the other hand, non-labile pesticides tend to be adsorbed into the soil surface and cannot be moved through leaching due to their strong affinity to soil particles and hydrophobicity [10,27]. These immobile or slowly mobile pesticides in the soil pose a risk for rotational crops and soil surface contamination. In brief, the applied pesticide can be persistent in the soil due to its non-degradability and strong bonding potential with the soil surface [34]. The persistence of pesticides and soil processes is illustrated in Figure 2.

4. Factors Governing Pesticide Degradation and Behavior in Soils

Pesticide degradation and environmental behavior are largely influenced by several key factors, including pesticide, soil, and plant factors, and climatic conditions [35]. The chemical properties of pesticides (such as solubility, logK_ow, logK_oc, ionization capacity, and functional chemical group) are the vital factors for controlling pesticide fate, including adsorption, degradation, uptake, and transport of pesticide from application sites to the target plants and surrounding environment [36,37,38]. In a nutshell, the hydrophobicity of pesticides is directly correlated with adsorption with soil particles and the subsequent leaching loss through the soil profile [10,15]. The second most important factor is the soil. In particular, soil organic matter, moisture content, soil texture, pH, redox potential, cation exchange capacity, and soil microbes are the key soil factors that control the adsorption, degradation, and mobility of pesticides in the soil [39,40]. Amongst the soil factors, the addition or removal of soil organic matter has a direct correlation with pesticide degradation [10,41]. Other triggering factors are the plants themselves, including plant root archeology, plant uptake capacity, and foliage shape, which are linked to pesticide deposition and govern the movement of the pesticide following foliar application [35].

Climatic conditions, including rainfall, wind, and temperature, affect the fate and degradation of pesticides at application sites [15]. Thus, the growing conditions, including climatic factors, are also considered vital factors in pesticide degradation and dissipation behavior in the environment [18,42]. For example, the pesticide behavior and degradation pattern under controlled growing conditions, either in a greenhouse or a polyhouse, are different than under open field conditions employing a similar pesticide [6,9] because the open field climatic conditions, including rainfall, wind, moisture, and temperature, cannot be controlled as compared to greenhouse conditions. On the other hand, soil salinity (a major abiotic stress condition of soils) can cause a comparatively higher persistence of pesticides at the application site. Salt ions, especially Na⁺, may interfere with the metabolic transformation and biodegradation of parent pesticides due to the disturbance of rhizospheric microbes in the soil vadose zone [43].

The key triggering factor for pesticide degradation and behavior at application sites is governed by the chemical nature of the pesticide applied. In general, the degradation and transport behavior of ionizable pesticides is governed by soil pH, moisture, soil organic matter, and clay characters, including cation exchange capacity (CEC) [44,45]. In contrast, the non-ionic pesticides and their degradation are mainly influenced by the soil organic matter content and pesticide–soil particle sorption potentials [10]. As reported by previous research, soil moisture is the initial factor for enhancing the pesticide degradation process regardless of its chemical type [46,47]. For example, the degradation half-life (DT₅₀) of phorate is 78 days (sandy loam soil) and 72 days (loam soil) for dry soil treatment, but the reduced half-life has been recorded for moisture and organic amendment treatments (i.e., 21 days for sandy loam, and 18 days for loam soils, respectively) [10]. It is noted that phorate, a non-ionic pesticide, is triggered by soil organic amendment and moisture during enhanced degradation in soils. however, the degradation of a total of 10 ionic pesticides (e.g., six acidic and four basic pesticides) is influenced by soil pH and organic carbon [36].

Pesticides are derived from various chemical structures having diversified functional chemical groups. Therefore, the degradation pattern of each pesticide is also very complex [45,48]. However, specific factors such as plant-, soil-, and climate-specific factors are vital for triggering the pesticide-specific degradation and environmental fate at application sites [38]. Briefly, pesticide degradation factors can be classified into two broad groups: (i) positive factors and (ii) negative factors. The underlying factors that can enhance the pesticide degradation process are termed positive factors. For instance, the addition of external soil organic matter, moisture contents, higher cation exchange capacity (CEC), catalytic agents, and soil pH act as positive factors for enhanced pesticide degradation and metabolic transformation [5,17,49]. On the other hand, the removal of soil organic matter, disturbance of soil microbes, excess soil acidity, and lower soil fertility act as negative factors [50,51]. Among the catalytic factors, heterogeneous catalysts, metal oxide, ozone, Fenton’s reagent, and phenolic mediators act as chemical catalysts [52]. In addition, biocatalysts (e.g., laccase derived from fungi, oxidase, peroxidase from horseradish-HRP, and esterase) are considered green catalysts to enhance pesticide degradation.

5. Pesticide Chemodynamics and Distribution

Chemodynanics is a combined, holistic term, including environmental fate, persistence, and transport of applied chemicals in the specified area, the fate of chemicals, including binding at the application sites, adsorption, and degradation under the influence of various vital factors, such as chemical properties, interactions of chemicals with soil surfaces, and climatic conditions (such as temperature, rainfall, and winds) [37,53,54]. Immediately after application, pesticide chemfate can be classified into (i) dissolved pesticides, (ii) soil-deposited pesticides, (iii) transformed pesticides, and iv) residual pesticides [15,55]. Further, the chemfate of a pesticide is mostly dependent on the soil properties, pesticide solubility, and the mobility and polarity of the pesticide [5,56]. Among the factors, soil organic matter is a vital factor that controls chemfate and metabolic transformation of pesticides in soils [10,35]. Soil pH, on the other hand, controls the overall soil reaction and determines the fate of ionic pesticides [36]. Soil moisture also triggers the initial breakdown of pesticides in the soil and indicates further movement into the soil–water system [57,58]. Among the chemfate mechanisms, oxidation, reduction, hydrolysis, photolysis, biodegradation, and mineralization have mostly been reported by earlier research [5,59].

Pesticide distribution and movement in global agricultural soils are highly dependent on the solubility of pesticides and the interaction of pesticides with soil particles [60,61]. Pesticides with high solubility and mobility tend to be captured by surface runoff or sub-surface movement, resulting in surface water contamination [61]. In contrast, pesticides with lower solubility and higher permeability tend to leach and pose a hidden threat to groundwater contamination [5]. The residual pesticides and their abundance are governed by pesticide mobility and solubility [62].

A comprehensive illustration regarding global pesticide use and the movement of residual pesticide with water through leaching and percolation is presented in Figure 3. According to Figure 3, the distribution and use of current use pesticides (CUPs) has increased in the South American, African, and Asian regions. In addition, contamination of groundwater has been reported in most European countries. Among the CUPs, herbicides accounted for 50% of total global pesticides, followed by insecticides (30%) and fungicides. The unscrupulous use of pesticides may lead to off-target toxicity and human health implications.

In addition, the agricultural surface slope and the application dose of the pesticide also act as vital factors in controlling global pesticide distribution and residual pesticide characteristics at the application sites [63]. In general, most pesticides are hydrophobic and cannot be dissolved easily in soil solution. Thus, there is a possibility of soil adsorption by soil particles for non-polar pesticides [21,64]. In particular, pesticide toxicity and persistence are controlled by the chemodynamics of the pesticide and its metabolic transformation [7,65]. To study pesticide toxicity and persistence, it is important to acquire a decent knowledge of pesticide chemodynamics. In addition, most of the herbicides are polar or semi-polar and can be dissolved in soil solution and moved through soil water as a result of surface runoff and leaching [63,64]. Thus, the chemodynamics of pesticides determines the overall distribution and abundance of the pesticides applied. The major events during pesticide chemodynamics are adsorption–desorption, dissipation and degradation, and transformation into structurally similar metabolites (Figure 4).

During pesticide fate and transport studies, the major events and processes are (i) adsorption–desorption, (ii) dissipation and degradation, (iii) leaching or infiltration, and (iv) runoff and volatilization [3,15,66]. Adsorption and desorption processes are the initial processes of pesticide movement and deposition in the soil particles. Hydrophobic pesticides, which have a higher affinity to soil particles, are mostly adsorbed. Once adsorbed onto the soil surface, the pesticide can be desorbed due to slow release into the soil solution [67]. Several acting forces, such as H-bonding, π-π bonding, and van der Waals forces, help with the adsorption of pesticides onto soil surfaces [39,67]. Dissipation and degradation of pesticides are controlled by several key factors, and this is the active process of pesticide management and transformation into metabolites [38,65].

On the other hand, leaching and runoff are the processes of pesticide movement either in a vertical or horizontal direction. Volatilization is another rapid process of volatile pesticide loss in the open air. In addition, evaporation and transpiration may occur together under tropical climatic conditions termed “evapotranspiration”. The pesticides remaining in the soil at the application sites after these losses or plant uptake are the residual pesticides. The overall chemodynamics of applied pesticides is illustrated in Figure 4.

6. Transformation of Metabolites from Parent Pesticide

The transformation of parent pesticides into structurally similar transformation products (TP) is a vital and critical metabolic process in pesticide behavior and the environmental fate of the applied pesticides [29,30]. Pesticides applied are initially deposited at the application sites (e.g., leaf foliage during foliar application and surface soils during soil application). After plant uptake and soil deposition, a vital process is the metabolic transformation of parent pesticides through various triggering factors [35]. This transformation is site-specific and controlled by several biological and geological factors [38]. The organic matter content of soils and microbes, including bacteria and fungi, are considered prime factors for enhanced transformation of pesticides into metabolites [45,60].

In general, the disappearance of the parent pesticide after the initial application was considered safe for the plants, particularly when the residual pesticide was below the maximum residue limit (MRL), which was recommended as a safe application rate of pesticide to farming land [68]. However, the metabolites or transformation product (TP) derived from the parent pesticide have been found to be more toxic and persistent in the soils or at the application sites [69]. Thus, toxic metabolic transformation is considered a critical concern for the safe application of pesticides. To address this research uncertainty, in recent years, the definition of residual pesticide has included the parent pesticide and transformed metabolites [20]. Moreover, the metabolites derived from parent pesticides show higher toxicity and persistence than parent pesticides, and the metabolic pathways and concerning factors in various environmental matrices (e.g., soil, sediment, water, and plants) are not yet fully understood [60]. Thus, toxic metabolites derived from parent pesticides are considered a hidden threat to food safety. Although recent studies have included pesticides and their toxic metabolites, this research area is still in its primary stage [9,70]. Furthermore, transformation products (TP) of parent pesticides are critical for off-target biota and groundwater contamination due to the leaching potential [21,65].

In soil, soil organic matter is the major triggering factor for the enhanced metabolic transformation of pesticides [21]. In particular, the chemical composition of the humic acid fraction of soil organic matter is the key driver for controlling pesticide transformation in soil [60]. The organic matter may present in soils in dissolved or bound forms [31]. However, the addition of external organic matter to soil can cause enhanced degradation and metabolic transformation [10]. On the contrary, the removal of organic matter from soil can cause a detrimental effect during the metabolic transformation of pesticides, resulting in hindrance to pesticide transformation [65]. Amongst other factors, soil moisture content, pH, temperature, and adsorption of parent pesticides are vital [38]. Biological factors, including soil-inhabiting microbes and their functional enzymes, are active biological agents fostering pesticide metabolism [71]. For instance, an obsolete persistent organochlorine pesticide can be converted into a toxic metabolite through various metabolic processes, including the dehydrochlorination process, and also be transformed into secondary metabolites over time. As a result, the residual concentration of the parent pesticide decreased, but the deposition of toxic metabolites increased simultaneously. Similarly, the transformation of the parent pesticide phorate into toxic oxidative metabolites such as phorate sulfoxide and phorate sulfone in two different soils has been documented [10]. The transformation of phorate into metabolites was driven by concurrent oxidation and hydrolysis reactions. However, the metabolic transfer and reaction pathways of pesticides in soils and plants are indistinctly evaluated due to many triggering factors [35]. Another previous study noted the robust transformation of endosulfan into endosulfan sulfate due to rapid oxidation [59]. Furthermore, hydrolysis, along with oxidation, can transfer endosulfan sulfate into endosulfan ether and endosulfan lactone. It is noted that the toxicity and persistence of endosulfan sulfate have been reported to be higher than the parent endosulfan. In an earlier study, atrazine, a triazine herbicide, was reported to be transformed into structurally similar metabolite deisopropylatrazine (DIA) and deethylatrazine (DEA) through dealkylation [58,72]. Additionally, the hydrolysis and photolysis of atrazine resulted in the transformation into hydroxyatrazine (HYA). Thus, a transformation of parent pesticides into toxic metabolites is considered a critical risk for pesticide safety guideline management.

Figure 5 illustrates the mechanism and process of the transformation of pesticides into toxic metabolites. Each process can be stimulated separately or in combination during transformation. In addition, a higher toxicity than the parent pesticides of the transformed metabolites has been identified [59]. The higher toxicity of transformed metabolites as compared to parent pesticides is presented in Figure 6. In particular, the higher toxicity of endosulfan sulfate has been reported than parent endosulfan (Figure 6A). As a result, the inclusion of toxic metabolite endosulfan sulfane during residual pesticide investigations has been suggested by the safety authority of global pesticide regulations. Similarly, 3,4-dichloroaniline (3,4-DCA), another toxic metabolite derived from dicarboximide fungicides including diuron and procymidone, was found to be toxic to tested animals and humans (Figure 6B). Moreover, 3-hydroxy carbosulfan (derived from parent carbosulfan) was found to be toxic to zebrafish. However, the study was limited in this area as it reported on the sustainable remediation of these toxic metabolites. Thus, sustainable remediation of toxic metabolites needs to be the focus of future studies.

7. Pesticide Monitoring and Risk Assessment

Residual pesticide monitoring is globally evaluated as a routine lab analysis of analytical chemistry laboratories [6]. Regular analysis is performed to revise safety guidelines and decipher the altered behavior of applied pesticides in specific regions and specific plants [20]. Regular monitoring is suggested by the global regulatory authorities to update the dissipation pattern and pesticide behavior in global soils. Due to variations in analytical instruments and analytical methods, the analytical findings are not unified for regional data and global data [43]. This is a key challenge for the unification of analytical approaches globally. Due to separate method verification, variations in analytical instruments, and dissimilar methods of pesticide extraction, a unified analytical approach has not been attained [70]. However, the prime goal of regular pesticide residue monitoring is to establish safety guidelines for pesticide application following good agricultural practice (GAP). Due to variations in methodology and dissimilarity in instrument sensitivity, a separate analytical protocol is adapted for each analytical laboratory during the validation of the analysis of pesticides [43].

In global analytical laboratories, the regular monitoring of residual pesticides revealed that pesticide fate and metabolic transformation is a complex process, and widely varied due to differences in the tested soils and climatic factors [15]. Thus, regular amendments to pesticide safety regulations are expected in compliance with good agricultural practices. However, earlier reports noted that global research uncertainty regarding residual pesticide monitoring and safety regulations still exists [73]. This gap in pesticide safety studies is considered a critical threat to sustainable agriculture. Furthermore, the study regarding risk assessment of residual pesticides and their effect on the food web has not been conclusively documented. However, global pesticide monitoring with salient findings is presented in

Table 1. Global monitoring of pesticides in different regions of the world with salient findings.

Region	Country	Study Site	Studied Pesticides	Analytical Method	Salient Findings	Reference
Asia	China	Vegetable fields and orchards near a river basin in North China	Atrazine, chlorpyrifos, tebuconazole, thiamethoxam, pymetrozine, and difenoconazole	Extraction by QuEChERS and analysis by HPLC-MS/MS	Among 47 studied pesticides, 6 pesticides showed higher concentrations	[73]
	China	Three soil samples of a nut-growing region in China	Six organochlorines, one organophosphate, and six pyrethroid group pesticides	A simple methanol extraction, followed by GC-ECD analysis	Among 29 pesticides, organochlorine was detected in 78.9% of soils and pyrethroid in 65.8%	[74]
	Korea	Minor and leafy vegetables from the Gwangju and Jeonnam regions of Korea	A multi-residue pesticide analysis strategy for 230 pesticides was performed	Modified QuEChERS extraction, followed by GC-MS and LC-MS/MS analysis	Among the samples, 1.4% of vegetables exceeded the Korean MRL and posed a dietary risk to Korean consumers	[75]
	Korea	Twenty types of fruit and vegetables were collected from the local market in Incheon (Korea)	Multi-residue pesticide analysis comprising the currently used pesticides (CUPs) in Korea	HPLC, GC-ECD, GC-MS/MS, LC-MS/MS analysis for multi-pesticide analysis	Chlorfenapyr, procymidone, etofenprox, pendimethalin, fluopyram, and azoxystrobin were found to be the most detected pesticides	[76]
	Nepal	Agricultural farms practicing traditional farming and IPM	Seven organophosphate and eight organochlorine pesticides	QuEChERS extraction, followed by LC-MS/MS analysis	IPM-derived soil showed negligible dietary health risks as compared to conventional farming soil	[1]
	Saudi Arabia	Drainage channel, lagoon wetland in the Arabian Gulf and eastern province of Saudi Arabia	Multi-residue pesticide analysis strategy	SPE extraction, methanol washing through an ultrasonic bath, followed by UHPLC-MS/MS analysis	Among the pesticides, chlorpyrifos, diazinon, and bifenthrin showed a risk to the studied biota	[77]
	India	Soil samples from different land uses on Andaman Island, India	Multi-pesticide residue analysis protocol from the soil was used	A robust QuEChERS extraction, followed by GC-MS analysis	Detection of endosulfan and DDT (41.7%), followed by Aldrin (16.7%), in the Andaman Island soil samples	[78]
	Pakistan	Local market-derived vegetables and fruits in Faisalabad, Pakistan	Thiamethoxam, imidacloprid, acetamiprid, thiacloprid, and carbendazim	HPLC-UVD and LC-MS/MS analysis for simultaneous pesticide residues	Local market vegetables and fruits were contaminated with pesticides and exceeded the Codex MRL	[79]
	Bangladesh	Vegetables, fish, and fish feed collected from a local market in Bangladesh	17 organochlorine, 5 pyrethroid, and 3 organophosphate pesticides	QuEChERS extraction and GC-MS analysis for simultaneous pesticide detection	Among the studied samples, the majority of vegetables and a few fish samples showed pesticides exceeding MRL	[80]
	Japan	A total of 12 vegetable and fruit samples collected in Osaka, Japan	Five neonicotinoids (Imidacloprid, thiacloprid, acetamiprid, thiamethoxam, and nitenpyram)	Methanol extraction, SPE clean-up, followed by LC-MS analysis	Five pesticides showed acceptable recovery (70–95%) during the 0.1 and 1.0 mg/kg spiking test	[81]
	Philippines	Topsoil, water, and eggplant fruit samples collected from the eggplant farm in Pangasinan	Malathion, cypermetrhin, chlorpyrifos, profenfos, and triazophos	Acetonitrile-based SPE extraction, followed by GC-ECD or GC-NPD analysis	Among the studied samples, eggplant fruit showed the highest concentration of pesticides, followed by plant and water samples	[82]
Europe	Czech Republic	Seventy-five arable soil samples collected from central Europe (Czech Republic)	A total of 53 pesticides and 15 transformation products (metabolites)	QuEChERS extraction, followed by LC-MS/MS analysis	Triazines and conazoles were the most detected pesticides, followed by simazine and their transformation products	[83]
	10 EU countries	Various crop samples such as cereal, root crops, vegetables, fruits, and beans from 10 EU countries	Multi-pesticides, including organochloride, organophosphate, pyrethroids, and triazole pesticides	QuEChERS extractions and LC-MS/MS, GC-HRMS analysis	Twenty percent of total eggplant samples tested positive for pesticides and posed a risk to human health due to continuous exposure to pesticide-contaminated fruit of eggplant	[84]
	Portugal	Strawberries are grown under organic and IPM farming technology	A total of 170 targeted pesticides through a multi-pesticide analysis strategy	QuEChERS, followed by GC-MS/MS and LC-MS/MS analysis	Strawberries from organic farming had no detectable pesticides, while nine pesticides were detected in IPM samples	[85]
	Greece	Soil samples from the olive farm located in Southern Greece	Glyphosate and its primary metabolite AMPA	QuEChERS extraction, followed by LC-MS/MS	Longer persistence of primary metabolite AMPA was evident over the parent glyphosate	[86]
	Spain	A total of 33 rice field sites during the rice production period	A total of 10 pyrethroid insecticides	Ultrasonic extraction followed by GC-MS analysis	The irrigation from the wastewater treatment plant was determined to be the pesticide contamination source in the paddy field	[87]
	Poland	Sediment, soil, and surface water from agricultural and forest field	Atrazine and triketone herbicides, including their metabolites	Acetonitrile extraction and HPLC-DAD analysis	Atrazine was not detected in the soil samples, but the transformation product was still detected (41%) in the soil	[72]
	France	Forest, agricultural soils, and grassland	Persistent organic pollutants (POPs) such as DDT, DDD, lindane	Acetone-assisted pressure liquid extraction and HPLC-MS	The wide transport of POPs was documented in the study regions	[88]
	Switzerland	Archived farming soils, orchards, vineyards	A total of 80 polar pesticides and 90+ transformation products	Pressurized liquid extraction and LC-HRMS analysis	A decade-long study noticed the transformation of 50% of parent pesticides into transformation products (TP)	[89]
Africa	Kenya	Nzoia sugarcane belt sub-catchment water samples and soil samples adjacent to a river	Organochlorine pesticides and herbicides	Soxhlet extraction, followed by GC-MS and LC-MS analysis	The concentrations of some detected pesticides crossed the limit of EU-MRL	[90]
	Nigeria	Vegetables and fruits available at the Nigerian market	Organochlorine and organophosphate group pesticides	DCM extraction and florisil clean-up, followed by GC-FPD analysis	Among the 38 tested pesticides, the levels of six pesticides were found to be over the MRL	[91]
	Ghana	Soil samples collected from cocoa plantation farm	Neonicotinoid pesticides	QuEChERS extraction, followed by LC-MS/MS	Multi-residue analysis was optimized for neonicotinoid determination	[92]
	Uganda	Fresh vegetables from the rural region in southwest Kabale district	Cypermethrin, dimethoate, malathion, metalaxyl, profenofos, dichlorvos, and mancozeb	AOAC suggested QuEChERS, followed by LC-MS/MS and GC-MS/MS analysis	The terminal residue was found to be over the MRL in sprayed and market-derived vegetable samples	[93]
	Egypt	Arable soils from vegetable fields near the Eastern Nile Delta region	Multi-pesticide analysis targeting 33 compounds through simultaneous analysis	QuEChERS extraction and LC-MS/MS and GC-MS/MS analysis	Chlorpyrifos and propamocarb were found to be the most detected residual pesticides	[94]
	South Africa	Fresh fruits and vegetables collected from the biggest South African market	A total of 74 pesticides are commonly used in the vegetables of African agriculture.	Modified QuEChERS extraction and analysis by GC-ECD	Boscalid, endosulfan, profenofos, and procymidone exceeded the national MRL	[95]
	Ethiopia	Vegetables and surface water near the Rift Valley in Ethiopia	DDT, α-cyhalothrin, profenofos, metalaxyl, β-endosulfan	QuEChERS extraction and analysis by GC-MS/MS	Approximately 2–12% of samples exceeded the EU MRL, and regular monitoring was suggested.	[96]
USA	Brazil	Fresh vegetables and fruits from respective regions in Brazil	Carbaryl, carbofuran, carbendazim, flutriafol, fuberidazole, and thiabendazole	AOAC QuEChERS and HPLC-DAD analysis as a new method	Seven studied pesticides showed good performance during a simultaneous analysis	[97]
	Argentina	Domestic market of Argentina (fresh fruits and vegetables)	Multi-residue pesticides, including 35 compounds	QuEChERS extraction and GC-MS analysis	Approximately 60% of samples were positive for pesticides but were within the MRL limit	[98]
	Chile, Mexico	Commonly consumed vegetables in Mexico and Chile	A total of 22 pesticides is analyzed for this study	QuEChERS multi-residue analysis by GC-MS/MS	Among studied pesticides, 10 residue pesticides were found in vegetables, and a greater number were in Mexican (7) samples than in Chilian (3)	[99]
	Louisiana	Two different soils from the traditional cropping system in Louisiana, USA	Synthetic and organic herbicides	Performance evaluation of herbicide for weed suppression	Synthetic herbicides were more efficient during weed control than organic herbicides	[100]
Australia	New South Wales	Sediments from the fast-growing farming land in Australia	Targeted for 97 pesticides from the sediments	Methanol extraction of sediments followed by LC-MS/MS	The detected pesticides numbered 10 out of 97 target compounds	[101]

.

8. Safety Guidelines for Pesticide and Good Agricultural Practice

In the global context, pesticide safety guidelines are governed by accredited regulatory bodies, including the US-EPA (United States-Environmental Protection Agency); the EU-CODEX (European Union-Codex Alimentarius); the MFDS-Korea (Ministry of Food and Drug Safety, Republic of Korea); and the MHLW-Japan (Ministry of Health, Labor, and Welfare, Japan) [2,102]. In addition, some joint regulations and guidelines have been established for global pesticide safety and application following good agriculture practice (GAP), such as the OECD (Organization for Economic Co-operation and Development) guidelines and the JMPM (FAO/WHO Joint Meeting on Pesticide Management) guidelines [20]. In general, the global guidelines for the safe application of pesticides are regularly updated and amended due to soil and climatic variations. The vital regulations include the establishment of global and regional maximum residue limits (MRL) and setting a safe pre-harvest interval (PHI) for the application of synthetic pesticides to edible crops [2,28]. The revision and amendment for new MRL and PHI are modified every year based on growing conditions and climatic factors [31,103]. The residual behavior of pesticides varies widely with climatic factors and growing conditions of crop plants. Under intensive agriculture, multiple crops are grown in sequence, employing the same field. Thus, a hidden threat may be overlooked for the following crops if there is no interval maintained between the first crop and the following crops [46,103]. Thus, a new regulatory indicator plant back interval (PBI) has been established to keep the farming land fallow (without planting) for a certain period until the residual pesticide concentration is below the MRL [104]. This PBI is an essential parameter considered during the regulatory setting of the positive list system (PLS) in Japan and is followed by the Republic of Korea [14]. Briefly, PLS is a new amendment for unregistered pesticides for minor vegetables considering a uniform tolerable limit (e.g., 0.01 mg/kg) where the MRL of specific pesticide is not registered for those crops [104].

Although pesticide regulations for safe application and handling are well established, regular monitoring of pesticides specific to crop species, growing conditions, and soil types should be performed per the suggestions of global regulatory bodies [38]. The pesticide behavior and dissipation pattern vary with growing conditions, plant species, and soil types. According to GAP, a regular amendment to the MRL recommended dose (RD) and pesticide monitoring regime has been suggested to comply with the safe application of pesticides in farming land [9,105]. A study noted that the bioconcentration factor (BCF) was the triggering factor in determining plant uptake of endosulfan in root crops [73]. Similarly, other factors, such as the root concentration factor (RCF), also affect plant uptake and subsequent translocation of the applied pesticides in cropping lands. Further, an integrated pest management (IPM) approach can be established to ensure the judicious application of pesticides in arable land. In particular, the adoption of an IPM could ensure the judicious application of pesticides while employing non-chemical measures (i.e., advanced cultivation techniques, crop rotation, resistant variety, and protection of beneficial organisms), including physical, chemical, and biological approaches for managing pest infestation on farming lands [106]. The safety guidelines and regulatory bodies are illustrated in Figure 7.

As per Figure 7, the strict management of residual pesticides in edible crops is critical due to food safety and subsequent health hazard concerns. Thus, the maximum residue limit (MRL) of pesticides can be combined with daily intake and body weight to calculate health risk hazard parameters. If the fresh vegetables are cooked or processed, then processing factors may be considered during health risk assessment [56]. Pesticides vary according to their chemical structure and behavior pattern at the application sites and different crops. Thus, revision of the existing regulations, including MRL, PHI, and PBI, is recommended by the global regulatory authorities and has been performed regularly, and amendments to those safety parameters must be published by the concerned authorities [68].

9. Sustainable Management of Pesticides in Soils

The overuse and misuse of pesticides are acting as barriers to Sustainable Development Goals (SDGs). In particular, SDG 2: Zero hunger can be affected by the misuse of pesticides, which can cause human health complications and soil health alteration. Similarly, accidental pesticide poisoning can affect SDG 3: Healthy lives and well-being. SDG 15: Sustainable use of terrestrial ecosystems can be disrupted through misuse of pesticides, leading to harm to ecosystems and biodiversity. As a result, research gaps in the fate and transport study of pesticides in soils, including uncertainty of pesticide fate, undetectable and bound pesticides, and desorption of pesticides, are crucial for setting safety guidelines for pesticide application [18,32,68]. Although pesticide behavior and environmental fate is a classic research theme meticulously documented by previously published papers, the regular monitoring and spatial assessment of pesticide fate and behavior in soils or application sites is still significant due to the wide variation in monitoring data under varied soil conditions [102]. A major concern of pesticide residue analysis is undetectable pesticides. In general, the multi-residue analysis of pesticides may have this uncertainty, leaving a research gap in the investigation of soils or environmental samples and is a hidden threat to the safety guidelines for pesticides [32]. Undetected pesticides still have not been addressed to optimize the multi-residue pesticide analysis. The second analytical gap is “bound” pesticide residue or the non-extractable fraction of pesticides in soils [31]. In modern analytical chemistry, the affordable extraction of pesticides is performed by using a QuEChERS extraction kit to process investigated samples rapidly. However, the bound residue of pesticides cannot be extracted from soils when employing commonly used extraction technology, such as the kit above [32]. This bound residue can be mineralized or transformed into another unknown compound over time. This is also a big challenge in establishing safety guidelines for pesticide application in soils. Pesticide dissipation, degradation, and persistence are highly varied with climatic conditions [71]. Thus, modeling data do not perfectly predict the dissipation rates and pesticide behavior in diversified growing conditions. This is a critical research gap concerning the prediction of pesticide dissipation using model-derived data. In addition, the crop characteristics (e.g., foliage feature, plant root architecture, root-shoot uptake potentials, and plant vascular movements) and soil characteristics (e.g., soil types, organic matter status, soil pH, CEC, and soil nutrient status) largely control pesticide behavior and environmental fate [16].

Pesticide behavior is strongly dependent on the climatic factors. The dissipation rate of a specific pesticide under controlled lab conditions or a polyhouse varies from the dissipation data under open field conditions [12]. Climatic factors, such as temperature, wind, rainfall, and soil moisture, are vital factors that control pesticide dissipation and residual behavior [15,66]. Thus, a wide variation in pesticide degradation data has been documented due to spatial variation. As a result, pesticide studies should be conducted separately based on growing conditions. In recent years, environmental and statistical modeling data have gained popularity for predicting pesticide persistence, safe preharvest interval (PHI), and maximum residue limit (MRL) [67,68]. However, a significant variation has been documented by previous research groups when comparing model-derived data and field-incurred data. The plant species, soil characteristics, and method of pesticide application were reported as the key factors in controlling pesticide behavior and persistence [38]. As a result, a significant research gap is evident in global pesticide and regional monitoring data. Even the globally accredited MRL, PHI, and other safety indicators may not perfectly match with regional pesticide monitoring experiments [20]. Thus, regular monitoring and collection of spatial data for pesticide behavior are highly recommended for the sustainable management of pesticides in soils (Figure 8).

Given the above-mentioned situations and global dilemma, the specific recommendations of pesticide monitoring and synchronization with global policy to ensure the sustainable management of pesticides in global agriculture are as follows:

• Revise the existing MRL and PHI and consider the plant back interval (PBI) (if any) to ensure the least impact of residual pesticides on the following crops on a regional basis. Pesticide behavior is varied due to regional factors and diversified climatic factors. Thus, micro-climatic pesticide monitoring is recommended to address this global concern. In addition, regular synchronization with global regulatory bodies is necessary to augment necessary amendments of safety guidelines regarding pesticide application to enable good agricultural practices (GAP) to be followed;
• Consider the transformation of parent pesticides into toxic metabolites and study the persistence of those toxic metabolites. The parent pesticide may be transformed into toxic metabolites. These toxic metabolites are considered hidden threats to global agriculture and off-target biota due to limited data and meticulous persistence monitoring. The global monitoring of pesticides should be revised to include transformed metabolites and their persistence until complete mineralization or transformation into non-toxic intermediates has occurred to ensure overall pesticide safety guidelines;
• In general, only a small fraction of the pesticide is required to control the target pests, whereas several-fold higher doses can be deposited at the application sites or off-target biota due to conventional applications. As a result, advanced technologies, including controlled released pesticide formulations, adopting integrated pest management (IPM) approach, and pesticide detox mechanisms, should be explored for sustainable management of pesticides in soils. The choice and maintenance of pesticide sprayer machines have a significant impact on spray drift and distribution at the application sites [107]. A technical check, regular calibration, types of spraying machine (e.g., aerial sprayer or machine-operated sprayer), and nozzle design all affect the quality of pesticide distribution. These crucial research gaps should be addressed to achieve sustainable management of pesticides in soils;
• Pesticide-contaminated soils can be managed through green and sustainable strategies, including biochar, biological enzymes, phytoremediations, and nanoremediation. These technologies are environmentally friendly and affordable for large-scale applications. A solitary application and/or combined technologies will be wondrous options for the sustainable management of pesticides. Thus, simultaneous studies can be employed for sustainable remediation of pesticides from contaminated sites using green and affordable approaches;
• A holistic synchronization of global reference laboratory-derived data with the regional analytical data should be a wondrous option for minimizing analytical error and certifying precise analysis of pesticide residues and their behavior at application sites. In recent years, statistical modeling data have become popular for predicting pesticide residue and environmental fate. However, there is a data gap between modeling-derived pesticide data and real-field incurred data. Thus, environmental modeling data should be corroborated with field-incurred data to establish reliable and reproducible analytical data. The overview of sustainable management of pesticide contamination is presented in Figure 9.

10. Conclusions and Future Outlook

In the global context, pesticides are key agricultural inputs for crop protection purposes when employed following safety guidelines. However, incidental overuse can cause groundwater contamination and pose potential risks to off-target organisms. Regular monitoring of residual pesticides is performed by global regulatory bodies and respective research groups due to the complex biogeochemical features of each pesticide group. In earlier research, a wide variation in environmental behavior, dissipation patterns, and metabolic transformation was documented. Several key factors, such as pesticide-, soil-, plant-, and climate-specific factors, trigger the overall pesticide behavior and environmental fate. In addition, the transformation of toxic metabolites from parent pesticides is regarded as a critical threat to rotational crops and food web contamination. Thus, pesticide fate, transport, and metabolic transformation at the application sites are controlled by complex biochemical interactions. The following points are listed for properly addressing pesticide fate, residual uncertainty, and risk assessment studies:

• Pesticide fate and degradation patterns are controlled by specific regional conditions and application periods (i.e., season). As a result, seasonal variations in pesticide dissipation may occur at the application field. Additionally, application methods and types of formulation may also govern the pesticide behavior and environmental fate. Thus, spatial and temporal variations in pesticide fate and transport should be considered during safety guideline assessment of pesticide application in the field;
• Poor monitoring of pesticides is another issue that neglects obsolete pesticides and their derivatives. For instance, DDT, endosulfan, and HCH and their derivatives may linger in the soils even decades after their last application. Therefore, regular monitoring is needed to detect traces of obsolete pesticides and their toxic derivatives in soils. To minimize the uncertainty of the persistence of pesticides in the soil, poor monitoring of pesticides must be improved and synchronized with global pesticide safety policy updates;
• Lack of public awareness and consciousness about the safe use of pesticides on farming land and ignorance of personal safety measures is another crucial concern with respect to developing countries. In general, farmers and end-users of pesticides neglect the safety measures during pesticide spraying and have limited access to the right information regarding MRL, recommended dose of pesticides, proper disposal of empty pesticide containers, and the use of personal protection equipment (PPE). In particular, the point source of pesticides from sprayers, disposed empty containers, and spills during handling are underestimated, as reported by the TOPPS–Life project [108]. Eventually, that negligence will emerge as the key factor for point source contamination of waters by plant protection products (PPP). Thus, increasing public awareness by providing proper information about safety guidelines for pesticides and the training of farmers should be included;
• Due to persistence and higher toxicity, several old formulations of pesticides have been banned or restricted for extensive use on farming lands. The rejection of such obsolete pesticide formulations was the initial step toward the sustainability of global agriculture and the introduction of new/novel formulations of pesticides, which have non-persistence and lower toxicity to mammals;
• During pesticide fate and transport studies, the major events, including environmental fate (e.g., adsorption, dissipation, degradation, plant uptake, and soil deposit), transport of pesticide (e.g., leaching, surface runoff, and sub-surface seepage), and loss of pesticide (e.g., photodegradation, volatilization, detoxification, and evapotranspiration), should be considered.