Changes in the Bioavailability of Ionizable Herbicides in Volcanic Soils Due to Soil Acidification by Urea as Fertilizer

Abstract

The application of urea as a nitrogen fertilizer and herbicide is a common practice in agricultural systems. However, there is no background information on how the soil acidification caused by urea might affect the herbicide bioavailability in volcanic soils. The persistence study was conducted under microcosm incubation conditions in two Andisol soils amended with a field equivalent nitrogen dose of 200 kg N ha⁻¹ and double dose of 400 kg N ha⁻¹. Clopyralid, fluroxypyr, picloram, and triclopyr, acidic ionizable herbicides, were applied at the field equivalent dose. Adsorption studies were also carried out on both soils at pH 4, 5, and 6. Clopyralid and picloram showed the greatest increase in half-life in the range of 20–80%. The application of twice the dose of urea resulted in minor changes. A higher adsorption implies a higher persistence of the herbicides. This was more evident for the Piedras Negras soil (PNS). The conclusion of this work is that soil acidification by urea increases the persistence of ionizable herbicides in Andisol soils and that this effect depends on the acidity of the herbicide and the physico-chemical characteristics of the soil, which are among the most determining factors.

1. Introduction

The application of urea and herbicides is part of agronomic crop management for effective weed control and plant nutrition to achieve higher yields and quality products. However, there is worldwide concern about the environmental impact of both agrochemicals, especially on water resources. Urea, the most widely used nitrogen fertilizer in the world, causes soil acidification that triggers several detrimental processes, affecting crop productivity and the environment. During the hydrolysis of urea, there is a short-term increase in pH due to the release of OH⁻. Subsequently, the release of H⁺ during ammonium nitrification leads to a sustained increase in soil acidity over time [1,2,3,4]. The preceding process leads to an increase in dissolved organic carbon, elevated levels of aluminum in the soil solution, and various effects on micro-organisms [5,6,7,8,9]. In the context of this study, the persistence and sorption processes of ionizable herbicides could be strongly affected due to mainly pH changes. There is a limited number of studies that assess the behavior of pesticides in soils fertilized with inorganic fertilizers, and various responses have been observed, depending on the soil properties, pesticide structure, and fertilizer dose. Regarding adsorption, a decrease in adsorption of 2,4-D, MCPA and flumetsulam, acidic herbicides, was reported in urea-fertilized Andisol soil. [1,10]. In addition, there was an increase in the adsorption of basic herbicides, such as terbuthylazine and metribuzin [11,12], and a neutral herbicide, such as metolachlor [13], in soils with a low organic matter (OM) content and acidic and basic pH levels.

With regard to the persistence of pesticides in fertilized soils, an increased persistence of atrazine and 2,4-D [14], terbuthylazine [15], and cypermethrin has been reported [16]. Although, for cypermethrin, a different answer has been observed depending on the fertilizer supply [17]. As well, no effect has been reported for 2,4-D when urea was applied in the equivalent field rate [1].

The herbicides included in this study, clopyralid, fluroxypyr, picloram, and triclopyr, are widely used to control weeds in agricultural and forestry soils being absorbed through plant foliage and roots. They are ionizable herbicides due to their acidic properties and are mainly present as anions in soil solution; they are also highly soluble in water [18] (

Table 1. Chemical structure and some properties of studied herbicides.

Herbicide	Formula	Dose a (g a.i. ha−1)	Sw b (g L−1)	Log P c	pKa d	α e (%)
						pH 4	pH 5	pH 6
Clopyralid		143	7.8	−2.63	2.0	0.99	0.1	0.01
Fluroxypyr		300	6.5	0.04	2.9	7.4	0.8	0.1
Picloram		244	0.5	−1.92	1.8	2.0	0.2	0.02
Triclopyr		720	8.1	4.62	4.0	50.0	9.1	1.0

^a Recommended herbicide field dose of active ingredient. ^b Water solubility. ^c Octanol–water partition coefficient. ^d Dissociation constant. ^e Neutral form of herbicide in soil solution (%)

). They are mobile in soil and generally have a high leaching potential. The GUS leaching potential index values for clopyralid, triclopyr, and picloram are 3.02, 3.30, and 4.14, respectively, and are classified as highly leachable, except fluroxypyr, which has a value of 1.03 and is classified as poorly leachable [18]. Their low affinity for binding to soils and relatively low persistence suggest that it has the potential to leach into groundwater and contaminate surface water resources. Laboratory and field studies conducted under various conditions indicate that the movement of these herbicides in soil is determined by several factors, such as the soil organic matter content, surface cover, and pH, among others, and that they are detected at low concentrations in soil and water [19,20,21,22,23,24]. The ecotoxicological effects of these herbicides on various species such as birds, mammals, bees, and aquatic organisms have been documented in reports prepared by the European Food Safety Authority (EFSA), an agency of the European Union (EU), which consider these products to be of low risk to these species, although further evaluation is required [25,26,27,28]. For ionizable herbicides, an inverse relationship has been established between the soil solution pH and adsorption due to the variation in the chemical forms of these substances as a function of pH and the adsorption mechanisms that would be acting [29]. In general, these herbicides are of low persistence, with the exception of clopyralid, which is classified as moderately persistent [18].

Several studies on soils with a generally low OM content and varying pH values show relatively low adsorption [29,30,31], and dissipation with values in a variable range depending on the study conditions being generally classified as low to moderate persistence [20,32,33,34,35,36,37]. No persistence studies were found in the presence of urea as a fertilizer for the herbicides under study. Studies of adsorption kinetics [38] and their effect on soil micro-organisms have been published by the authors of this work [39].

This study was carried out in two Andisols, which are volcanic soils that are widely distributed around the world. They are variable surface charge and acidic pH soils, high in OM and allophane content. In these soils, variations in the pH of the soil solution modify its surface charge, potentially influencing the adsorption of ionizable herbicides. The mineral fraction has a high content of allophone, imogolite, and ferrihydrite, and, in the most superficial horizons, Al-humus and Fe-humus complexes. They have a high specific surface area, a low bulk density, and a high-water retention capacity [40].

The aim of this study was to assess how the application of urea affects the persistence of ionizable herbicides in volcanic soils. Additionally, considering the pH changes resulting from the entire process of urea mineralization and recognizing the role of pH in adsorption processes, which are closely linked to herbicide persistence in soils, adsorption studies were conducted at three different pH values.

2. Materials and Methods

2.1. Herbicides

The analytical standards for the following herbicides were used: clopyralid (3,6-dichloropyridine-2-carboxylic acid), fluroxypyr (4-amino-3,5-dichloro-6-fluoro-2-pyridyloxyacetic acid), picloram (4-amino-3,5,6-trichloropyridine-2-carboxylic acid), and triclopyr (3,5,6-trichloro-2-pyridyloxyacetic acid). The analytical standards of herbicides were provided by Chem Service (West Chester, PA, USA, purity > 98.8%). All reagents used were analytical or HPLC grade (Merck-Sigma (St. Louis, MO, USA)).

2.2. Soils

The used soils were two Andisols from southern Chile, belonging to the Freire family (FS; 38.50° S, 72.35 W), which is medial, mesic, Typic Placudands, with a silty loam texture, and Piedras Negras family (PNS; 40.23° S, 72.30° W), which is medial mesic, Acrodoxic Hydric Melanudands, with a sandy loam texture [41]. Soil samples were sampled from the top 20 cm, air-dried, and sieved through a 2 mm mesh and analyzed in Soil Analysis Laboratory of the University [42]). Briefly, the OM content was measured using the Walkley–Black method. The pH was measured in soil suspensions with deionized water at 1:2.5 (w/v) ratio. Cation exchange capacity (CEC) was calculated from the total exchangeable bases (Mg, Ca, K, and Na), which were analyzed by flame atomic absorption spectrophotometry.

Table 2. Physico-chemical properties of soils.

Soil Order	Location a	pH	OM b (%)	CEC c (cmol(+)/kg)	Sand (%)	Silt (%)	Clay (%)	Texture
Andisol	FS	5.68	12.0	8.20	39.7	42.9	17.3	silty loam
Andisol	PNS	5.14	20.0	2.97	54.8	41.0	4.2	sandy loam

^a Location: Freire soil (FS), Piedras Negras soil (PNS). ^b Organic matter. ^c Cation exchange capacity. Mean values, n = 3.

shows the main properties of both soils.

2.3. Microcosm Set-Up and Persistence of Herbicides

The used soils have been kept in incubation conditions at 20 °C and 60% of the water-holding capacity in darkness for a month before setting up the microcosms. For the microcosm set-up, approximately 500 g of soil (dry-weight basis) for each treatment, in three replicates, were placed into plastic vessels. Urea was applied at an equivalent dose of 200 kg N ha⁻¹ (2N) and 400 kg N ha⁻¹ (4N), equivalent to 573 mg kg⁻¹ and 1143 mg kg⁻¹, respectively. After 24 h, the standard of each herbicide was applied in soil microcosms: clopyralid 0.19 mg kg⁻¹ (1C), fluroxypyr 0.40 mg kg⁻¹ (1F), picloram 0.32 mg kg⁻¹ (1P), and triclopyr 0. 97 mg kg⁻¹ (1T). The amount of herbicide used has been estimated on the basis of doses traditionally used in cereal crops of active ingredient (a.i.), under field conditions corresponding to clopyralid 143 g ha⁻¹, fluroxypyr 300 g ha⁻¹, picloram 244 g ha⁻¹, and triclopyr 720 g ha⁻¹. Aqueous solutions of urea and herbicides were applied to the soils with a small spray bottle and thoroughly mixed and shaken manually, and the soil moisture content was adjusted by weight. About 20 g (dry-weight basis) of sample was taken from microcosms at 1, 5, 10, 20, and 30 days after treatments and they were kept at −20 °C for further analysis [43]. A total of 36 microcosms for each soil were established in this experiment considering three N doses (0N, 2N, and 4N) and one herbicide dose, in triplicate. The extraction of herbicides from soil microcosm samples involved three successive extractions with methanol:water (60:40 v/v) acidified with 0.1% H₃PO₄ extractive solution. Briefly, sub-sample of 10 g of fresh soil and 25 mL of solvent extractive solution were placed into a centrifuge tube, vortexed for 1 min, sonicated for 10 min, and centrifuged at 3000 rpm for 15 min. The supernatants were filtered, combined, and concentrated for analysis.

2.4. Adsorption Experiments

Duplicate sample of 2.0 g soil were placed in 50 mL centrifuge tubes (polypropylene copolymer), and mixed with approximately 18 mL of a 0.01 M CaCl₂ aqueous solution. Small volumes of HCl and NaOH 1 M were added to adjust pH at 4.0, 5.0, or 6.0. Preliminary tests were conducted in presence of herbicides to determine the minimum volume of acid or alkali solution required to reach these pH values. A volume of stock solution of each herbicide prepared in 0.01 M CaCl₂ was added to result in a concentration of 0.05, 0.25, 0.5, 1.0, 2.0, 5.0, 10.0, and 20.0 mg L⁻¹.in a final volume of 20 mL. The tubes were shaken at 20 ± 1 °C in a rotary shaker for 24 h. The tubes were centrifuged at 3000 rpm for 15 min, filtered using 0.22 µm membranes (PVDF), and transferred to HPLC vials for analysis [44]. For all experiments, the final pH was 4, 5, or 6 ± 0.1.

2.5. Herbicide Analysis

The herbicides were analyzed using a Shimadzu Prominence HPLC chromatograph LC-20AT (Shimadzu Scientific Instruments, Columbia, MD, USA), with a diode array detector (SPD-M20A), using a prontoSil column RP-C18 (250 mm length and 4.6 internal diameter, Bischoff Chromatography, Leonberg, Germany). The mobile phase used was a 50:50 (v/v) mix of acetonitrile and water acidified to pH 2 with phosphoric acid. The detection wave-length was 225 nm for all herbicides. The injection volume was 20 µL, the flow 1.0 mL min⁻¹, and the oven temperature 30 °C. Calibration curves were prepared for each herbicide [38]. The detection limit (LOD) for clopyralid, fluroxypyr, picloram, and triclopyr were 0.003, 0.007, 0.003, and 0.008 mg L⁻¹, respectively. The quantification limits (LOQ) were 0.012, 0.023, 0.010, and 0.027 mg L⁻¹, respectively. The precision (relative standard deviation) was <5%, and the chromatographic response for the calibration curve was linear up to 25 mg L⁻¹ (R² = 0.999). Blank soil samples without herbicide were used to evaluate soil matrix effect. No significant interference was recorded. Additionally, recovery experiment in a soil samples were performed in triplicate using four fortified levels of herbicide (0.2, 0.4, 1.0, and 2.0 mg kg⁻¹), with recovery ranging between 90–102% [45,46].

2.6. pH Changes in Soil–Urea Microcosms

A microcosm under the same conditions as the persistence study was established for each soil in order to measure the pH changes produced due to the application of urea. Approximately 50 g of soil (dry-weight basis) for each treatment in three replicates were placed into plastic vessels. Urea was applied at an equivalent dose of 0 (0N), 100 (1N), 200 (2N), and 400 kg N ha⁻¹ (4N), equivalent to 0, 285, 570, and 1140 mg kg⁻¹, respectively. The soil pH was measured in 1:2.5 soil–water suspension in deionized water using a glass electrode. Samples for each treatment were taken over 35 days.

2.7. Data Analysis

All experiments were conducted in triplicate and the data were expressed as mean values with their standard deviations. Fitting of data to the model was performed by least-squares regression analysis and the quality of fitting was evaluated through R² values. For persistence studies, the data were analyzed using the first-order kinetics Equation (1), where C (mg kg⁻¹) corresponds to the concentration at time t (d), C₀ (mg kg⁻¹) corresponds to the initial herbicide concentration, and k (d⁻¹) is the degradation constant. The rate constant is determined by linearizing Equation (2) and the half-life time (t_1/2) calculated through Equation (3).

$${C=C_{0}e}^{-kt}$$

(1)

$$ln{\displaystyle \frac{C_0}{C}}=kt$$

(2)

$$t_{1/2={\displaystyle \frac{0.693}{k}}}$$

(3)

Sorption data were described by the Freundlich model (4), and the constants were determined by linear fitting of Equation (5), where C_s (mg kg⁻¹) is the herbicide adsorbed by the soil, and C_e (mg L⁻¹) is the equilibrium concentration in the solution. K_f and 1/n are empirical constants. The distribution coefficients K_d were also calculated from the relationship C_s/C_e for a C_e of 10 mg L⁻¹ because the isotherms were nonlinear. K_f or K_d values were normalized for organic matter (OC) content (6).

$${\mathrm{C}}_s=K_f{{\mathrm{C}}_e}^{1/\mathrm{n}}$$

(4)

$${\mathrm{logC}}_s={\log \mathit{K}}_f+1/\mathrm{n} \mathrm{l}\mathrm{o}\mathrm{g}{\mathrm{C}}_e$$

(5)

$$K_{OC}={\displaystyle \frac{K_d}{\%OC}}100$$

(6)

3. Results and Discussion

3.1. Persistence of Herbicides

The results of the herbicide persistence study in soils, with and without urea application, are presented in

Table 3. Kinetic parameters according to first-order model (k, t_1/2) for the dissipation of studied herbicides in two Andisols with urea application.

	FS			PNS
Treatment	k	t1/2 (d)	R2	k	t1/2	R2
Clopyralid
0N1C	0.053 ± 0.01	13.1	0.996	0.045 ± 0.02	15.6	0.942
2N1C	0.029 ± 0.01	23.9	0.981	0.037 ± 0.03	18.7	0.986
4N1C	0.031± 0.02	22.4	0.995	0.025 ± 0.02	27.7	0.989
Fluroxypyr
0N1F	0.088 ± 0.02	7.9	0.988	0.023 ± 0.03	30.1	0.982
2N1F	0.074 ± 0.02	9.4	0.994	0.023 ± 0.03	30.1	0.993
4N1F	0.068 ± 0.02	10.2	0.991	0.022 ± 0.02	31.5	0.989
Picloram
0N1P	0.036 ± 0.03	19.3	0.997	0.037 ± 0.01	18.5	0.948
2N1P	0.022 ± 0.02	31.5	0.998	0.034 ± 0.03	20.4	0.940
4N1P	0.019 ± 0.03	36.5	0.996	0.025 ± 0.03	27.7	0.967
Triclopyr
0N1T	0.116 ± 0.02	6.0	0.992	0.019 ± 0.02	36.5	0.937
2N1T	0.121± 0.03	6.2	0.994	0.016 ± 0.03	43.3	0.952
4N1T	0.109 ± 0.04	6.4	0.994	0.015 ± 0.03	46.2	0.948

Standard deviation of mean (n = 3). SF: Freire soil, PNS: Piedras Negras soil.

. This table includes kinetic parameters based on the first-order kinetic model, specifically the rate constant (k) and half-life time (t_1/2).

The model fit well with the obtained results, as indicated by the coefficients of determination (0.937 ≤ R² ≤ 0.998). In the Freire soil (FS), treatments without urea application (0N) revealed that clopyralid and picloram exhibited relatively higher t_1/2 values compared to fluroxypyr and triclopyr. In the Piedras Negra soil (PNS), clopyralid and picloram had half-lives similar to those in FS. For fluroxypyr and triclopyr, the t_1/2 values were significantly higher than in the FS soil, by a factor of 4 and 6, respectively. There is a limited number of publications on the half-lives of these herbicides, which are classified as low or moderate persistence. Typical t_1/2 values have been reported in the range of 5–20 days in studies conducted on soils of varying pH and with an OM content of less than 10% [18,20,32,35,37,47], although there are other studies and conditions with a broader half-life range [19,25,26,27,28,48,49]. The results of this study are comparable to those reported in the literature cited above. Regarding the effect of urea application on the persistence of these herbicides, it can be generalized that the half-life increases due to the application of fertilizers in both soils, and there may or may not be an effect of a higher dose of fertilizer. In the FS soil at the field equivalent dose (2N), an increase in t_1/2 of approximately 80, 19, and 60% was observed for clopyralid, fluroxypyr, and picloram, respectively. For triclopyr, it was less than 3%. In the PNS soil, the t_1/2 values increased by about 20, 9, and 19% for clopyralid, picloram, and triclopyr, respectively; for fluroxypyr, there was no difference. In general, it was observed that the effect of nitrogen fertilization at the field equivalent dose (2N) had a minor effect on herbicide persistence in PNS soil. When twice the recommended dose of nitrogen (4N) was applied, differences of approximately 3–16% were observed in the FS soil compared to 2N. In the PNS soil, there was a significant increase in the half-life for clopyralid and picloram of approximately 48 and 36% with respect to 2N. This was not the case for fluroxypyr and triclopyr, whose variation was less than 5–7%. In general, the application of double the field dose of N (4N) had a greater impact on the PNS soil.

The effect of nitrogen application on herbicide persistence is varied and one of the factors that seems to have the greatest influence is soil pH, which regulates the herbicide bioavailability. Figure 1A and Figure 1B show the pH variations in a microcosm of FS and PNS, respectively, without urea application (0N) and with urea application, at equivalent doses of 100 (1N), 200 (2N), and 400 (4N) kg N ha⁻¹, respectively. For the equivalent dose of 200 kg N ha⁻¹ (2N), in the Freire soil, there was initially an increase of approximately 0.4 pH units two days after the start of the experiment, with a progressive decrease in pH until acidification with values close to 4.7 (Figure 1A). For the Piedras Negras soil, the maximum initial increase in pH was 0.7 units six days after urea application, for its subsequent acidification like FS, but a slower process (Figure 1B). For the 400 kg N ha⁻¹ (4N) dose, a similar behavior in pH changes was observed. As discussed below, soil acidification due to urea application influences the chemical forms or speciation of these ionizable herbicides, modifying adsorption and, therefore, their persistence. Additionally, pH changes produce other effects in the soil that could modify the persistence of herbicides. In this regard, inorganic nitrogen fertilization reduces the microbial biodiversity and abundance due to soil acidification, particularly bacterial populations that are strongly affected at a pH below 5 [8,9,50]. Generally, an alkaline or slightly acidic soil pH increases the biodegradation of a variety of organic substances, including some pesticides, while an acidic environment will limit biodegradation [5]. On the other hand, the concentration of aluminum, a toxic element for micro-organisms, increases due to the increased acidity of the soil as a result of the application of nitrogen fertilizers [7,51].

3.2. pH Effect on Adsorption of Herbicides

Considering the pH changes in the soil caused by the application of urea during the persistence study, adsorption studies in soils were carried out on specific pH soils with the aim of estimating their bioavailability in the soil solution. Thus, the adsorption of these herbicides was studied in both soils at pH 4, 5, and 6, artificially adjusted. Figure 2 and Figure 3 show the adsorption isotherms as a function of the pH of the herbicides in the FS and PNS soils, respectively.

The adsorption parameters, according to the Freundlich model, are shown in

Table 4. Adsorption of herbicides on two Andisol soils at different pH values. Freundlich parameters (K_f and 1/n), distribution coefficient K_d, and organic carbon distribution coefficient K_oc.

Soil	FS					PNS
Herbicide	Kf	1/n	Kfoc	R2	Kd	Koc	Kf	1/n	Kfoc	R2	Kd	Koc
Clopyralid
pH 4	1.89 ± 0.07	0.87± 0.01	27	0.992	1.79	26	7.65 ± 0.05	0.92 ± 0.01	66	0.998	6.83	59
pH 5	0.95 ± 0.05	0.90 ± 0.02	14	0.999	0.87	12	2.77 ± 0.06	0.97 ± 0.02	24	0.996	2.95	25
pH 6	0.54 ± 0.02	0.90 ± 0.02	8	0.992	0.53	8	1.28 ± 0.05	0.88 ± 0.01	11	0.976	1.28	11
Fluroxypyr
pH 4	29.89 ± 0.15	0.80 ± 0.01	428	0.999	23.57	338	115.61± 0.90	0.73 ± 0.01	994	0.946	139.12	1196
pH 5	13.66 ± 0.18	0.77 ± 0.01	196	0.998	9.90	142	47.04 ± 0.80	0.75 ± 0.01	405	0.991	44.25	380
pH 6	5.32 ± 0.15	0.86 ± 0.01	76	0.999	3.77	54	23.18 ± 0.20	0.67 ± 0.01	199	0.971	17.90	154
Picloram
pH 4	8.65 ± 0.02	0.87± 0.02	124	0.998	6.74	97	32.23 ± 0.20	0.90 ± 0.02	277	0.995	27.75	239
pH 5	3.92 ± 0.03	0.90 ± 0.01	56	0.999	3.11	45	11.80 ± 0.08	0.89 ± 0.01	101	0.998	8.82	76
pH 6	1.79 ± 0.01	0.95 ± 0.01	26	0.999	1.62	23	5.83 ± 0.09	0.96 ± 0.02	50	0.995	5.12	44
Triclopyr
pH 4	21.84 ± 0.06	0.75 ± 0.01	313	0.991	19.36	277	109.40± 0.40	0.94 ± 0.01	940	0.992	103.95	894
pH 5	8.73 ± 0.07	0.84 ± 0.02	125	0.997	7.48	107	31.72 ± 0.12	0.80 ± 0.03	273	0.997	26.99	232
pH 6	3.83 ± 0.06	0.88 ± 0.02	55	0.994	3.75	54	17.50 ± 0.07	0.82 ± 0.01	150	0.997	14.46	124

K_d values were obtained for a 10 mg L⁻¹ herbicide solution. Standard deviation of mean (n = 3). K_f unit: mg^1−1/n L^1/n kg⁻¹. K_d unit: L kg⁻¹.

. The adsorption data in both soils and for all herbicides fitted well to the Freundlich equation (0.946 ≤ R² ≤ 0.999). The values of 1/n yielded a result different from unity varying between 0.75–0.95 and 0.67–0.97 for the FS and PNS soils, respectively. Considering the nonlinearity, K_d was calculated for an initial concentration of 10 mg L⁻¹. The adsorption in the FS soil is lower than in the NPS soil and the value of K_f or K_d decreased with increasing pH. The adsorption order in the FS soil, at pH 6, which is closest to soil pH (pH 5.7), was the following: fluroxypyr, triclopyr > picloram > clopyralid. For the PNS soil at pH 5 (soil pH 5.1), the order was fluroxypyr > triclopyr > picloram > clopyralid. The physicochemical properties of the soil are among the factors that have a strong influence on the behavior of ionizable herbicides in soils. The Piedras Negras soil presents a sandy loam texture, with a high OM content (20%) and acid pH (5.1). The Freire soil has a silt loam texture, with a lower organic matter content (12%), and is less acidic (pH 5.7). Another soil property to be considered is the higher value of the cation exchange capacity (CEC) of the FS soil, which is a measure of the total negative charges within the soil (Table 2). All herbicides used in this study are characterized for possessing a carboxylic group in their structure, as well as polar atoms such as N, O, Cl, and F, and a pyridine or picolinic aromatic cyclic structure (Table 1). The interaction of these herbicides with organic matter and clays, depending on soil pH, occurs mainly through hydrogen bridge interactions, ion exchange, and hydrophobic interactions. The effect of pH on the ratio of molecular–anionic forms at the pH studied was estimated by using the Schwarzenbach equation [52].

Table 1 shows the values of αia, which represents the fraction of acid present in the acidic form at pH 4, pH 5, and pH 6. At pH 6 for all herbicides, the anionic forms predominate; the K_f values are the lowest and are comparatively lower in the Freire soil (FS). On the other hand, at pH 4, the molecular form of the acid is approximately 50, 7, 2, and 0.1% for triclopyr, fluroxypyr, picloram, and clopyralid, respectively. These values agree with the K_f values obtained, being higher for the more acidic herbicides (Table 1). Likewise, at this pH, the higher K_f and K_oc values for fluroxypyr and triclopyr in the NPS soil, which contains 20% OM, confirm the interaction of the molecular forms with organic matter.

The adsorption studies of ionizable herbicides such as 2,4-D, fluroxypyr, dicamba, and metsulfuron-methyl have been negatively correlated with the soil pH and positively correlated with the organic carbon content [29]. The authors of this work concluded that pH dependence derives mainly from the different proportions of ionic and neutral forms of the pesticide present at each pH level and from differences in their sorption strength. In addition, it is necessary to take into account the fact that the adsorption of anionic species at a lower pH can be explained through electrostatic interactions with amorphous minerals, mainly allophane and iron and aluminum oxide present in Andisols [53,54]. On the other hand, the influence of the pH variation on the surface charge of soil particles also plays an important role, since some soil matrix microenvironments could have a pH lower than that of the soil solution [29,55].

The bioavailability of pesticides in soil depends on physicochemical processes such as adsorption, transport, and biological processes, with adsorption being recognized as the key process. Defining the bioavailability of a herbicide requires an understanding of the strength of its interaction with soil and the concentration of the herbicide in the soil solution. In general, adsorption is often considered to reduce degradation by limiting the availability of organic chemicals to microbial or chemical transformations. This relationship has been observed for most of the acidic herbicides, such as 2,4-D, dicamba, fluroxypyr, fluazifop, metsulfuron-methyl, and flupyrsulfuron [32,55,56]. On the other hand, a positive relationship between adsorption and degradation has also been described for some pesticides such as triclopyr, mesotrione, metsulfuron-methyl, and 2,4-D [55,57,58]. This behavior can be explained, particularly in soils with a higher OM content, due to the micro-organisms are not uniformly distributed in the soil and there are regions on the soil surface with a higher microbial activity. Besides pesticides, biodegradation may not only occur in solution [56].

4. Conclusions

The main conclusion of this work is that inorganic nitrogen fertilization, specifically the application of urea, increases the persistence of ionizable herbicides in Andisol soils and that its magnitude depends on the acidic nature of the herbicide and the physico-chemical characteristics of the soil, among other factors. Clopyralid and picloram, the most acidic herbicides included in this study, showed the greatest increase in half-life. Changes in pH during the urea hydrolysis process determine the ratio of the molecular and anionic forms of these herbicides in the soil solution, modifying their adsorption. In general, it is possible to establish a relationship between the adsorption and persistence of these herbicides in both soils. A higher adsorption implies a higher persistence of the herbicides, which is more clearly seen for the PNS soil. Although, in several studies, it is possible to establish a direct relationship between both processes for ionizable herbicides, this is not always the case due to the multiple factors that influence both processes. Although both soils show similar trends, both K_oc and half-life times are higher in the NPS soil, particularly due to its higher acidity and higher organic matter content. The application of twice the urea dose produces similar pH changes in both soils, compared to the equivalent field dose producing minor or no changes in herbicide persistence. Other known factors that may contribute to understanding the behavior of these herbicides in the soil–nitrogen fertilizer system, caused by the higher soil acidity, is their effect on soil micro-organisms modifying their abundance and biodiversity and the toxicity of aluminum on micro-organisms, due to its higher availability. These results show that soil acidification caused by nitrogen fertilizers can have a significant impact on the bioavailability of acid herbicides and can affect the environment and agricultural production. These results, obtained under controlled conditions, will contribute to a better understanding of the behavior of both agrochemicals under field conditions.