Article pubs.acs.org/JAFC
Time-Dependent Movement and Distribution of Chlorpyrifos and Its Metabolism in Bamboo Forest under Soil Surface Mulching Yihua Liu, Danyu Shen, Donglian Zhong, Runhong Mo, Zhanglin Ni, and Fubin Tang* Research Institute of Subtropical Forestry, Chinese Academy of Forestry, Fuyang 311400, People’s Republic of China ABSTRACT: The dissipation and distribution of chlorpyrifos (CHP) granule formulation in bamboo forest under soil surface mulching conditions (CP) and noncovered cultivation conditions (NCP) from soil to product were investigated. In the CP treatment, the CHP granule with slow-release effect leached from the topsoil to the subsoil. Conversely, the CHP was fixed in the topsoil (0−5 cm layer) in the NCP treatment, and no obvious leaching effect could be observed. The residue of CHP could be found in bamboo shoots from CP treatment, mainly at the bottom part (5 cm length). CHP could be degraded into 3,5,6trichloro-2-pyridinol (TCP) in the soil and bamboo shoots. In addition, the straw used as the mulching material with higher OM and pH had some regulatory role in changing the pH and OM characteristics of the soil. Thus the straw could indirectly affect the adsorption and degradation behavior of CHP and TCP in the soil. KEYWORDS: chlorpyrifos, 3,5,6-trichloro-2-pyridinol, degradation, bamboo shoot, soil surface mulching
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INTRODUCTION
bamboo shoots is important for human health and environmental protection. The proper monitoring of pesticide residues is an important component of food safety control in agricultural products. Information on dissipation kinetics of pesticides in food crops and other plants is a key aspect in current risk and impact assessment practice, since human exposure to pesticides is predominantly caused by residues in agricultural crops grown for human and animal consumption. Due to the production pressure for food, CHP has been widely used in agricultural soils. Its degradation and metabolism had been studied in rice6 and different vegetables7 under field conditions. In recent years, some scientists have found that the cultivation process has a notable effect on the pesticide dissipation. For example, the dissipation rate of cyprodinil under greenhouse conditions was much faster than under field conditions either in strawberry or soil,8 and the half-lives of CHP in greenhouse cucumber9 and rice (field conditions)10 were 1.60 and 4.28 days, respectively. To the best of our knowledge, the behaviors of pesticides in soil and products under surface mulching cultivation are still poorly understood. Especially the metabolism of CHP (or other pesticides) in bamboo shoots and its cultivation soil is not available. Hence, in the current study, the fate of CHP in bamboo shoots under two cultivation patterns (soil surface mulching conditions (CP) and noncovered cultivation conditions (NCP)) from soil to product was studied. It is well known that the extent to which pesticides are susceptible to transport through and from soil and the contribution to non-point-source pollution are dependent on the process of degradation and adsorption, which determine the longevity and mobility of the pesticide in soil, respectively.11 Degradation and adsorption of pesticides in soils evidently
Bamboo shoot is one of the most popular types of nontimber forest product in Asian countries. China is the world’s largest producer and exporter of bamboo shoots. The bamboo shoots are exported to Japan, USA, and EU in large quantities every year. In China, soil surface mulching during bamboo shoot production (bamboo leaf, straw, and/or rice chaff are covered on the soil to increase the soil temperature and bring the harvest forward about 1 month) emerged as an important industry in the 1990s and is becoming more and more popular. Undoubtedly the fight against insects is the main difficulty in the cultivation of bamboo shoots, especially in soil surface mulching conditions. High relative humidity, dense bamboo, and scare sunshine favor insect reproduction. Although various practices are performed in order to minimize the incidence of these insects during bamboo growing, the most effective method to fight them is still the application of insecticides. Chlorpyrifos (O,O-diethyl O-3,5,6-trichloropyridin-2-yl phosphorothioate, CHP) is one of the most important pesticides used for insect control in bamboo growing. CHP is moderately toxic, and chronic exposure has been linked to neurological effects, development disorders, and autoimmune disorders.1 Furthermore, it is a suspected endocrine disruptor and has been listed as a candidate for priority review under the National Registration Authority’s Existing Chemical Review Program by the U.S. National Drugs and Poisons Schedule Committee in 2000.2 The main product of the hydrolysis and biodegradation of CHP is 3,5,6-trichloro-2-pyridinol (TCP). It was reported that TCP is 2−3 times more toxic to developing chick embryos than CHP,3 and it is more mobile compared to the parent molecule due to its higher water solubility (80.9 mg/L), thus causing widespread contamination of soil and aquatic environments.4 In addition, CHP and TCP can interact synergistically, resulting in an increase in the overall toxicity of the mixture compared to individual compounds.5 Therefore, monitoring the behavior of CHP and its metabolite TCP residues in © 2014 American Chemical Society
Received: Revised: Accepted: Published: 6565
April 8, 2014 June 19, 2014 June 27, 2014 June 27, 2014 dx.doi.org/10.1021/jf501540e | J. Agric. Food Chem. 2014, 62, 6565−6570
Journal of Agricultural and Food Chemistry
Article
was extracted with 50 mL of acetonitrile by homogenization with a high-speed blender (Ultra-Turrax T18, IKA, Germany) for 2 min. After the addition of 6 g of MgSO4 and 4 g of NaCl, each mixture was shaken intensively for 1 min and centrifuged for 5 min at 8000 rpm. An aliquot of the organic phase (25.0 mL) was transferred and concentrated by a rotatory evaporator at 40 °C to near dryness, then dissolved in 1.0 mL of methanol. Prior to analysis, the methanolic analyte was filtered through a 0.22 μm PTFE filter (Milford, MA, USA). A 2.0 g sample of soil (sieved through a 2 mm mesh) was mixed with 1 mL of distilled water for 5 min. Then methanol (5 mL) was added and vortexed for 2 min, and the sample was centrifuged for 5 min (4000 rpm). A 2.0 mL portion of the superstratum was transferred and filtered through a 0.22 μm PTFE filter. Chemical and Biological Analyses of Soils. Soil characteristics were examined including moisture content, pH, and organic carbon. Moisture content was measured according to Bending et al.12 Soil pH was measured in water with a soil to water ratio of 1:5 (w/v). Organic carbon was determined by dichromate oxidation and titrated with ferrous ammonium sulfate, followed by converting to soil organic matter content using the Van Bemmelen factor 1.724.14 Field Experiments and Sampling Procedure. The trial field (red soil) was located in Fuyang, Zhejiang Province, China. The recommended dosage is 120 kg/ha for 3% chlorpyrifos granules (Tianyi Agriculture Chemistry Company, Zhejiang, China). The field experiment was started on Nov 6, 2012. After the pesticide application, the surface soil was turned to mix uniformly. On the next day, compound fertilizer was applied by hand (1000 kg/ha), and the soil surface was covered by straw (20 cm in thickness). Representative soil samples were collected by random sampling, and CHP and its metabolic residues were measured using the method described above. The soil (0−20 cm) was divided into three layers (0−5, 5−10, and 10−20 cm) and collected (1 kg). Samples were collected randomly from each plot at 0, 6, 13, 21, 29, 38, 62, 83, 111, 140, 170, 201, 250, and 300 day intervals after pesticide application. The bamboo shoot samples were collected randomly from each plot at 83, 111, 115, 119, 125, 129, 132 day intervals after pesticide application. Each sample (1 kg) was chopped and divided into two samples. Data Analysis. To determine the kinetics of the degradation, plots of concentration against time were made, and an exponential regression analysis (first-order rate equation) was then performed on each data set.
show three-dimensional variability with highly complex relationships between bioavailability and biodegradation in both subsoil and topsoil.12 Understanding the extent of this three-dimensional variation and its controlling factors is essential in the risk assessment of environmental exposure. Additionally, pesticide quantitative-diversely distributes at different parts of plants (cuticle and internal parts or aboveground and underground parts, etc.), which results in differences in human intake risk. It is necessary to discuss the distribution of pesticides in the different parts of the plant. Wang et al. reported that the plasma of tomato initially accumulated the highest pesticide concentration, and the concentrations of chlorothalonil and chlorpyrifos in tomato followed the order cuticle > plasma > pulp.13 However, few studies thoroughly focused on the distribution of pesticides in soils and plants. Another purpose of this work is to determine the time-dependent dynamic distribution of CHP in different parts of bamboo area (bamboo shoots and soil) and the influencing factors. This study would help to provide basic information for developing regulations to safeguard the use of CHP in bamboo shoot cultivation and to prevent health problems in consumers. This may further lead to the development of optimized pesticide removal procedures.
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MATERIALS AND METHODS
Reagents and Solutions. Pesticide analytical standards were purchased from National Information Center for Certified Reference Materials (Beijing, China), certified quality. Individual pesticide stock solutions (100 mg/L) were prepared in methanol or acetonitrile and stored at −20 °C. Then, a series of dilutions containing the mixture of standards were prepared (10 mg/L) in methanol. HPLC-grade acetonitrile and methanol were obtained from Merck (Darmstadt, Germany). A Milli-Q-Plus ultrapure water system from Millipore (Milford, MA, USA) was used throughout the study to obtain the HPLC-grade water used during the analyses. Other solvents were from Shanghai Sanying Chemical Reagents (Shanghai, China), pesticide residue analysis quality. HPLC-MS/MS. The LC system consists of a high-performance liquid chromatograph (Waters, Milford, MA, USA) with an HSS T3 column (5 μm, 100 mm × 2.1 mm, i.d., Waters). A mobile phase consisting of solvent A (0.1% formic acid, in water) and solvent B (methanol) was used with the following gradient program: 80:20 A:B (initial), 10−90% A with 90−10% B (0−5 min), 10:90 A:B (5−10 min), 10−80% A with 90−20% B (10−14 min), 80:20 A:B (14−15 min). A subsequent requilibration time (3 min) was performed after each injection. The flow rate was 0.3 mL/min, and the injection volume was 10 μL. The column and sample temperatures were maintained at 35 °C. MS/MS was performed on a Waters Quattro Premier triplequadruple mass spectrometer equipped with an ESI source (Waters, Milford, MA, USA). MS/MS detection was performed in positive ion mode for CHP and in negative mode for TCP separately. The monitoring conditions were optimized for target compounds. Acquisition parameters were as follows: capillary voltage 3.5 kV, cone voltage 45 V, source block temperature 80 °C, cone gas 50 L/h, desolvation temperature 450 °C, desolvation gas (nitrogen gas) 550 L/h, respectively. As the precursor ion for CHP, 352 (m/z) was selected, and its quantitative and qualitative product ions were 97 (m/ z) and 198 (m/z), respectively, when the collision energies were both 30 V. As for TCP, 196/35 was selected as the quantification ion transition, and 198/37 were selected as the confirmatory ion transition with all the collision energies at 15 V. Multireaction monitoring mode was selected as the scan mode. Under the described conditions, the retention times of CHP and TCP were approximately 9.92 and 7.88 min, respectively. Pesticide Analysis. A portion (20.0 g) of prehomogenized sample (bamboo shoot) was weighed in a 250 mL glass breaker. The sample
c = c0e−kt where c represents the concentration of pesticide at time t, c0 represents the initial concentration, and k is the rate constant.
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RESULTS AND DISCUSSION Efficiency of Analysis Method. The accuracy and precision of the method were obtained with recovery studies by spiking tested compound-free soil and bamboo shoots at three levels (Table 1). The control samples from untreated plots and reagent blanks were also processed in the same way so as to find out the interferences, if any, due to the substrate and reagents, respectively. The peaks of CHP and its metabolic TCP were free from interfering peaks of soil and bamboo shoots. For the blank sample substrate, there was no peak detectable in the range of runtime. The recovery of CHP and TCP from soil and bamboo shoots ranged from 70.5% to 108.2% with RSD (relative standard deviation) ranging from 3.4% to 10.5%. The lowest spiked concentration of the analyte, which could be determined with acceptable precision and accuracy, was considered as the limit of detection (LOD) of the method. From the validation data presented in Table 1, it can be observed that the LODs were 0.10 mg/kg (soil) and 0.01 mg/kg (bamboo shoot), respectively. 6566
dx.doi.org/10.1021/jf501540e | J. Agric. Food Chem. 2014, 62, 6565−6570
Journal of Agricultural and Food Chemistry
Article
days, the slow-release effect weakened or disappeared. From 83 to 250 days, the degradation equation in Tl was Ct = 11.52e−0.0164t (R2 = 0.87), with a half-life 42.3 days. From 29 to 140 days, the CHP residue could be detected in Ml (Table 2), with the concentrations ranging from 1.25 to 0.15 mg/kg.
Table 1. Recoveries and RSD of Fortified Soil and Bamboo Shoot Samples matrix soil
bamboo shoot
compound CHP
CHP
soil
TCP
bamboo shoot
TCP
fortified level (mg/kg) 0.1 0.25 0.5 0.01
RSD (%)
recoveries (%) 85.2 93.2 93.2 100.2
89.6 95.7 98.7 95.7
80.3 105.6 106.7 93.4
5.5 6.7 6.8 3.6
0.05 0.1 0.1 0.25 0.5 0.01
91.3 108.2 93.6 73.2 89.4 79.3
86.7 100.7 82.3 86.7 96.3 70.5
92.5 99.3 85.1 89.6 87.2 85.4
3.4 4.7 6.8 10.5 5.2 9.6
0.05 0.1
88.2 92.4
76.8 88.6
85.1 83.7
7.1 4.9
Table 2. Distribution of CHP in the Middle Layer (Ml) and Bottom Layer (Bl) of CP Soil time (day) 0−21 29 38 62 83 111 140 170−250
Ml (±SD, mg/kg)
