Migration behavior of benzobicyclon hydrolysate and associated influencing 1 factors in different agricultural soils 2

Abstract. Benzobicyclon is a triketone pro-herbicide that needs to be hydrolyzed to form an active compound benzobicyclon hydrolysate (BH). This study aimed to investigate the migration behavior of BH in different types of agricultural soil and the associated influencing factors. Soil thin-layer chromatography and column leaching tests were used to study the migration behavior of BH in these soils. Based on the mobility retention factor (Rf = 0.34–0.90), the mobility of BH in thin soil layers was ranked in the order Lixisols > Anthrosols > Ferralsols > Phaeozems. The Rf value of BH was linearly positively correlated with soil sand content and pH, and negatively correlated with other physical and chemical properties of soil. BH was difficult to leach in Phaeozems, less difficult to leach in Ferralsols, and easy to leach in Anthrosols and Lixisols. Increasing the BH dosage and rainfall amount or adding humic acid and anionic (dodecyl benzene sulfonic acid) or nonionic (Tween-80) surfactant blocked BH migration in soil columns. In contrast, increasing the leaching solution pH and adding cationic surfactant (cetyl trimethyl ammonium bromide) promoted BH migration in soil columns.BH application has a low risk of groundwater pollution in Phaeozems and Ferralsols, but poses a potential threat to groundwater in Anthrosols and Lixisols.



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In rice production, the presence of weeds is a serious problem for rice crop growth and 46 high food yields, because weeds and rice plants coexist in paddy fields and compete for 47 Agilent Technologies). The chromatographic conditions were as follows: Mobile phase, 155 methanol-0.2% phosphoric acid water (55:45, v/v); flow rate, 1 mL/min; detection wavelength, 156 286 nm; column temperature, 40 ℃; injection volume, 10 µL; and the retention time of mL) was added. After stirring using a glass rod, the slurry was applied onto a glass plate (10 177 cm ´ 20 cm) and dried at 25 °C (±2 °C), with the thickness of the soil controlled between 0.5 178 and 1.0 mm. Subsequently, BH stock solution (10 µL, dissolved in methanol, 1000 mg/L) was 179 spotted at 2.5-cm intervals from the bottom of the glass plate at 25 °C (±2 °C) under light-proof 180 conditions. Three parallel solutions were prepared for each treatment. After the solvent had 181 evaporated, the thin plate was placed into a chromatography tank (20 cm ´ 30 cm ´ 30 cm) 182 with distilled water as the developing agent. The thin plate was unfolded to 15.0 cm and then 183 taken out from the tank. After drying, the soil on the thin plate was divided into six equal 184 segments (2.5 cm each). The BH content in the soil from each segment was measured using 185 HPLC analysis and its distribution on the thin plate was analyzed.  188 To explore the downward movement of BH in different types of agricultural soil, soil 189 column leaching tests were conducted following the OECD-312 standard method issued by the 190 World Economic Cooperation Organization (OECD 2004). The soil column was prepared using 191 a PVC pipe (inner diameter, 4 cm, length, 40 cm). A piece of filter paper was spread at the 192 bottom of the soil column, which was overlaid with a nylon mesh (180 µm). Next, a 1-cm-thick 193 quartz sand layer was added to the column, followed by the sieved soil sample (600-700 g) to 194 form a 30-cm deep soil column. A 0.01 mol/L CaCl2 aqueous solution was added to the soil 195 column from the bottom to saturate the soil and remove the air through reverse osmosis.

Test with different types of soil
where D is the dosage of BH added to each soil column (µg), M is the maximum recommended 203 dosage of BZB (kg/hm 2 ), d is the soil column diameter (cm), and p is 3.14. According to the 204 herbicide registration announcement, the maximum recommended dosage of BZB in rice-205 growing areas is 450 g/hm 2 . Therefore, the dosage of BH for the leaching test was calculated 206 to be 56 µg, and 56 µL of BH stock solution was added dropwise on the surface of each column.

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After application of BH, the soil surface was covered with a 1-cm-thick quartz sand layer, 208 followed by a piece of filter paper and then a small amount of coarse sand. The soil column 209 was leached with a 0.01 mol/L CaCl2 solution to simulate artificial rainfall (250 mL for 48 h) 210 and the leachate was collected. After leaching was complete, the soil column was evenly cut 211 into three sections. The BH content in the leachate and each section of the soil was measured.  213 To analyze the influence of BH application dosage on BH leaching in the soils, three 214 different dosages of BH (56, 84, and 112 µg) were added to the soil columns, corresponding to 215 1.0, 1.5, and 2.0 times the maximum recommended dosage of BZB in rice-growing area. Briefly,

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BH stock solution (56, 84, or 112 µL) was added dropwise to the surface of the soil columns.

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The test procedure and sample handling were the same as described in Section 2.7.1. Each 218 treatment was repeated three times.

2.7.3．Test with different rainfall amounts
To investigate the depth of BH leaching in the soils after rainfall and assess the risk of BH 221 residues to groundwater and drinking water, the test was performed using three different 222 rainfall amounts (250, 500, and 1000 mL) for 48 h. The test procedure and sample handling 223 were the same as described in Section 2.7.1. Each treatment was repeated three times.  225 To investigate the influence of different pH values on BH leaching in the soils, the test 226 was performed at different pH levels. Owing to acid rain caused by industrial pollution in some 227 areas throughout the year, rain can reach a pH as low as ~3.5. During crop production, it is 228 critical to improve soil fertility and increase crop yield. To increase income, fertilizers are often 229 applied to soil, some of which are alkaline and, therefore, increase the pH of the local soil. In 230 this study, pH values of 3.0, 5.0, 7.0, and 9.0 were selected as the initial leaching solution pH.

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Before the test, the pH of 0.01 mol/L CaCl2 solution was adjusted to 3.0±0.2, 5.0±0.2, 7±0.2, 232 and 9±0.2 with 0.01 mol/L HCl or NaOH. The test procedure and sample handling were the 233 same as described in Section 2.7.1. Each treatment was repeated three times.  To investigate the influence of humic acid on BH leaching in the soils, the concentration 243 of humic acid (≥90%, Macklin Biochemical Co., Ltd., Shanghai, China) was set to 0.5%, 1%, 244 and 2% under the range of organic matter content in the experimental soils (0.23-2.04%; Table   245 1). The soil column without humic acid added was used as a control. The test procedure and 246 sample handling were the same as in Section 2.7.1. Each treatment was repeated three times.  248 The mobility retention factor (Rf) of BH on the thin plate was calculated using equation (2):

Data analysis
where i is the number of segments in which the plate is divided, Zi is the distance of BH in 251 segment i from the origin, Zw is the distance of the solvent front from the origin, and Mi is the

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The BH contents in each soil section and the leachate were calculated as the percentage 254 of BH recovered from the total amount of BH added using equation (3): where Ri is the mass fraction of BH in section i of the soil or leachate in the total mass of BH 257 added; mi is the mass of BH in each section of soil and leachate (mg); i = 1, 2, 3, and 4, which 258 represents the 0-10 cm, 10-20 cm, and 20-30 cm soil sections, and the leachate, respectively; 259 m0 is the total mass of BH added (mg). According to the value of Ri, the mobility of BH in the  varied considerably with different soil types (Fig. 2.). After leaching with CaCl2 solution (250 301 mL), BH in the S1 soil columns was mainly distributed in the 0-20 cm sections, with the 302 maximum BH content in the 10-20 section. In the S2 soil columns, BH was also mainly 303 distributed in the 0-20 cm sections, but the BH content in these sections was markedly higher 304 than that of S1, while the maximum BH content occurred in the 0-10 cm section. For soils S3 305 and S4, BH was mainly distributed in the leachate, with the leaching rate of BH in S4 greater 306 than that in S3.

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Based on the Ri values, the leaching rate of BH in the four agricultural soils was in the order S4 > S3 > S1 > S2 (  changed considerably (Fig. 3.). When 56 µg of BH was applied, BH in the S1 soil columns was 324 mainly distributed in the 0-20 cm sections and the maximum BH content was located in the 0-  344 The migration behavior of herbicides in soil mainly includes upward, downward, and showed that BH leaching and migration in the four types of agricultural soil were influenced 349 by the rainfall amount (Fig. 4). When the simulated rainfall was 250 mL, BH was mainly 350 distributed in the soil sections of S1, with the maximum BH content in the 10-20 cm section.

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In S2, all BH was distributed in the soil sections, and the maximum BH content was located in     Rf 0-2.5 cm 2.5-5 cm 5-7.5 cm 7.5-10 cm 10-12.5 cm 12.5-15 cm S1     Table 1). Ri represents the mass fraction of BH recovered from the 0-10, 10-20, and 20-30 684 cm soil sections or the leachate in the total amount of BH added, respectively. Values are the 685 means ± standard error (n = 3). under different application dosages of benzobicyclon hydrolysate (S1 to S4 are defined in Table   688 1). Values are the means ± standard error (n = 3). amounts (S1 to S4 are defined in Table 1). Values are the means ± standard error (n = 3). levels (S1 to S4 are defined in Table 1). Values are the means ± standard error (n = 3).
693 Fig. 6. The distribution and content of BH in soil columns after leaching in the presence of 694 different surfactants (S1 to S4 are defined in Table 1). Values are the means ± standard error (n 695 = 3).
696 Fig. 7. The distribution and content of BH in soil columns after leaching with the addition of 697 different concentrations of humic acid (S1 to S4 are defined in Table 1). Values are the means ± 698 standard error (n = 3).  Table 1). Ri represents the mass fraction of BH recovered from the 0-10, 10-20, and 20-30 under different application dosages of benzobicyclon hydrolysate (S1 to S4 are defined in Table   715 1). Values are the means ± standard error (n = 3). amounts (S1 to S4 are defined in Table 1). Values are the means ± standard error (n = 3). levels (S1 to S4 are defined in Table 1). Values are the means ± standard error (n = 3). 724 725 Fig. 6. The distribution and content of BH in soil columns after leaching in the presence of 726 different surfactants (S1 to S4 are defined in Table 1). Values are the means ± standard error (n 727 = 3).