Field-aging biochar influence on bensulfuron-methyl behavior in rice under sprinkler irrigation in high water stress regions

1 Introduction

Agricultural systems are seriously affected by climate change, which causes great uncertainty about future availability of food (Joshi et al., 2024). It is therefore necessary to intensify efforts to improve the resilience of these systems, thus guaranteeing food security for the expanding world population (Gaydon et al., 2023). In this context, rice (Oryza sativa L.) is one of the most important staple crops worldwide, providing food for the world’s population (Danso Ofori et al., 2025). China in one of the largest rice producers and consumers in the world (Ding et al., 2026), and rice is also a key crop in Mediterranean Europe (Egea et al., 2026). However, its production is particularly threatened under traditional flooded irrigation practices and conventional tillage (Savari et al., 2025; Zhou et al., 2024). This situation is even more critical in Mediterranean countries such as Italy and Spain, Europe’s largest rice producers, due to the scarcity of water resources (Gharsallah et al., 2023; Peña et al., 2022). Therefore, there is an urgent need to develop agricultural managements to improve the resilience of rice (Dass et al., 2025). Various irrigation strategies (e.g., alternate wetting and drying, drip, furrow irrigation, sprinkler) have been investigated in rice cultivation with the aim of improving water productivity (Loaiza et al., 2025; Rato-Nunes et al., 2023; Yu et al., 2021). Among these, sprinkler irrigation has proven to be a viable alternative to flood irrigation in rice cultivation (Hu et al., 2025), especially in soils with adequate levels of organic matter. In this regard, sprinkler irrigation supports the adoption of no-tillage practices and the application of organic amendments (Dominguez-Bohorquez et al., 2025; Nasseri, 2019; Peña et al., 2022), both key strategies for enhancing soil health, and aligns well with several sustainable development goals, including responsible consumption and production and climate action.

However, agronomic practices in rice crop can significantly modify soil properties (e.g., organic matter content, pH, microbial activity), and consequently influence herbicide behavior, whose use is crucial for weed management in rice crop, affecting processes including adsorption-desorption, leaching, and persistence. Generally, as total organic carbon increases, the adsorption of herbicides onto soil particles also increases, which may reduce their mobility down the soil profile (e.g. Yavari et al., 2022), although measurements of these trends have frequently been contradictory, due to effects of water-soluble organic carbon (Gámiz et al., 2019). In addition, herbicide adsorption may also depend on pH; however, the effect of pH is not very consistent, as it depends mainly on the physicochemical properties of the herbicide (Kaur and Kaur, 2025). Thus, non-ionic herbicides have shown a greater tendency to be adsorbed by soil colloids than acidic herbicides (Jamshidi et al., 2022). Therefore, the impacts of agronomic practices on an active matter will depend on the properties of the soils, as well as the characteristics of herbicide.

One of the most widely used herbicides for weed control in rice fields worldwide is methyl α-(4,6-dimethoxypyrimidin-2-ylcarbamoylsulfamoyl)-o-toluate, commonly known as Bensulfuron-methyl (BM). BM is considered as a safe herbicide; however, it is very toxic to aquatic organisms and might cause adverse effects in the aquatic environment (Sabater et al., 2002). In this context, BM exhibits high persistence in soil due to its non-volatile nature, weak acidity, and limited photodegradation (López-Piñeiro et al., 2019; Takagi et al., 2012). As a result, it can readily contaminate aquatic environments through leaching or surface runoff (Delgado-Moreno et al., 2007). In fact, previous studies have shown that BM is frequently detected in aquatic systems. For example, Ismail et al. (2015) reported that water samples collected from various rice field areas in Malaysia contained BM at concentrations of 0.55 μg L^-1. Similarly, Pareja et al. (2011) reported that in most paddy field water samples from Spain, BM concentrations exceeded 0.1 μg L^-1, which is the maximum concentration permitted for individual pesticides in water intended for human consumption and in groundwater under European Union legislation. Therefore, exploring the influence of different rice management practices on BM behavior is essential to reduce its potential risks to ecosystems and human health.

The use of biochar as an organic amendment is widely recognized for its ability to improve soil properties and to mitigate pesticide pollution and crop nutrition (Twagirayezu et al., 2024; Qadir et al., 2024) and supports the production of rice (Chen et al., 2025). However, inconsistent results about its effects on pesticide behavior have been obtained, mainly due to variations in soil and biochar properties, as well as the pesticides used (Siedt et al., 2021; Sun et al., 2025). In this sense, many studies have demonstrated immobilization of pesticides in agricultural soils amended with biochar (Cheng et al., 2022). However, Zhelezova et al. (2017) reported that biochar derived from hardwood (Betula sp.) enhanced the sorption capacity of diuron in both clay and sandy soils, while reducing glyphosate sorption in sandy soil. Similarly, soil amendment with biochar is expected to reduce pesticide availability and slow their degradation rate in soil (He et al., 2025). In contrast, some studies have shown that biochar modify the composition and activity of microbial communities, thereby promoting the degradation of herbicides in the soil (López-Piñeiro et al., 2022; Yavari et al., 2023). In relation to the leaching process, several studies have shown that the application of biochar is an effective strategy for reducing the mobility of pesticides in soil. Thus, Li et al. (2013) indicated that the use of biochar derived from wood residues and produced at relatively low temperatures (350 °C) as an organic amendment could be a good strategy to control the leaching losses of acetochlor and 2,4-D. Nevertheless, Cederlund et al. (2017) showed that hardwood biochar (380-430 °C) did not reduce the leaching of glyphosate. Furthermore, although most studies have examined the effects of fresh biochar, once incorporated into the soil, biochar can undergo aging processes that alter its physicochemical properties, which in turn could affect its impact on herbicide behavior (Gámiz et al., 2019; Wang et al., 2020). Therefore, it is also essential to determine the effects of biochar aging, preferably under real field conditions (Cheng et al., 2024). Regarding the effects of biochar on BM behavior, although the limited results obtained so far are promising from the perspective of sustainable management and pollution reduction (Xie et al., 2024; Yang et al., 2023), further research is needed, particularly because most studies have focused on modified biochar, which often entails high costs and could limit economic feasibility (Martin-Franco et al., 2023; Murtaza et al., 2024).

For biochar to be a viable organic soil amendment, it must be accessible, affordable, and producible on a large scale (Marousek et al., 2024). In Mediterranean regions, the dehesa system, dominated by the holm oak (Quercus ilex L.), generates pruning waste whose value has declined (Teutscherova et al., 2018). Therefore, its use for biochar production could reduce reliance on expensive adsorbents and contribute to a more sustainable, low-waste environment (Khan et al., 2026; López-Piñeiro et al., 2022), while also creating opportunities for rural development and poverty alleviation (Senadheera et al., 2025). Furthermore, biochar derived from holm oak pruning (BHO) has a high TOC content and a high capacity to absorb water and pesticides compared to other raw materials, making it particularly relevant when combined with water-saving irrigation practices in rice cultivation, where water and pesticide use is high.

Despite the extensive use of BM in rice fields around the world and its potential threat to ecosystems and human health, the only study we have found on the impact of different cropping systems on BM behavior has been one of our own previous publications (López-Piñeiro et al., 2019), carried out on soils with properties considerably different from those in the present study. Furthermore, we found no studies evaluating how biochar can affect the environmental fate of this compound under different water managements and tillage strategies in rice cultivation. Thus, the objective of this study was to assess the effects of BHO on the sorption–desorption, dissipation, and leaching of BM in rice cropping systems under different water management and soil tillage practices. Additionally, to evaluate the field-aged effects of BHO, the experiments were also conducted during the second year following its application.

2 Materials and methods

2.1 Herbicide

Bensulfuron-methyl (BM) analytical standard (98% purity) was obtained from Dr. Ehrenstorfer GmbH (Germany). The compound has a molecular weight of 410.4 g mol^-1, a water solubility of 6.65 mg L^-1 at 20 °C, a pKa of 5.20, and a vapor pressure of 4.99 × 10–³ mPa at 20 °C. BM concentrations were determined using high-performance liquid chromatography (HPLC) with a Waters 600E chromatograph coupled to a Waters 996 diode-array detector. Separation was carried out on a Nova-Pack C18 column (150 mm × 3.9 mm i.d., 4.5 μm particle size). The mobile phase consisted of acetonitrile and water (55:45 v/v) with 0.1% H₃PO₄, delivered at a flow rate of 1 mL min^-1. The column temperature was maintained at 30 °C, and the injection volume was 25 μL. Detection was performed at 234 nm. Quantification was based on external calibration using BM standard solutions in the range of 0.05–20 μM. Under these conditions, the limits of detection and quantification were 0.0068 μM and 0.0223 μM, respectively.

2.2 The biochar

The biochar (BHO) used in the field experiment, conducted in a rice paddy located in southwestern Spain, was obtained from Carylevere Co., Ltd (Zahinos, Spain). It was produced from holm oak prunings through pyrolysis (550 °C for 48 h). Before analysis and application, it was milled and sieved to a particle size of 2 mm. The physicochemical properties of BHO, including total carbon (TC), hydrogen (TH), nitrogen (TN), ash and oxygen contents, water-soluble organic carbon (WSOC), specific surface area (SSA), pH, and electrical conductivity, were determined following the procedures described by López-Piñeiro et al. (2022). The characteristics of field-aged BHO were assessed in the second year. Biochar particles were recovered from soil samples collected to a depth of 20 cm corresponding to the amended treatments (PFT-BHO, ST-BHO, and S0T-BHO). Suspensions of these samples were prepared in distilled water at a 1:10 (w/v) ratio and shaken to remove any adhering soil particles; this process was repeated four times prior to analysis (Koide et al., 2011). Table 1 (adapted from López-Piñeiro et al., 2022) summarizes selected properties of both fresh (initial) and field-aged BHO under different management systems.

Table 1. Selected properties of the fresh (initial) and field-aged BHO.

EC, electrical conductivity; WSOC, water-soluble organic carbon; SSA, specific surface area. All the data are mean values (n=3). *Total Oxygen calculated assuming < 1% of S without ash content. Values with the same letter within a row are not significantly different at the p< 0.05 level of probability.

2.3 Design of the experiments, sampling, and assay

A field experiment was conducted during 2018 and 2019 in a rice paddy located in the province of Badajoz (38°55′N, 6°57′W; southwestern Spain), an area characterized by a Mediterranean semi-arid climate with a mean annual temperature of 16.2 °C and average rainfall of 460 mm. The site had been managed for long-term (> 10 years) under conventional rice cultivation, involving permanent flooding irrigation and conventional tillage. The topsoil layer (0–20 cm) exhibited a loam texture, consisting of 20.8% clay, 28.9% silt, and 50.3% sand. The experimental design comprised 18 plots arranged in six treatments with three replicates each. Individual plots measured 10 m × 18 m (180 m²). The treatments included: permanent flooding irrigation with conventional tillage (PFT), sprinkler irrigation with conventional tillage (ST), sprinkler irrigation without tillage (S0T), and their corresponding counterparts amended with biochar applied in the first year (PFT-BHO, ST-BHO, and S0T-BHO, respectively). Before rice sowing, biochar (BHO) was applied at a rate of 28 Mg ha^-1 (April 2018) by surface spreading over the PFT-BHO, ST-BHO, and S0T-BHO plots, followed by a single minimum tillage operation with a rotary hoe, strictly required to incorporate the amendment into the soil. In the S0T-BHO plots, no-tillage was maintained throughout the rest of the study. After rice harvests (September in 2018 and 2019), different subsamples of topsoil (0–20 cm) were collected within each plot and combined to form one composite sample per plot. These samples were used to determine BM sorption–desorption, dissipation and leaching experiments under laboratory conditions. Data collected during the first and second years after BHO application were used to evaluate its effects over time, with the biochar being progressively aged under field conditions from the first to the second year. The properties of soils of different treatments are provided in Supplementary Table 1 of the Supplementary Material.

2.4 Experiments of sorption and desorption

The BM sorption-desorption experiments were carried out using a batch equilibration method. Briefly, for each treatment, triplicate soil samples (5 g) were treated by mechanical shaking at 20 ± 1 °C for 24 hours with 10 mL of solutions of BM in 0.01M CaCl₂ at different initial concentrations (0.5, 2.5, 5, 10, and 20 µM). Desorption was measured immediately after sorption by successive dilution from the 20 μM initial concentration points. Thus, the 5 mL of supernatant removed for the sorption analysis was replaced with 5 mL of 0.01 M CaCl₂. The samples were resuspended, shaken for another 24 hours, centrifuged, and the equilibrium concentration in the supernatant was determined by HPLC. This desorption procedure was repeated thrice. The data were fitted to the empirical Freundlich equation, Cs = K_fCe^nf, where Cs (μM kg^-1) is the amount of BM sorbed at the equilibrium concentration Ce (μM L^-1), and K_f and n_f are constants that characterize the relative sorption capacity. The desorption percentage was obtained from the expression % D = ((Csa − Csd)/Csa) × 100, where Csa denotes the amount of BM retained at the end of the sorption phase and Csd the amount still adsorbed after the desorption process.

2.5 Studies of the dissipation

For the dissipation study of BM, soil samples were incubated under two moisture regimes, flooded and non-flooded conditions. Samples from ST, ST-BHO, S0T, and S0T-BHO treatments were incubated at 80% of field capacity using distilled water, simulating sprinkler irrigation conditions, whereas samples from PFT and PFT-BHO treatments were maintained under flooded conditions with a soil-to-water ratio of 1:1.25 (w/v). Prior to BM application, soils were pre-incubated in the dark at 20 ± 1 °C for 7 days to allow microbial communities to adapt to incubation conditions, and to promote the development of reducing conditions in flooded soils. Then, BM was applied at a rate of 1.7 µg g^-1 (8.5 µg per tube). For each treatment, triplicate 5 g soil samples were placed in 50 mL glass tubes and incubated in the dark at 20 ± 1 °C for 49 days. Soil moisture was maintained throughout the experiment by periodic addition of distilled water. At designated sampling times (2 h; 2 and 5 days; and then every 7 days up to 49 days), three replicate tubes per treatment were collected for herbicide analysis. Soil samples (5 g) were extracted with 10 mL of a 55:45 acetonitrile/water (v/v) containing 0.1% H₃PO₄ solution by end-over-end shaking at 20 ± 1 °C for 24 hours, followed by centrifugation. Herbicide residues in the extracts were quantified using HPLC, with recovery rates exceeding 95% of the applied amount. Under flooded conditions, BM residues in water samples were also analyzed by HPLC. Dissipation data for BM in soil and water were fitted to a first-order kinetic model, C = C₀e⁻^kt, where C is the concentration at time t (days), C₀ is the initial concentration, and k (day^-¹) is the degradation rate constant. Half-lives (t_1/2) were subsequently calculated. Additionally, dehydrogenase activity (DHA) under both incubation conditions was determined using INT as a substrate, following the method described by García et al. (1993).

2.6 Studies of the leaching

For the BM leaching studies, soil columns were prepared using PVC (5 cm internal diameter × 30 cm length), with three replicates per treatment. To minimize soil loss during the experiment, the upper 5 cm of each column was filled with sea sand, while the lower 5 cm contained a mixture of sea sand and glass wool. The remaining 20 cm was hand-packed with air-dried soil from the respective treatments. The columns were then oversaturated with 0.01 M CaCl₂, and excess solution was allowed to drain for 24 hours. BM was subsequently applied to the surface of the soil columns at a rate of 0.5 kg ha^-1 (98.1 µg per column). Each column was leached daily with 50 mL of 0.01 M CaCl₂ solution until no BM was detected in the leachates, and the collected leachates were analyzed by HPLC. At the end of the leaching period, the columns were sectioned into 5 cm increments to determine the remaining BM. Extraction of BM was performed following the same procedure used in the dissipation studies described above.

2.7 Statistical analyses

Statistical analyses were performed using IBM SPSS Statistics software (version 25). A two-way ANOVA model was applied to evaluate the interaction between management and year. Additionally, one-way ANOVA was conducted for each factor separately (management and year) to assess statistical differences among management treatments within each year and within the same management across different years. Prior to ANOVA, all data were checked for homogeneity of variances and normality of residuals. When significant effects were detected, Duncan’s post hoc test was applied to define homogeneous groups. Furthermore, bivariate correlation analyses were carried out using Pearson’s correlation coefficient to determine statistically significant relationships between variables.

3 Results and discussion

3.1 Sorption-desorption experiments

The BM sorption–desorption parameters are listed in Table 2. The Freundlich equation adequately described the experimental sorption data across all treatments, with R² values ranging from 0.937 to 0.993 during the first year and from 0.962 to 0.995 during the second year of the study. All n_f values were below unity (0.508–0.986), indicating L-type isotherms in every case and, therefore, a non-linear sorption process in which the highest-affinity sites are preferentially occupied at low equilibrium concentrations (Figure 1).

Table 2. Bensulfuron-methyl sorption-desorption parameters.

The data for n_f, K_f, and desorption (D) are mean values. The management included: permanent flooding irrigation with conventional tillage (PFT), sprinkler irrigation with conventional tillage (ST), sprinkler irrigation without tillage (S0T), and their corresponding counterparts amended with biochar applied in the first year (PFT-BHO, ST-BHO, and S0T-BHO, respectively). ANOVA factors: M, management; Y, Year; M × Y, Interaction Management × Year. and *significant at p levels of 0.01and 0.001, respectively; NS, not significant. Different letters indicate differences (p < 0.05) between treatments in the same year (lower-case letters) and between years within the same treatments (upper case letters). R² is the coefficient of determination. †The percentage of D was calculated after three desorption cycles.

BM sorption was significantly influenced by the management regime (p < 0.001) and by the year (p < 0.01), and a significant management regime × year interaction was also detected (p < 0.001), indicating that the magnitude of the management effects differed between the two study years (Table 2). In the original soils, BM K_f values ranged from 3.60 to 14.6 in the first year and from 3.47 to 10.6 in the second year. Among the management treatments, the flood-irrigated (PFT) displayed markedly higher K_f values than the sprinkler-irrigated one under the same conventional tillage (ST), exceeding it by factors of 2.61 in 2018 and 2.11 in 2019, which reflects the effect of the irrigation system under conventional tillage. This finding could pose a conflict from an agronomic perspective, as the increased retention of BM in PTF soils can be beneficial by reducing its potential for environmental pollution. However, this same effect may also decrease herbicide efficacy in weed control, thereby requiring higher application rates of soil-applied herbicides to achieve effective weed management (Mendes et al., 2019; Obregón-Álvarez et al., 2021).

This pattern can be attributed to differences in soil pH, which was consistently higher with sprinkler irrigation than with flood irrigation, with values differing by more than one pH unit (5.53 in PFT versus 6.73 in S0T; Supplementary Table 1). In this respect, BM is a weak acid (pKa = 5.2), so that pH becomes one of the soil properties most strongly governing its retention through the effect of ionization: as pH rises, the anionic form of the herbicide predominates in solution, being more water-soluble and displaying a lower affinity for the solid phase, which reduces sorption and enhances its leaching potential (Espinoza et al., 2009). Supporting this interpretation, a significant negative correlation between K_f and pH was found when the analysis was restricted to the original soils (r = −0.935; p < 0.01). These results are in agreement with those of López-Piñeiro et al. (2019), who linked greater BM sorption to lower soil pH and with the findings of Kumari et al. (2020), who reported a higher adsorption capacity for flucetosulfuron (another sulfonylurea herbicide) in Indian rice-growing soils of low pH (4.25) than in more alkaline ones (7.89), further underlining the key role of soil pH in controlling BM sorption.

Biochar application modified BM sorption in soils managed under both conventional and alternative production systems. Both the fresh and the aged amendment significantly reduced BM retention in all treatments relative to their unamended counterparts (Table 2). The magnitude of this reduction was nevertheless governed by the management system and by the year: in the first year (2018), BM K_f values declined by factors of 1.34, 2.73 and 5.89 for S0T-BHO, ST-BHO and PFT-BHO, respectively, whereas in the second year the corresponding reductions amounted to factors of 1.50, 1.35 and 2.37. This behavior indicates that the effect of the amendment on BM sorption depended not only on the management practice applied, but also on the temporal changes undergone by the biochar properties during its residence in the field (Table 1), in agreement with the findings of López-Piñeiro et al. (2022). In fact, the importance of biochar aging in modifying its physicochemical properties and, consequently, its interaction with contaminants has likewise been highlighted by Khan et al. (2026). Thus, the increase in specific surface area (SSA) observed in aged biochar, particularly in the ST-BHO and PFT-BHO treatments in the second year, compared with fresh biochar (Table 1), could explain the greater BM sorption capacity of aged biochar (3.73 and 4.47) relative to fresh biochar (2.05 and 2.48) in ST-BHO and PFT-BHO, respectively (Table 2). In line with this finding, Binh and Nguyen (2020) indicated that SSA is a key parameter for estimating the adsorption capacity of biochar.

The lower sorption recorded in the amended soils may be related to the BHO-induced rise in soil pH, which is attributable to the high pH of the fresh biochar (9.08; Table 1). Relative to the unamended managements, soil pH increased by 0.37, 0.82 and 0.87 units in the first year, and by 0.57, 0.86 and 0.85 units in the second year, for S0T-BHO, ST-BHO and PFT-BHO, respectively (Supplementary Table 1). Thus, following the same trend as the decrease in sorption capacity, the increase in soil pH was consistently smaller under the no tillage practices. This may be attributed to the higher soil pH under S0T (Supplementary Table 1), associated with the greater capacity of plant residues retained in this cropping system to adsorb H⁺ and Al³⁺ ions, thereby contributing to soil pH regulation (Rato-Nunes et al., 2023). Accordingly, a significant negative correlation was found between K_f and soil pH (r = -0.836; p < 0.01), confirming the decisive role of this property in BM retention. In this respect, Delgado-Moreno and Peña (2008), in a soil amended with fresh and composted olive cake, found pH to be the main property responsible for the changes in the retention of three sulfonylurea herbicides, including BM itself, although in their case the amendment slightly promoted sorption.

As previously found for sorption, the Freundlich equation provided a satisfactory description of the desorption data, with R² values of 0.912-0.996 in 2018 and 0.904-0.987 in 2019 (Table 2). BM desorption (D) was significantly affected by the management system, the sampling year and their interaction (Table 2; p < 0.001). A clear trend emerged among the unamended managements, where herbicide release consistently followed the order S0T > ST > PFT in both years. Thus, D values ranged from 22.4 to 21.2% under S0T, from 7.30 to 16.4% under ST, and from 3.59 to 12.3% under PFT (2018 and 2019, respectively). These results are closely tracked soil pH, with the highest D values corresponding to S0T, the management regimen exhibiting the most elevated pH values (Supplementary Table 1). Accordingly, D was significantly and positively correlated with soil pH across the unamended treatments (r = 0.728; p < 0.01), a relationship that mirrors the negative K_f and pH correlation reported above and therefore points to a common speciation-driven mechanism. Similar behavior has been documented for other sulfonylureas, such as Kaur and Kaur (2025), who reported greater desorption hysteresis in alkaline soils, which they attributed to the irreversible binding of herbicide molecules (mesosulfuron-methyl and iodosulfuron-methyl) to soil particles.

BHO application brought about a generalized increase in herbicide desorption. In its first year effect, desorption percentages rose from 7.30 to 23.6% in ST-BHO and from 3.59 to 19.1% in PFT-BHO, representing increases of 3.2- and 5.3-fold relative to their unamended counterparts. As regards the second year effect of BHO, this increase was maintained in both treatments, albeit to a lesser extent (factors of 1.2 and 1.9 for ST-BHO and PFT-BHO, respectively), which may be attributed to the significant increase in K_f values in the ST-BHO and PFT-BHO treatments, observed under the influence of the biochar in the second year (2019) compared to the first year (2018). Likewise, this behavior could also be related to changes in the properties of biochar during its aging under field conditions (Table 1). Thus, the lower EC values observed in aged BHO (0.373–0.603 dS m^-¹), compared with fresh BHO (3.54 dS m^-¹), together with the lower WSOC values in aged BHO (258–273 mg kg^-¹) relative to fresh BHO (368 mg kg^-¹) (Table 1), could explain the smaller increase in D values following BHO field ageing. Indeed, D values were significantly and positively correlated with WSOC (r = 0.415, p < 0.01), indicating that greater amounts of WSOC in soils could reduce BM retention. This is consistent with reports that indicated that WSOC might enhance herbicide sorption reversibility (Cabrera et al., 2014; Carpio et al., 2020), including sulfonylurea herbicides (Földényi et al., 2013).

In addition, the generalized increases in D values in BHO-amended management is consistent with the BHO driven rise in soil pH and responds to the same speciation mechanism described for sorption: the anionic form of BM, predominant at high pH, is repelled not only by the negatively charged soil colloids but also by the biochar particles themselves, thereby weakening retention and favoring herbicide release (Dechene et al., 2014). In fact, D values were significantly and positively correlated with soil pH (r = 0.661; p < 0.01).

3.2 Dissipation experiments

Figure 2 shows the BM dissipation curves together with the dehydrogenase activity (DHA) values recorded throughout the dissipation experiments. The experimental data fitted a first order kinetic model, obtaining high coefficients of determination in both the first (R² ≥ 0.803) and the second (R² ≥ 0.876) year. The half-life of the herbicide (t_1/2) was significantly affected by the management system, the sampling year and their interaction (Table 3; p < 0.001). In the unamended soils, t_1/2 values under aerobic conditions were 56.1 and 51.1 days for S0T and ST in 2018, as compared with 28.4 days in the anaerobic treatment (PFT). The same pattern was reproduced in 2019, with t_1/2 values of 67.9 and 59.9 days under aerobic conditions (S0T and ST, respectively) and of 31.2 days under anaerobic ones (PTF, Table 3). BM dissipation was therefore faster under flooding (PFT) than under sprinkler irrigation (ST), by a factor of 1.8 in 2018 and 2019 in agreement with the findings reported by Takagi et al. (2012) in rice soils in Japan. A similar difference was observed with respect to S0T (factors of 2.0 in 2018 and 2.2 in 2019), however, as PFT and S0T differ in both irrigation and tillage, this comparison cannot be ascribed to the irrigation system alone.

Table 3. Bensulfuron-methyl dissipation parameters.

Half-lives: t_1/2; DHA: dehydrogenase activity calculated as the cumulative sum of the values recorded throughout the dissipation study. The management included: permanent flooding irrigation with conventional tillage (PFT), sprinkler irrigation with conventional tillage (ST), sprinkler irrigation without tillage (S0T), and their corresponding counterparts amended with biochar applied in the first year (PFT-BHO, ST-BHO, and S0T-BHO, respectively). ANOVA factors: M, management; Y, Year; M × Y, Interaction Management × Year. *** significance at p levels of 0.001. Different letters indicate differences (p < 0.05) between treatments in the same year (lower-case letters) and between years within the same treatments (upper case letters). R² is the coefficient of determination. † The percentage of D was calculated after three desorption cycles.

DHA values were significantly influenced by the management system, the sampling year and their interaction (Table 3), with markedly lower values being recorded under aerobic than under anaerobic conditions, in line with the results obtained by López-Piñeiro et al. (2019). This behavior may be ascribed to the shift of the microbial community towards anaerobic populations brought about by the decrease in soil aeration and redox potential, as described by Wolińska and Bennicelli (2010). Accordingly, the t_1/2 of BM was negatively correlated with DHA (r = −0.847; p < 0.01), pointing to a relevant contribution of biological processes to herbicide degradation in these soils, in line with Delgado-Moreno and Peña (2008), who likewise identified biological processes as the main mechanism responsible for BM dissipation in soil.

Nevertheless, the faster BM dissipation under flooding is favorable in terms of reducing soil residues. In any case, although the sprinkler-irrigated systems displayed higher half-lives, of up to 67 days, in no instance did these exceed the length of the crop cycle, which ranges from 100 to 145 days depending on the variety, so that the greater persistence associated with these systems would not entail a risk of residue accumulation for subsequent crops in rotation scenarios either. This said, the lower persistence under flooding carries an unfavourable agronomic reading: the flood-irrigated treatment (PFT) combined the lowest herbicide persistence with the highest sorption capacity (Table 2), so that BM remained available in the soil solution for a shorter time and in a smaller amount. Since BM is a systemic herbicide that must be taken up by the roots and foliage of weeds to exert its action, this lower availability might, under field conditions, weaken its efficacy under flood irrigation, a hypothesis that would require validation in the field. In this respect, dissipation is regarded as one of the key processes governing both the environmental fate of herbicides and their weed control efficacy (Kalsi and Kaur, 2019). Indeed, in an analogous study with Bispyribac-sodium herbicide, López-Piñeiro et al. (2022) observed poorer control of Echinochloa crus-galli L. under flood irrigation than under sprinkler irrigation, in agreement with the lower persistence and uptake capacity of the herbicide in the flooded system.

In contrast to the overall reduction in persistence observed following BHO application (in both the first and second years), this behavior may be related to the stimulation of microbial activity induced by the amendment. This interpretation is supported by the increase in DHA in all amended treatments, which rose by factors of 1.84, 1.65, and 1.98 under the first year effect and by 1.63, 1.28, and 1.06 under the second year effect for S0T-BHO, ST-BHO, and PFT-BHO, respectively. Similarly, Qadir et al. (2024) found that biochar application could be a promising agronomic strategy for improving soil microbial activity. In addition, our results are consistent with those reported by Udomkun et al. (2025), who found that atrazine dissipation increased in biochar-amended soils, likely due to enhanced microbial activity. Indeed, t_1/2 was significantly and negatively correlated with DHA (r = −0.871; p < 0.01), which suggested the important role of biological processes in BM dissipation” Similar findings were described by Kaur et al. (2026), who attributed the faster dissipation of the sulfonylurea herbicides (Mesosulfuron-methyl and Iodosulfuron-methyl) in soils amended with press mud, farmyard manure, vermicompost, or poultry manure (half-lives of 3.58–16.2 days), compared with unamended soils (half-lives of 10.7–22.5 days), to higher dehydrogenase activity.

This effect nevertheless depends largely on the biochar application rate. Thus, Obregón-Álvarez et al. (2021) recorded higher t_1/2 values for Sulfometuron-methyl (another sulfonylurea) in soils amended with plant-residue biochar aged in the field for ten years, as a consequence of a lower microbial activity in the presence of the biochar. This effect was conditioned by the rates applied, of 40 and 80 Mg ha^-¹, far higher than the one used in the present study, which would explain why in their case the biochar reduced microbial activity rather than stimulating it. Conversely, Kaur and Kaur (2025), in soils from India, observed that the addition of different organic amendments accelerated the degradation of two sulfonylurea herbicides, in association with a higher DHA. Similarly, Terrón-Sánchez et al. (2023) reported a higher DHA in biochar amended soils for Imazamox (a herbicide widely used in rice cultivation), which translated into a lower persistence relative to the unamended soils. Overall, these results indicate that BHO amendment, depending on its characteristics and on the application rate, may contribute to reducing BM residues in rice soils and, thereby, its persistence in the agroecosystem.

3.3 Leaching experiment

The breakthrough and cumulative leaching curves of BM obtained under laboratory column conditions for the soils from the different management systems are shown in Figure 3. In the PFT treatment, the herbicide began to leach at a pore volume below unity in both years (0.943 and 0.953; Table 4), lower than the values recorded for S0T and ST, which indicates an earlier appearance of BM in the leachate and, therefore, a high initial mobility under flooding management. The application of BHO reduced this mobility in the ST-BHO and PFT-BHOby delaying the onset of leaching, as evidenced by their higher pore volume values, both in the fist year (2018) and in the second year (2019). However, in the no-tillage system (S0T-BHO), no significant differences were observed compared to its counterpart without amendment,in either year. Similar findings were reported by Joshi et al. (2019), who suggested that organic amendments can be recommended to control sulfosulfuron (a member of sulfonylurea herbicides) leaching, associated with increased on TOC. In fact, in our study a significant and positive correlation between TOC and Initial pore volume values was found (r = 0.350; p < 0.05), suggesting that higher levels of organic matter prevent the BM from leaching rapidly.

Table 4. Bensulfuron-methyl leaching parameters.

The data are presented as means. The management included: permanent flooding irrigation with conventional tillage (PFT), sprinkler irrigation with conventional tillage (ST), sprinkler irrigation without tillage (S0T), and their corresponding counterparts amended with biochar applied in the first year (PFT-BHO, ST-BHO, and S0T-BHO, respectively). ANOVA factors: M, management; Y, Year; M × Y, Interaction Management × Year. * and *** significant at p levels of 0.05 and 0.001, respectively; NS, not significant. Different letters indicate differences (p < 0.05) between treatments in the same year (lower-case letters) and between years within the same treatments (upper case letters). ^†Pore volume for initiation of the herbicide’s leaching.

The maximum leached concentrations of BM in the unamended soils varied between 0.045 µM in PFT and 0.104 µM in S0T in 2018, and between 0.064 µM and 0.143 µM for those same treatments in 2019. Under sprinkler irrigation, the maximum leached concentration was higher without tillage (S0T) than with conventional tillage (ST), which in turn exceeded that of the flooding irrigation (PFT), which may be related to the lower retention capacity of these soils and to the more reversible nature of such retention. Supporting this, the maximum leached concentration was significantly and negatively correlated with K_f (r = −0.801; p < 0.01) and positively correlated with D (r = 0.766; p < 0.01).

BM leaching was significantly influenced by the management system, the year and their interaction (p < 0.001). In the unamended soils, the leached percentages were moderate, reaching maximum values of 5.19% in 2018 and 10.5% in 2019 (Table 4), nevertheless higher than the values below 4% reported by López-Piñeiro et al. (2019) in Mediterranean soils with pH values ranging from 5.15 to 5.92, slightly lower than those of the present study (Supplementary Table 1). Under sprinkler irrigation, BM leaching losses under conventional tillage (ST) exceeded the flood-irrigate PFT, by a factor of 4.18 in 2018 and 3.02 in 2019, whereas the comparison between S0T and ST varied between years (Table 4). These results indicate that sprinkler irrigation, may promote BM leaching, a behavior that can be ascribed to the lower retention capacity of these soils and to the higher maximum concentrations recorded in the leachate. Indeed, the values of total leached percentage were significantly and negatively correlated with K_f (r = −0.808; p < 0.01) and positively correlated with the maximum leached concentration (r = 0.825; p < 0.01). Similar findings have been described for other sulfonylurea herbicides, such as flucetosulfuron and chlorsulfuron, in different agricultural soils (Kumari et al., 2020; Ahmad et al., 2016).

Throughout the two years of study, BHO application significantly reduced BM leaching losses in the sprinkler irrigated treatments. Thus, the first year effect of the amendment led to decreases by factor of 2.2 and 1.4 in S0T-BHO and ST-BHO relative to S0T and ST, respectively, while for the second year effect these decreases were by factors of 1.6 and 1.3, respectively (Table 4). This behavior could be related to the reduction in the t_1/2 of BM induced by BHO in all treatments involving soil amendments. Indeed, the total leached BM correlated significantly and positively with t_1/2 (r = 0.816; p < 0.01), thereby evidence that dissipation and leaching are two mutually dependent processes in determining the environmental fate of the herbicide. In line with this, Terrón-Sánchez et al. (2023), working with Imazamox (another herbicide widely used in rice cultivation), observed that an increase in Imazamox persistence led to a significant rise in the amount of herbicide leached. In addition, Cessna et al. (2010) demonstrated that the susceptibility of three sulfonylurea herbicides to leach into groundwater depends, in part, on their persistence in soil. However, their study neither considered BM herbicide nor evaluated the effect of BHO.

Conversely, under flood irrigation, the total amount of BM leached increased by a factor of 1.9 in PTF-BHO compared with PTF under the first year effect, whereas no significant differences were observed under the second year effect (Table 4). This may be attributed to the low leaching of BM in the PTF treatment, which is associated with its higher adsorption capacity and/or lower persistence (Tables 2, 3), leaving little scope for BHO to further reduce BM leaching.

At the end of the leaching trial, the amount of BM retained in the column was significantly affected by the management system (p < 0.05), with no management × year interaction detected, indicating that this effect remained constant across both years of the study (Table 4). The herbicide was distributed evenly across the four sections of the column in all treatments and in both growing seasons. In the unamended soils, there was a clear trend towards lower BM recovery under the sprinkler-irrigated treatment with conventional tillage (ST) than under flood irrigation (PFT), with reductions of up to 1.30-fold in 2018 and 1.35-fold in 2019, while the comparison between S0T and ST varied between years. This finding is related to the fact that the sprinkler-irrigated treatments (S0T and ST) were those that recorded the major sorption capacity and irreversibility (Table 2) and the highest leaching losses of BM (Table 4). In fact, the total amount of BM extracted was significantly correlated with K_f (r = 0.394; p < 0.05) and D values (r = −0.367; p < 0.05), suggesting that the sorption–desorption process plays a key role in the leaching behavior of sulfonylurea herbicides (Kaur et al., 2026). The effect of BHO on the total amount of BM extracted depended on the irrigation system. Thus, relative to the unamended treatments, BHO application under sprinkler irrigation increased the total amount of BM extracted in both tillage systems (S0T-BHO and ST-BHO) in both years. In contrast, under flood irrigation (PTF-BHO), the total amount of BM extracted decreased throughout the study (Table 4). These results agree with those found in the leaching process. Thus, the increase in the amount of herbicide extracted from the leaching columns in amended sprinkler treatments was associated with a corresponding decrease in BM leaching losses. In fact, a significant and negative correlation between total BM extracted and total BM leached (r = −0.444; p < 0.01).

4 Conclusions

The present study demonstrates that the environmental fate of bensulfuron-methyl (BM) in rice soils is strongly influenced by soil properties, particularly soil pH, which are, modified by the implementation of sprinkler irrigation, with or without holm oak biochar (BHO). These effects are further modulated by the extent of biochar ageing in the field. Thus, compared with the traditional flood-irrigation system, sprinkler irrigation, reduced the sorption capacity and increased the persistence of BM, which might favour weed control under field conditions, although this require field validation. In addition, the application of BHO consistently reduced BM sorption and enhanced its desorption by increasing soil pH. Furthermore, likely due to the stimulation of soil microbial activity following BHO application, BM persistence was slightly reduced in all the rice cropping systems studied. Finally, although the implementation of sprinkler irrigation, led to more BM being leached from soil, a reduction in herbicide leaching was observed in soils enriched with BHO, although this effect was more pronounced in those subjected to sprinkler irrigation. Therefore, the combination of sprinkler irrigation and BHO application could represent a promising sustainable agronomic strategy for rice production, with a potential to improve soil health and mitigate BM pollution of water resources, particularly in rice-growing regions under semi-arid conditions, where adaptation to the current challenges posed by climate change is especially important. Nevertheless, although this study provides valuable information on the effects of these alternative crop management strategies on BM behavior under laboratory conditions, further studies under field conditions are needed to confirm these findings, as well as to evaluate their impact on herbicide efficacy and weed control.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fagro.2026.1953203/full#supplementary-material

Supplementary material Supplementary material at the source

Footnotes

Edited by: Davide Farruggia, University of Palermo, Italy

Reviewed by: Abazar Ghorbani, Nanjing Forestry University, China

Khuram Shehzad Khan, China Agricultural University, China

Footnotes

BM, Bensulfuron-methyl; BHO, Biochar; DHA, Dehydrogenase activity; D, Desorption; SSA, Specific surface area; WSOC, Water-soluble organic carbon; EC, Electrical conductivity; TOC, Total organic carbon.