Adsorption and degradation behavior of six herbicides in different agricultural soils

This study focuses on the assessment of herbicide adsorption and degradation in three soils (Haplic Chernozem, Haplic Fluvisol, and Arenic Regozem) from different agricultural regions of the Czech Republic where sunflower is cultivated. Soil samples were used in laboratory batch adsorption and degradation experiments for six herbicides commonly used on sunflower crops. The findings are used to examine the effect of soil and herbicide properties on adsorption and degradation, as well as to determine the possible relationship between the two processes. The (Kf) sorption coefficient ranged from 1.07 to 135.37 cm3/nμg1−1/ng−1, and sorption increased in the following order: dimethenamid-p < pethoxamid < S-metolachlor < flurochloridone < aclonifen < pendimethalin. Adsorption of all six herbicides was positively correlated with soil organic matter content (p < 0.001), and cation exchange capacity (p < 0.001). pH was negatively correlated with the adsorption of all six compounds (p < 0.001). Degradation rates of herbicides ranged from 0. 012 to 0. 048 day−1, which corresponding to the half-lives (DT50) between 14 and 57 days, respectively. The highest half-life values were found in Haplic Fluvisol (a loam with higher organic matter content and lower pH). Results showed that both adsorption and degradation of herbicides are mainly controlled by soil organic matter. A negative relationship was found between the sorption coefficient and the rate of degradation. It can be concluded that the Freundlich sorption coefficient (Kf) can be a good predictor for the degradation of the studied herbicides.


Introduction
European Union sunflower production volume accounts third largest after Ukraine and Russia, contributing about 10% of the world production amount in 2019 (Kandel et al. 2020). Sunflower cultivation areas are expected even more to be expanding in near future with its potential use as green energy for sustainable transport systems and as alternative cash crop in Europe (Fontaras et al. 2012;FAO 2020). To meet the increasing demands, considerable amount of pesticide application in sunflower seed production to control of more than 80 diseases and harmful insects of about 60 species in a particular season is required (EC 2009). Herbicides are the most common agrochemicals used in world agriculture, in Europe their usage share exceeded 45% (Braschi et al. 2011). The herbicides of pendimethalin, aclonifen, flurochloridone, S-metolachlor, pethoxamid and dimethenamid-p are most frequently used for pre-and post-emergence control of various annual grasses including dicot weeds and broad-leaf weeds in annual and perennial crops (Kilinc et al. 2011;Jursík et al. 2015). Many of them are used both individually and in combination to achieve maximum efficacy of weed control which is likely going to be expanding trend (Gunstone et al. 2021).
Increasing pesticide use in agricultural production raising the concern of pesticide accumulation in soil and thus its further behavior and degradation have been targeted by numerous studies (Oliveira and Brighenti 2011;FAO 2020). Several factors, including physicochemical characteristics of the pesticides and soil properties (e.g., pH, soil organic matter (SOM), clay content, soil texture content) are the critical factors responsible for the persistence, adsorption, and dissipation/degradation of the herbicides in soil (Ferreira et al. 2001;Quintero et al. 2005;Kah and Brown 2007;Kodešová et al. 2011;Sondhia 2013;Janaki et al. 2015). However, the importance of SOM has been extensively stressed than other soil components in retention or dissipation of herbicides in soil (Takeshita et al. 2019;Bonfleur et al. 2015;Tejada and Benítez, 2017). Authors reported that about 40% of the soil CEC is maintained by SOM in heavy soils, while almost 60% of it is dependent on the SOM in light soils (Alves et al. 2001;Alvarez-Puebla et al. 2005;Bayer and Mielniczuk 2008;Bedmar et al. 2011). Therefore, cation retention by SOM affects the behavior of herbicides and cause high adsorption through which reduces their leaching into groundwaters and thus to be subjected into degradation (Bedmar et al. 2011;Palma et al. 2016).
Soil processes, such as adsorption and degradation, remain critical tools in assessing the environmental behavior of the pesticides in soils (Quintero et al. 2005). Even, SOM tend to be of great importance in controlling soil herbicide adsorption, there are several factors affecting to herbicide behaviors in soil including pH and clay content (Villaverde et al. 2008). Several studies found that sandy soils, which usually have lesser sorption capacity that permit herbicides to migrate in soil profile (Krauss and Wilcke 2002) and thus cause more herbicide contamination, while clay soils are less permeable, thus retain more herbicides so that preventing them further leaching (Renaud et al. 2004;Alamgir 2016). Kah and Brown (2007) found that most herbicides adsorption and degradation are pH-dependent. The degradation may be intensive at higher pH and when pH is weakly alkaline, herbicide degradation tends to go under microbial presence. Steckel et al. (2013) reported that maize crop can be damaged from isoxaflutole and flufenacet in soil under less SOM and high pH. Besides, herbicide sorption and pH may have correlated in presence of weakly basic and acidic ions of the herbicides (Kah and Brown 2007). In general, high level of soil pH decreases herbicides adsorption in soil, meanwhile higher adsorption can be seen when soil pH is lower (Hiller et al. 2012;Palma et al. 2016).
Despite some herbicides have less water solubility and thus leaching, and may be adsorbed by SOM and clay (Chopra et al. 2010;Shaner 2012), several literatures reported their detection in ground and surface waters which may occurred mostly due to porous flows, runoff or erosion (Goncalves et al. 2007). Along with stated above knowledge, there is little known about several herbicide behavior under sunflower plants and their adsorption and degradation relationships with various soil types and their characteristics especially stressing on SOM, pH and soil texture (Wauchope et al. 2002;Coquet et al. 2004), and comprehensive knowledge is of importance to accurately predict their environmental persistent and behavior to prevent further concerns (Boivin et al. 2005).
Therefore, this research aims: (i) to characterize the adsorption and degradation of herbicides in three soils stressing different properties; (ii) to identify main soil parameters that influence the adsorption and degradation of the selected herbicides employing the correlation analysis, and (iii) to evaluate how the adsorption and degradation processes of those herbicides affect each other.

Soils
Three representative soils used in this study were obtained from three different agricultural sites near the sunflower field in the Czech Republic. The soils were sampled from the top layer (0-20 cm) and used for both adsorption and degradation experiments. The selected sites had no history of pesticide treatment in the recent past. The soils were classified as Haplic Chernozem with silty clay loam (Collected in Suchdol), Haplic Fluvisol with loam (in Dobroměřice), and Arenic Regozem with sandy loam (collected in Volárna). Soil profile pictures with descriptions are given in Supporting information, Pictures S1, S2, S3. The physicochemical properties of soils were measured in laboratory conditions and summarized in Table 1.

Adsorption experiment
The batch equilibrium method was used to conduct the sorption experiment (OECD 2000). Stock solutions were prepared using 0.01 M CaCl 2 aqueous solution containing 1, 2.5, 5, 10 and 25 μg mL −1 of each herbicide. 10 g of airdried, ground, and sieved (≤ 2 mm) soil samples were put in 50 mL of glass bottles in triplicates and 20 mL of stock solutions were added. The bottles were then placed on a mechanical shaker for 24 h at room temperature (20 °C) and centrifuged at (13,000 rpm) for 10 min. Supernatants were filtered by a glass syringe filter (0.7 μm) and then analyzed by HPLC-UV (Dionex, USA). The amount of herbicide adsorbed to soil particle (μg g −1 ) was calculated from the difference between the initial and final concentrations (μg cm −3 ) of the solution. The sorption isotherms were described using the Freundlich equation as follows: where C s is the amount of herbicide adsorbed to the soil (μg g −1 ), C e is the equilibrium concentration in solution (μg cm −3 ), K f and 1/n are empirical constants. The distribution coefficient (K d ) for all soils was calculated (see Supporting information S2). (1)

Degradation experiment
The degradation experiment was carried out using a standard procedure (OECD 2002). The soil samples were dried in the laboratory, plant parts, and the seeds were removed before grinding and the soil was passed through a 2 mm sieve. Fifty grams of soil were placed in 50 cm 3 glass bottles and the herbicide solution was added separately. The preparation time for soil samples did not exceed 5 days. The herbicide solubility in the initial solution was increased by methanol and a stock solution of 0.01 M CaCl 2 was prepared. The treated samples were mixed and incubated in a thermostat at 20 °C constant temperature. During the incubation period, the moisture content of the samples was monitored once a week and the caps were not tightly closed to prevent anaerobic conditions. The incubation period was 0, 1, 2, 5, 12, 23, 46, 80 and 120 days. Samples were collected at predetermined intervals and immediately stored in a freezer at -20 °C until further processing of the extractions. The soils were dispersed in 50 mL of methanol and the bottles were shaken for 24 h (at 200 rpm). The extracts were centrifuged for 10 min (at 4700 rpm) and then filtered using a 0.7 μm glass syringe filter and transferred to 1.5 ml vials. The modified method by Kočárek et al. (2016) was used to determine the residual concentration of herbicides in soil solution (μg cm −3 ) using the HPLC (Dionex; USA) technique. The rate of degradation and half-life (DT 50 ) of herbicides were calculated using the following equation: where C t is the residual amount of herbicide in soil (μg g −1 ), C 0 is the initial concentration of herbicide in solution (μg g −1 ), t is time (days), and k R is degradation rate constant (day −1 ). The half-life (DT 50 ) of herbicides (day) was calculated as follows:

HPLC conditions and herbicides' calibration lines
The samples from adsorption and degradation studies were analyzed by HPLC (Dionex, USA), using a PDA-100 photodiode array detector equipped with a P680 HPLC pump and ASI-100 automated sample injector. The guard columns (Security Guard Cartridge AQ C18 4 × 2.00 mm) precolumn, connected to a Kinetex 2.6µ, C18, 100 A column, 50 × 4.6 mm (Phenomex) was used for separation of the studied herbicides. The injection volume (μL), retention time (min), and detection wavelengths (nm) for each herbicide are shown in Table 3. The flow rate of the mobile phase was 1 mL min −1 and the column temperature was set at 25 °C. The mobile phase was prepared separately for each herbicide by mixing acetonitrile, redistilled water, and formic acid. (2) For pendimethalin, 700 mL of acetonitrile, 300 mL of water and 1 mL L −1 of formic acid, for aclonifen 850 mL of acetonitrile, 150 mL of water and 1 mL L −1 of formic acid, for flurochloridone, 800 mL of acetonitrile, 200 mL of water and 1 mL L −1 of formic acid, for S-metolachlor 500 mL of acetonitrile, 500 mL of water and 1 mL L −1 of formic acid, for pethoxamid, 900 mL of acetonitrile, 100 mL of water and 1 mL L −1 of formic acid, for dimethenamid-p 525 mL of acetonitrile, 475 mL of water and 1 mL L −1 of formic acid. The herbicides' limits of detection (LOD) were determined as the lowest injected concentrations of the pesticides yielding signal-to-noise ratios of 3. The herbicides' limits of quantification (LOQ) were determined based on the standard deviation (STD) of the UV detector response and the slope of the calibration line (s) (LOQ = 10 STD/s). The tested herbicides' recovery was ranged from 84.9% to 118.1.6% for all herbicides, respectively.

Statistical analyses
Statistical analysis was carried out to determine the relationship between treatment variables. A simple and multiple correlation analyses between soil properties and absorption coefficient and degradation experiment results were evaluated using the Pearson correlation coefficient and p value. The soil properties (Table 1) were the main predictors of correlation analysis and the statistically significant level was always 0.05 or lower. All analyses were performed using the Statistica® 13 (StatSoft, Inc. Oklahoma, USA).

Sorption isotherm
The herbicides Freundlich sorption isotherms are presented in Fig. 1, and sorption parameters (K f and n) together with the regression coefficients and R 2 values calculated for herbicides with three soils are given in Table 4. The K f values ranged from 1.07 to 135.37cm 3/n μg 1−1/n g −1 and sorption of herbicides increased in the order dimethenamid-p < pethoxamid < S-metolachlor < flurochloridone < aclonifen < pendimethalin. The slopes (n > 1) of isotherm of herbicides calculated for three soils indicated a high level of linearity as n values, being higher than 1, excluding flurochloridone. Flurochloridone adsorption is defined as an S-type curve, since n values were < 1 in all three soils and varied from 0.76 to 0.85, respectively. The K f values of pethoxamid for three soils are within the range of 1.88-5.61 cm 3/n μg 1−1/n g −1 , while its highest sorption was indicated in Haplic Fluvisol (Table 4). Among the six herbicides, the highest K f value reported was systematically observed in Haplic Fluvisol (a loam with high SOM and lower pH, Table 1). The K f values in this soil ranged from 2.41 to 135.37 cm 3/n μg 1−1/n g −1 for dimethenamid-p and pendimethalin. Dimethenamidp showed the lowest K f values in the studied soils and was relatively smaller than the value in PPDB (2019), (K f = 3.69 cm 3/n μg 1−1/n g −1 ). The lowest sorption values were found in the Arenic Regozem (a sandy loam with low SOM and higher pH) for dimethenamid-p and pethoxamid. It should be noted that pendimethalin displayed the highest K f values and was more strongly adsorbed on all three soils than any of the other herbicides (Table 4). Table 5 shows the statistical correlation between herbicide's K f coefficients and soil properties. The K f coefficient of herbicides was strongly correlated with SOM and CEC (at p < 0.001). This indicates that they are the major predictors of sorption and increase affinity to soils with a high SOM content. Besides, a positive correlation was observed between SOM and CEC (r ≥ 0.989, p < 0.001), and no relationship was found between clay content and CEC (Tables 6, 7). The K f was negatively correlated to soil pH H2O (r ≥-0. 829, p < 0. 001). A negative, but not significant correlation was also observed between K f and pH KCL for all herbicides (Table 5).

Herbicides degradation
Degradation rate and half-life (DT 50 ) values for herbicides are shown in Table 8 and degradation curves for six herbicides in three soils are presented in Fig. 2. The degradation rate decreased in the order aclonifen < pethoxamid < dimethenamid-p < pendimethalin < flurochloridone < S-metolachlor. Degradation rate constants of herbicides were in the range of 0.012-0.048 d −1 which, corresponds to half-lives between 14 and 57 days. The longest DT 50 values for herbicides were observed in Haplic Fluvisol, while the shortest values were in Haplic Chernozem. The dissipation half-lives for dimethenamid-p in the three soils ranged from 19 to 34 days, which were almost within the range of DT 50 (7.7-31.5 days) reported in EFSA (2018). The calculated half-life values were extremely short, especially in Haplic Chernozem (more than 10 times) and Arenic Regozem (around 8 times). Despite this, the average pendimethalin half-life value was within the same range ( The average values were 18 days for aclonifen and 20 days for pethoxamid. Shortest values were found in Haplic Chernozem and Arenic Regozem. The dissipation halflives (from 38 to 51 days) for flurochloridone in three soils were consistent with those previously reported, ranging from 9 to 66 days (EFSA, 2010). A longer dissipation half-life (51 days) was found in Arenic Regozem. The observed halflife values for S-metolachlor ranged from 37 to 57 days, and the highest value was found in Haplic Fluvisol (Table 8).
The findings of herbicide degradation were statistically analyzed and the rate of degradation with clay content was positively correlated and a negative correlation was found with the SOM and CEC contents (Table 9).

Sorption behavior of herbicides
In terms of K f parameters, our findings are in agreement with those shown by Wauchope et al. (1992) and Rytwo et al. (2005), which addressed the strong affinity of pendimethalin to solid phases due to its hydrophobic nature. Kočárek et al. (2018) previously studied sorption isotherms of pendimethalin under laboratory conditions and obtained a higher K f value (270.1 cm 3/n μg 1−1/n g −1 ) in a silt loam soil. Aclonifen showed significantly lower K f values than that shown in PPDB (K f = 138.1 cm 3/n μg 1−1/n g −1 ). On the other hand, Trevisan et al. (1999) tested the sorption of aclonifen in 9 different soil types corresponding to our soil characteristics and determined significantly higher K d values between 8.54 and 602.60 mL g −1 . Flurochloridone, as previously stated, has a relatively higher affinity for all three soils at low concentrations, while its adsorption may decreases at high concentrations (Pinna et al. 2014). Adsorption coefficient (K f ) values (Table 4) for s-metolachlor, pethoxamid, and dimethenamid-p showed that they have a considerably lower sorption behavior compared to the other three herbicides. This was probably due to their physicochemical properties, such as a smaller polar surface area (Å2) and high-water solubility (Table 2). However, the K d values for S-metolachlor were within the range of previously reported findings for soils with similar properties, e.g., 0.51-3.40 cm 3 g −1 (Si et al. 2009), 0.76-16.67 cm 3 g −1 (Westra et al. 2015), and 0.6-5.7 cm 3 g −1 . (Weber et al. 2003). Pose-Juan et al. (2018) recently investigated the sorption of pethoxamid in amended and un-amended sandy loam soil, resulting in significantly low K f values 0.20 cm 3/n μg 1−1/n g −1 for the un-amended soil. Furthermore, a high level of linearity was observed in the un-amended soil by the pethoxamid adsorption curve (n f = 1.68), whereas its sorption by amended soil was near to the linearity of (C-type) (  μg 1−1/n g −1 ) for dimethenamid-p in silt loam soil. Westra et al. (2015) examined dimethenamid-p sorption in 25 different soils and the mean K d values calculated for soils were 2.3 cm 3 g −1 .

The effect of soil properties on herbicide sorption
Based on the herbicides' properties and pKa values (Table 2), their molecules are in neutral form and behave as     a non-ionic organic compound. Recently published authors have stated that hydrogen bonding is a key mechanism for the adsorption of non-ionic molecules on SOM and clay minerals. The effect of SOM on the adsorption of non-ionic herbicides was evaluated under different soil conditions. For the compounds flurochloridone, S-metolachlor, dimethenamid-p, and pethoxamid, our findings support the predicted interactions described in the literature (Weber et al. 2004;Pinna et al. 2014;Pose-Juan et al. 2018).
In general, organic matter has a high adsorption capacity, in particular compounds with lower water solubility (Carringer et al. 1975), even though the structure of the pendimethalin and aclonifen molecules have considerably low water solubility (0.33 and 1.4 mg L −1 ) and high K foc values (13,792 and 7,126 cm 3/n μg 1−1/n g −1 ) ( Table 2). However, the findings suggested that the adsorption of all six herbicides was positively related to SOM (r ≥ 0.981, p < 0.001) regardless of their water solubility and K foc values. It should  be noted that some of the hydrophobic pesticide mobility can be increased in soils with a high SOM content due to increased solubility of compounds in the presence of dissolved organic carbon (DOC) as shown by Kodešová et al. (2012) for chlorotoluron and in the case of DDT. Soil minerals and organic components are used to determine the soil adsorption potential of pesticides, particularly non-ionic compounds such as atrazine (Spark and Swift 2002;Ben Hur et al. 2003). Ben Hur et al. (2003) the contribution of clay minerals to pesticides can be significant, when the ratio between clay minerals and a fraction of organic carbon is greater than 30. A small ratio was observed between soils, except for Haplic Fluvisol (the ratio clays/SOC was 35). However, the effect of clay content on herbicide adsorption was not observed for all tested soils, as K f values and clay content was not significantly correlated (r ≤ 0.228) ( Table 5).
On the other hand, it is difficult to assess the influence of a single soil property on soil adsorption, since they are always correlated each other. In our study, soil pH H2O was negatively correlated with SOM (r ≥ − 0. 905, p < 0. 001), and CEC (r ≥ − 0. 919, p < 0. 001) ( Table 6). Weber et al. (1989) indicated that pesticide adsorption by hydrophobic bonding is pH-independent. Ahmad et al. (2014) reported that the adsorption behavior of the molecule is influenced by the pKa of the herbicide and the pH of the soil solution.
According to Kan and Tomson (1990), adsorption of nonionic molecules is less affected by soil pH. In our study, the pKa values of pendimethalin and aclonifen appeared to be insignificant for soil sorption. Based on the information provided in PPDB (2019), both pendimethalin and aclonifen are acidic herbicides and the dissociation of pendimethalin is somewhat inconsistent (Kočárek et al. 2018), but stable in the pH range 4-9 (Sakaliene et al. 2007). Aclonifen, which has a pKa of -3.15 (PPDB, 2019), was neutral in the current experiment, as the pH of soil solutions ranged from 7.8 to 8.2 (Table 1). This means that those two herbicides cannot be completely dissolved. In this case, adsorption processes could not have occurred through anionic-cation pathways, but through hydrogen bonds, Van der Waals force, or hydrophobic partitioning (Trevisan et al. 1999;Shetti et al. 2019). There was a negative correlation between the K f coefficient and the CaCO 3 content (r ≥ − 0.513, p < 0.05) for all herbicides. Adsorption of 2,4-D was studied by Rodriguez-Rubio et al. (2006), and found that soil with a very large amount of CO 3 2− content was necessary for adsorption. In our case, however, the opposite effect has occurred, and it is predicted that the adsorption of herbicides would not be favored by CaCO 3 . Also, Kodešová et al. (2011) suggest that the CaCO 3 is not sufficient for the evaluation of herbicide adsorption, as the resulting correlation may be based on extremely high or very low CaCO 3 values. As stated above, the clay and silt content were not correlated with the adsorption coefficient and a slightly negative but not significant correlation was observed with sand content (Table 4). This indicates that they were not the main predictors of the adsorption of the studied herbicides, and support the findings of (Weber et al. 2004;Alletto et al. 2013;Westra et al. 2015), while the findings of Peter and Weber (1985) suggest that the clay content has a direct influence on the adsorption of alachlor and metolachlor. The correlations between K f values in all six herbicides are summarized in Table 6. A strong positive correlation was observed between the sorption co-efficient of the herbicide. As already discussed, this may be due to the sorption mechanism of these herbicides, which prevails as non-ionizable compounds and are mainly present in their neutral form. In addition, similar behavior of herbicides can be predicted under the same soil conditions.

The effects of soil properties on herbicide degradation
Several authors have proposed that adsorbed chemicals are less available for transport and degradation (Selim et al. 1999;Koskinen et al. 2001). It can be assumed that the longest DT 50 values were possibly due to the higher SOM content in the soil (Table 1). The high DT 50 values could also be explained by the sorption coefficient (K f ) of herbicide. The DT 50 values may also be increased if microbiological degradation is the key dissipation mechanism (Felsot and Dzantor 1995;Munoz-Leoz et al. 2013).
The degradation rate of two herbicides significantly correlated only with soil pH; a positive correlation for pethoxamid, and a negative, but, not significant correlation for flurochloridone. For other herbicides, weak positive correlations were observed between soil pH and degradation rate. The obtained half-life for aclonifen was much shorter than the value presented (62.3 days) in PPDB (2019), and for pethoxamid, it is significantly higher than those stated in the literature (Pose-Juan et al. 2018;Rodriguez-Cruz et al. 2019). Vischetti et al. (2002) also observed a high aclonifen half-life (40.3 to 49.1 days) in the laboratory under different temperature and soil moisture conditions in sandy clay loam soil. As shown in Table 8, the rate of degradation of aclonifen was positively correlated with clay content (r ≥ 0. 880, p < 0.001), positive but not significantly correlated with soil pH, and a negative but not significant correlation was observed with SOM, CaCO 3 , and CEC for all three soils. Based on data from the Pesticide Properties DataBase (PPDB), aclonifen is stable between pH 5 and 9 at 22-70 °C and photolysis is not a major route of degradation (PPDB 2019). It is known that microbiological degradation takes more time and the results suggest that degradation of aclonifen was mainly caused by chemical hydrolysis and/or oxidation due to its shortest half-life among the herbicides. The faster degradation rate and shorter DT 50 values obtained for pethoxamid could be explained by its low sorption capacity (Table 3), which were more available in dissolved organic carbon (DOC). According to Marín-Benito et al. (2012), DOC can adsorb herbicides and may increase the bioavailability for degradation. Pethoxamid dissipation in soil was studied by Dhareesank et al. (2005), and stated that rapid dissipation could be explained by its irreversible adsorption and hydrophobic character. Rodriguez-Cruz et al. (2019) investigated the dissipation of pethoxamid at different initial concentrations, and the results indicated that organic residues would decrease the dissipation rate at the lowest dosage rate (2 mg kg −1 ).
Based on the EFSA (2018) data, dimethenamid-p is characterized as stable in hydrolysis, while photolysis in water and soil takes place relatively quick (3-5 weeks). In this study, an increase of dissipation half-life of herbicides was observed mainly in soils with higher SOM content, and this has also been observed for dimethenamid-p in similar conditions, even though dimethenamid-p has higher water solubility and lower K foc values (Table 2) in comparison with other herbicides. It should also be noted that our findings are compatible with the previous study, which indicates that dimethenamid-p appears to be bound immediately to soil particles or dissolved in soil solution and then desorbed into the soil solution, where readily degraded by soil microorganisms (Kočárek et al. 2018). The pendimethalin's half-life reported here was shorter than the value of 182.3 days as presented in PPDB (2019). The findings indicate that pendimethalin was rapidly degraded in the studied soils; thus, the rate of dissipation was not related to the hydrophobicity and the degree of adsorption, even though the pendimethalin was strongly adsorbed by all three soils (Table 3). The degradation rate of pendimethalin was positively correlated with clay content (r ≥ 0. 536, p < 0.05) and, a negatively wit h SOM (r ≥ − 0. 741, p < 0. 01) and CEC (r ≥ − 0. 737, p < 0. 01) and, a negative but not significantly correlated with CaCO 3 (Table 9).
Available data on flurochloridone soil persistence under laboratory conditions is scarce. Walker (1987) obtained a similar DT 50 value (40 and 90 days) for flurochloridone incubated in the laboratory at 20 and 10 °C in sandy loam soil. According to information presented in PPDB (2019), flurochloridone is moderately persistent and its main degradation pathway is by chemical hydrolysis. The degradation rate of flurochloridone positively correlated with clay content (r ≥ 0. 994, p < 0. 001), and negatively with CaCO 3 (r ≥ − 0. 783, p < 0. 05), and a positive but not significant correlation was observed with SOM and CEC. A slightly negative correlation was found with soil pH for all three soils (Table 9). The clay content seems to be the predominant factor in the determination of the dissipation of flurochloridone due to its strong correlation with the rate of degradation. Indeed, there was not a significant relationship between the SOM content and the rate of degradation.
In addition, it could also be hypothesized that the degradation of flurochloridone may decrease (longer half-life) in soils with higher pH. The degradation of S-metolachlor has previously been evaluated in several studies. Our results were comparable to those reported by Rice et al. (2002) for a half-life of 9. 6-81 days and slightly higher than those reported by Wu et al. (2011) (37.9-49.5) days in five different soils. Assuming that S-metolachlor is more persistent in high SOM than in low SOM. Therefore, a negative correlation between the degradation rate and SOM content (r ≥ − 0. 583, p < 0.05) and CEC (r ≥ − 0. 579, p < 0.05) we re observed, while a significant positive correlation was found with clay content (r ≥ 0. 702, p < 0.01) for all soils (Table 9). Consequently, a decrease in SOM content and an increase in clay content result in a reduction in the half-life of S-metolachlor in soils.
In summary, it seems likely that the degradation of herbicides in soils was affected by several parameters, including SOM content, clay content, CEC, and soil pH. However, in most cases, SOM content showed a statistically significant relationship between the herbicides adsorption coefficient and the rate of degradation. This suggests that they may be inversely related. Correlation analysis shows that soil pH was negatively correlated with herbicide adsorption (Table 4), and according to Villaverde et al. (2008) it is often estimated that degradation is supported by a higher amount of chemical present in the solution. Finally, as predicted, there was a negative association between the adsorption coefficient (K f ) and the rate of degradation (Table 9). The correlation has shown that the K f sorption coefficient is useful in predicting the degradation of the studied herbicides.

Conclusion
The sorption of herbicides increased in the order dimethenamid-p < pethoxamid < S-metolachlor < flurochloridone < aclonifen < pendimethalin. The largest K f values for all herbicides were obtained in Haplic Fluvisol. In general, there was a strong positive correlation between the adsorption coefficient and SOM content (r ≥ 0. 981, p < 0. 001) and the CEC (r ≥ 0. 974, p < 0.001) for all herbicides. Moreover, the Freundlich isotherm (slope) n values indicated a high affinity to the herbicides on all three soils, with only relatively little affinity to flurochloridone. The results concluded that the presence of a greater concentration of SOM may increase the adsorption of these herbicides. Degradation of herbicides was followed by first-order kinetics and increased in the order aclonifen < pethoxamid < dimethenamid-p < pendimethalin < flurochloridone < S-metolachlor. The dissipation half-lives of herbicides were significantly longer in Haplic Fluvisol (20-57 days) than compared to