Introduction

The use of insecticides is a worldwide practice to control insect pests and enhance the yield and quality of food crops. The use of these compounds in agriculture in disagreement to label direction can have detrimental effects on surface and groundwater resources and soils, as well as wildlife, aquatic ecosystems, and human health (Guardo et al. 1994; Rani et al. 2014).

The European Water Framework Directive (2000/60/EC) and the Groundwater Directive (2006/118/EC) have been developed to prevent and control surface and groundwater pollution and deterioration. Therefore, it is of paramount interest to understand the phenomenon governing the movement of pesticides through the soil profile. Moreover, in Spain, the Law 22/2011 on waste and soil pollution, which is the transposition of the Waste Framework Directive 2008/98/CE, forced the administration to declare and define polluted soils, regulating certain aspects on remediation of contaminated soils. In this respect, solarization, a soil disinfection technique based on solar heating of the covered soil with a thin clear plastic film (Katan and DeVay 1991), and biosolarization, a soil disinfection technique based on combination of organic soil amendments with solarization (Ros et al. 2008), have been used for the bioremediation of soil containing pesticide residues (Flores et al. 2008; Fenoll et al. 2011a, b). Nowadays, the addition of organic amendments is a common practice extensively used in some Mediterranean areas to improve the physicochemical and biological properties of soil to enhance its fertility and to modify the fate of pesticides in the environment (Briceño et al. 2007).

The partition of pesticides between the soil and water occurs when these molecules are introduced into the soil, affecting other aspects of their behavior, such as persistence, movement, and disappearance. Leaching process can be defined as the movement of water and dissolved pesticides through the soil. This movement is determined by sorption and degradation processes (Kreuger 1998; Spark and Swift 2002). Leaching, sorption, and degradation processes can be noticeably influenced by one or more factors, such as soil and pesticide properties, unsaturated zone characteristics, land use and management, weather conditions, and irrigation (Spadotto et al. 2005; Grondona et al. 2014). The leached pesticides and their transformation products cause pollution of surface and subsurface water. To avoid pesticide leaching through the bulk soil, a possible mitigation measure can be the increase of the degradation rate of these compounds in soil by bioremediation techniques, including solarization and biosolarization.

Spirodiclofen (SD), spiromesifen (SM), and spirotetramat (ST) are three spirocyclic tetronic/tetramic acid (ketoenol) derivative insecticides that are extensively used for the control of a wide spectrum of sucking insects in fruits and vegetables. These compounds act as inhibitors of acetyl-coenzyme A carboxylase (ACCase) (Bretschneider et al. 2007; Nauen 2005; Nauen et al. 2003). According to the groundwater ubiquity score (GUS) index (Gustafson 1989), spirocyclic tetronic/tetramic acid derivatives (STADs) are classified as insecticides with low persistence and leachability (PPDB 2016), although their enol transformation products are relatively more mobile than the parent compounds and may leach into groundwater (Mate et al. 2014). However, to date, there is no scientific information available on the degradation of STADs in soil by solarization and biosolarization treatments. Thus, the main objectives of this work were as follows: (i) to study the mobility of three insecticides, SD, SM, and ST, and their enol transformation products present in the commercial formulations using packed soil columns under laboratory conditions; (ii) to evaluate the efficacy of solarization and biosolarization on nematode population abundance; and (iii) to assess the use of solarization and biosolarization as remediation techniques for soils containing residues of these compounds under greenhouse conditions.

Materials and methods

Chemicals

Residue analysis grade acetonitrile was purchased from Scharlab (Barcelona, Spain). Formic acid (98% purity) and sodium chloride were ordered from Fluka-Sigma-Aldrich (Steinheim, Germany). Pesticide standards, ST (cis-4-(ethoxycarbonyloxy)-8-methoxy-3-(2,5-xylyl)-1-azaspiro[4.5]dec-3-en-2-one), SM (3-mesityl-2-oxo-1-oxaspiro[4.4]non-3-en-4-yl) 3,3-dimethylbutyrate), and SD (3-(2,4-dichlorophenyl)-2-oxo-1-oxaspiro[4.5]dec-3-en-4-yl) 2,2-dimethylbutyrate), were purchased from Dr. Ehrenstorfer (Augsburg, Germany) with a purity >98%. The chemical structures of the three insecticides and main physical-chemical properties of the active ingredients are shown in Supplementary Table 1. Experimental values of aqueous solubility (S W), octanol/water partition coefficient (K OW), aqueous hydrolysis, soil degradation, and soil/organic partition coefficient (K OC) were taken from The Pesticide Properties DataBase (PPDB 2016). Stock solutions (1000 μg mL−1) of each compound were prepared by dissolving 0.025 g of each in 25 mL of acetonitrile. Standard mix solutions (10 μg mL−1) for the calibration were prepared in acetonitrile from the stock standards and were kept at −18 °C before use.

Test soil for leaching and solarization experiments

A Haplic Calcisol (clay 32.9%, silt 30.2%, and sand 36.9%; bulk density 1.33 g cm−3, pH 7.3, organic matter 3.1%, and electrical conductivity 7.3 dS m−1), a typical soil from the Campo de Cartagena (southeastern Spain), was chosen for leaching and solar heating experiments.

Leaching study

The downward movement of pesticides was investigated following a previously published experiment protocol (Fenoll et al. 2011b). Briefly, polyvinyl chloride columns of 40 cm (length) × 4 cm (i.d.) were packed with 200 g of soil. The pore volume (PV) of the packed column estimation was 79.3 ± 1.6 mL. The top of each column was spiked with 2.5 mL of a methanol/water solution (10 + 90, v:v) containing 200 μg of each compound using commercial formulations (Envidor 24%, SD; Oberon 24%, SM; and Movento 15%, ST supplied by Bayer CropScience). Eight hundred milliliters of 0.01 M CaCl2 was added during 16 days for leaching of these compounds. The leachates were quantitatively collected on daily bases (50 mL day−1). At the end of the experiment, the column was opened and the soil was separated into two layers (10 cm each).

Nematode origin

A locally occurring Meloidogyne spp. population was used in the present study. Meloidogyne incognita race 2, originally isolated from a single egg mass of pepper plant, was grown in a greenhouse in Murcia. Species and race identifications were performed on differential host tests and with isozyme electrophoresis (Esbenshade and Triantaphyllou 1990; Robertson et al. 2006) by the Reference Laboratory in Madrid. Population was maintained on tomato cv. “Marmande Claudia” (Clausse Tezier) in climatic chamber at 23–24 °C, with 45–60% relative humidity during the light period and 85–100% relative humidity during darkness with 14-h light cycle. Pieces of infested tomato roots were placed on a Baermann funnel to hatch, and second-stage juveniles (J2) were collected 72 h later and used. Juvenile suspensions were then adjusted to contain 4500 ± 50 J2 in 10 mL and added to each pot. At the end of the experiment, the population density of M. incognita was estimated, extracting the nematodes on Baermann trays and counting the juveniles (J2) (Flegg 1967).

Soil remediation study

The soil remediation experiments were conducted during the summer season (July–September, 2013) using the methodology proposed by Fenoll et al. (2014). Treatments were as follows: (i) non-mulched soil (control, C); (ii) soil covered with plastic film (solarized, S); (iii) soil + composted sheep manure covered with plastic film (biosolarized, BS1); (iv) soil + sugar beet vinasse covered with plastic film (biosolarized, BS2); and (v) soil + meat-processing waste with plastic film (biosolarized, BS3) (25 pots per treatment). Meat-processing waste is a meat meal composed of pork skin, viscera, and bone. First, the manure and meat-processing waste were applied to the BS1 and BS3 pots at a rate of 32 and 3 g pot−1, respectively, according to the Nitrate Directive (Council Directive 91/676/EEC 1991) which fixes the maximum limit for manure application at 170 kg N ha−1 year−1 in nitrate vulnerable zones and then thoroughly mixed with the soil. In BS1 pots, only 4.47 kg of soil was used to avoid a “dilution effect” following the application of organic matter. The organic matter used for BS1 and BS3 was a composted sheep manure obtained by Indore method (Howard 1943) over a period of 4 months (pH = 7.8, electrical conductivity = 13.2 dS m−1, organic matter = 373.2 g kg−1, total N = 9.3 g kg−1, P = 10.8 g kg−1, and K = 29.7 g kg−1) and meat-processing waste (pH = 6.7, electrical conductivity =3.1 dS m−1, organic matter = 420.1 g kg−1, total N = 100.1 g kg−1, P = 1.0 g kg−1, and K = 0.4 g kg−1) supplied by El Pozo SA (Murcia, Spain), respectively. The organic amendment used for BS2 was a sugar beet vinasse (pH = 5.8, electrical conductivity = 3.5 dS m−1, organic matter = 592.3 g kg−1, total N = 33.3 g kg−1, P = 0.4 g kg−1, and K = 35.1 g kg−1) supplied by Azucarera del Ebro (Madrid, Spain). Vinasse was added to water and applied to the BS2 pots at a dose of 8 mL pot−1 according to the Nitrate Directive (Council Directive 91/676/EEC 1991). Next, the soil samples of each container were treated with a sprayer (Matabi) using Envidor (24% SD), Oberon (24% SM), and Movento (15% ST) formulations. The initial concentration of each insecticide in the soil was approximately 1 mg kg−1. All the pots were treated with 10 mL of juvenile nematode suspension and irrigated to field capacity. Solarization and biosolarization were initiated on the 10th of July. At the same time, the pots were treated with nematodes. Five pots per treatment were collected initially and 15, 30, 60, and 90 days after the application.

Insecticide analysis

Soil and water samples were extracted and analyzed according to the procedures described by Fenoll et al. (2009, 2011c). The separation of SD, SM, and ST was carried out using an HPLC system (Agilent Series 1200, Agilent Technologies, Santa Clara, CA, USA) equipped with an Agilent G6410A triple quadrupole mass spectrometer. A preliminary study of optimal selected reaction monitoring (SRM) transitions for each compound was carried out by infusing directly individual analytes (2 μg mL−1) into the ion source. Supplementary Table 2 lists the analytes, along with their optimized SRM transitions and their retention times.

Recovery studies were carried out in triplicate using soil and water samples spiked at two concentration levels (5 and 20 μg kg−1 for soil and 2 and 20 μg L−1 for water). The recoveries of the insecticides from spiked soil and water samples varied between 69 and 112%, and limits of quantification (LOQ, calculated as ten times the signal-to-noise ratio) ranged from 0.1 to 0.5 μg kg−1 and 0.02 to 0.1 μg L−1 in soil and water, respectively.

Statistical analysis

The main effect (disinfection treatment) and differences between means were analyzed statistically following ANOVA and Fisher’s least significant difference (LSD) post hoc test, respectively, using the IBM SPSS Statistics version 19.0 software (IBM Corporation, Armonk, NY) package.

Results and discussion

Leaching of pesticides through the soil

The distribution between soil and water for the pesticides applied to soil columns and their breakthrough curves (BTCs) are shown in Figs. 1 and 2, respectively. The results indicated that enol metabolites are more mobile than the parent compounds. Thus, under the assay conditions, SM and SD were found in small quantities in leachates, whereas ST did not leach out of the column and was therefore not detected in the leachates. However, all the studied enol metabolites were found in the leachates. The percentage of insecticides remaining in soil and water ranged from 0.003 to 10% of the initial mass for SD and ST, respectively (Fig. 1). Such differences between the initial and the recovered amounts can be attributed to biochemical and hydrolytic degradation during the experiment (Supplementary Table 1). The SD and SM levels found in the leachates were of 0.2 and 1.9%, respectively, of the total mass applied to the column while 0.5 and 1.5% of their residue levels were recovered from the bottom soil layer, respectively. The residual levels of SD, SM, and ST recovered from the upper soil layer were 9.7, 7.9, and 0.003%, respectively, of the initial mass; SM leaching began around 0.6 PV. The relative BTCs show that ST-enol leached rapidly (maximum about 2 PV) while for SD-enol and SM-enol, the maximum peaks were found around 4 PV (Fig. 2). As regards ST-enol, leaching ended at about 7 PV, probably due to its rapid hydrolytic degradation.

Fig. 1
figure 1

Distribution of insecticides and their enol transformation products after leaching process. The error bars denote standard deviation

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Fig. 2
figure 2

Relative breakthrough curves (BTCs) of insecticides and their enol transformation products found in leachates. The error bars denote standard deviation

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Bearing in mind the total amount found in the leachates, SD-enol, SM-enol, and ST-enol showed moderate leaching potential under our experimental conditions, meaning that these intermediates represent a potential risk for groundwater pollution. In contrast, SD, SM, and ST behaved as non-leaching pesticides.

Effect of treatments on nematode population

The effect of biosolarization and solarization on the population of nematodes (Meloidogyne sp.) was assessed by reference to a control. Annual variations in the effectiveness of biosolarization have been attributed to the kind of amendment used and the starting dates of solarization (Guerrero et al. 2013), both being related to soil temperatures during the disinfection process. In our test, the cumulative times with temperatures above 40 °C during soil disinfection at 15 and 30 cm depth were approximately 650 and 950 h, respectively, while in the control, only 70 and 4 h were recorded. Those temperature-time regimes caused 100% reduction in the number of juveniles in disinfected soils and 98.2% in the control, according to the threshold values (33 h at 40 °C for juveniles) set by Wang and McSorley (2008). The reduction in the number of juveniles obtained in the control treatment was also similar to that obtained by Wang et al. (2006) and Wang and McSorley (2008). Therefore, besides the demonstrated suitability of these non-chemical alternatives to methyl bromide for reducing the number of juveniles, they could be used as a remediation tool for soils polluted by pesticides.

Dissipation of pesticides in soil by solarization and biosolarization

Table 1 shows the evolution of SD, SM, and ST residues in non-disinfected, solarized, and biosolarized soils. Just after application, SD, SM, and ST residues ranged from 0.9 to 1.2 μg g−1. In the control treatment, similar degradation rates were observed for all the studied insecticides. In addition, differences between the C, S, and BS treatments were low, probably due to the high rate constant of these insecticides per se, which would disguise the effect of S and BS. However, as can be seen in Table 1, higher degradation rates were observed in disinfected soils than in the control, which agrees with previous findings (Fenoll et al. 2010a, b). The results showed significant differences (P < 0.05) between the control (C) and disinfected (S and BS) soils. In the case of mulched soils, dissipation was rapid, lowering residual levels to below 10 ng g−1 in the first 15 days. At this time, the residue levels found for the non-mulched soil were about 50, 60, and 80 ng g−1 for SD, SM, and ST, respectively. After 60 days, no residues of these insecticides were found in the mulched soils, while 3, 2, and 1 ng g−1 for SD, SM, and ST, respectively, were recovered from the non-mulched soil after 90 days. As far as ST is concerned, this compound appeared to be more readily degradable than the other two insecticides in both S and BS treatments. However, no significant differences between S and BS treatments were observed for any of the compounds, except for SD between the BS3 and the S and BS2 treatments.

Table 1 Dissipation of insecticide residues (mg kg−1) in non-disinfected, solarized, and biosolarized soils
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Table 2 shows the kinetic parameters obtained for C, S, BS1, BS2, and BS3 soils according to the first-order model. Fitting with the simple first-order equation (R t = R 0 × exp[−Kt]) provides R values in all cases higher than 0.999 for the studied STADs. This model allows kinetic parameters, such as the dissipation rate constant (K) and half-life (t ½), to be determined in a simple way and can be recommended for assessing dissipation of these compounds in non-mulched and mulched soils. The half-lives for non-mulched soils were 3.5, 3.7, and 4.1 days for SD, SM, and ST, respectively, while for mulched soils, the half-lives ranged from 1.9 to 2.2 days for SD, 1.6 to 1.9 days for SM, and 1.6 to 1.9 days for ST. As a consequence, higher degradation rates for SD, SM, and ST were observed in the treated soils, which can be attributed to the rapid hydrolysis of these compounds (PPDB 2016).

Table 2 Calculated values of constant rates (K, days−1) and half-lives (t ½, days)
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In the soil, pesticides may be transformed by biotic or abiotic processes, leading to transient organic intermediates. In this study, the evolution of enol derivative residues detected during the degradation of SD, SM, and ST in non-disinfected, solarized, and biosolarized soils was followed using LC-MS/MS. As can be seen in Table 1, these compounds appear at the beginning of the experiment, probably as impurities in the commercial formulations used. As a general rule, for both the solarization and biosolarization treatments, the initial concentration of SD-enol and SM-enol increased during the first 2 weeks of the trial and decreased by the end of the experiment. However, the initial concentration of ST-enol exponentially decreased in all cases, with its residues completely disappearing after 90 days except in the non-disinfected treatment. Both solarization and biosolarization treatments also enhanced the degradation rates of enol derivatives compared with the observed in the control soil. However, a similar degradation rate with solarization and biosolarization treatments was also observed for these transformation products. Based on the intermediates identified in the reaction system for tetronic/tetramic derivatives, the main mechanism of degradation involves electron transfer reactions with ●OH attacking the carbonyl group. Therefore, all intermediates (enol derivatives) may appear as a consequence of the cleavage of alkyl-oxygen bond of the parent compounds, as suggested by some authors for phthalates (Xu et al. 2009; Wang et al. 2012).

The results obtained indicate the effectiveness of using solar heating techniques (solarization and biosolarization) to reduce the persistence of STADs and their enol transformation products in the soil. This may be attributed to the increase in the soil temperature and the greater number of accumulated hours at high temperature in the mulched soils compared with the non-mulched soil (Fig. 3). Thus, an increase of 10 °C in the soil temperature increases the reaction rate 2.2-fold for activation energy of 54 kJ mol−1 (FOCUS 1997). In this work, the temperature reached by the soil with a maximum number of accumulated hours was 26 °C, in the control treatment increasing to 31 °C for solarized and biosolarized soils. In addition, the solarized and biosolarized soils were subject to a greater number of hours at temperatures above 27 °C. In those conditions, some thermophilic and thermotolerant organisms can survive and a number of bacterial taxa can decrease or increase (Stapleton and De Vay 1995). Therefore, soil temperatures during solarization can create a new microbial balance, which may also contribute to pesticide biodegradation in the soil. In addition, these temperatures can also increase the action of catalytic substances and the desorption of pesticides (Navarro et al. 2007).

Fig. 3
figure 3

Accumulated hours for the different soil temperatures reached during the experiment. Temperature measured at two depths: 0–15 and 15–30 cm

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In biosolarization treatments, exogenous organic matter can have contrary effects on degradation through an increase in adsorption or in degradation as a result of microbial activity. Thus, some studies have found that the introduction of exogenous amendments increases the pesticide degradation rates attributed to soil microbial activation (Pascual et al. 1997). However, other studies have suggested that pesticide degradation decreases due to an increase in adsorption following the addition of exogenous amendments (Rouchaud et al. 1994; Sánchez et al. 2003). In this work, the studied insecticides showed similar behavior in the S, BS1, BS2, and BS3 treatments. This can mainly be attributed to the high degradation rate of these insecticides. In addition, other factors can promote similar behavior, such as the inactivation of some soil microbes by solarization, the ability of the soil to adsorb these compounds in soil (bound residues), or the chemical reactions occurring on the active surfaces of mineral particles and humus (Lal and Saxena 1982; Nawab et al. 2003; PPDB 2016). In other cases, similar degradation rates were observed for solarized and biosolarized (composted sheep manure and sugar beet vinasse) treatments for linuron (Fenoll et al. 2014).

Conclusions

The study indicates that STADs are practically immobile in soil. However, the enol derivatives that appear during their hydrolysis are more mobile than the parent compounds. On the other hand, the results obtained in this work indicated the effectiveness of using different soil disinfection techniques (solarization and biosolarization) to reduce the nematode population and persistence of these compounds in the soil, probably due to the increased microbial activity resulting from changes in temperature and soil moisture. This study also revealed the similar effect of mulching combined with any of the organic residues used (composted sheep manure, sugar beet vinasse, and meat-processing waste) on the dissipation of these insecticides in agricultural soil.