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Article

Differential Molecular Interactions of Imidacloprid with Dissolved Organic Matter in Citrus Soils with Diverse Planting Ages

by
Junquan Chen
1,
Yawen Zhang
1,
Yanqi Guo
1,
Kai Jiang
2,
Duo Li
1 and
Taihui Zheng
1,*
1
Key Laboratory of Agricultural Resources and Ecology in Poyang Lake Basin of Ministry of Agriculture and Rural Affairs in China, School of Land Resources and Environment, Jiangxi Agricultural University, Nanchang 330045, China
2
School of Environment and Resources, Xiangtan University, Xiangtan 411105, China
*
Author to whom correspondence should be addressed.
Agriculture 2025, 15(9), 997; https://doi.org/10.3390/agriculture15090997
Submission received: 24 March 2025 / Revised: 26 April 2025 / Accepted: 2 May 2025 / Published: 4 May 2025
(This article belongs to the Section Agricultural Soils)
Browse Figures
Versions Notes

Abstract

:
The interactions between dissolved organic matter (DOM) and agrochemicals (e.g., neonicotinoid insecticides, NIs) govern the distribution, migration, and potential environmental risks of agrochemicals. However, the long-term effects of agricultural management on the DOM components and structure, as well as their further influences on the interactions between DOM and agrochemicals, remain unclear. Here, spectroscopic techniques, including Fourier transform infrared spectroscopy, two-dimensional correlation spectroscopy, and three-dimensional excitation–emission matrix fluorescence spectroscopy were employed to delve into the interaction mechanism between the DOM from citrus orchards with distinct cultivation ages (10, 30, and 50 years) and imidacloprid, which is a type of pesticide widely used in agricultural production. The findings revealed that the composition and structure of soil DOM significantly change with increasing cultivation age, characterized by an increase in humic substances and the emergence of new organic components, indicating complex biodegradation and chemical transformation processes of soil organic matter. Imidacloprid primarily interacts with fulvic acid-like fractions of DOM, and its binding affinity decreases with increasing cultivation age. Additionally, the interactions of protein-like fractions with imidacloprid occur after humic-like fractions, suggesting differential binding behaviors among DOM fractions. These results demonstrate that cultivation age significantly influences the composition and structural characteristics of soil DOM in citrus orchards, subsequently affecting its sorption capacity to imidacloprid. This study enhances the understanding of imidacloprid’s environmental behavior and provides theoretical support for the environmental risk management of neonicotinoid pesticides.

1. Introduction

The red soil region of China is a crucial citrus cultivation base; however, the high temperatures and humidity during the rainy season frequently lead to the outbreak of citrus Huanglongbing [1,2]. Imidacloprid, a leading neonicotinoid pesticide with multiple action modes (contact, stomach, and systemic toxicity), has been widely adopted in citrus production due to its low mammalian toxicity and high efficacy against pests such as aphids, leaf miners, and psyllids [3,4,5,6]. However, the red soil region experiences heavy rainfall and severe soil erosion, leading to the potential migration of imidacloprid through runoff, soil erosion, and leaching [7,8]. Although soil sorption significantly influences the migration of imidacloprid [9], research on the variation in imidacloprid sorption affinity of agricultural and forestry soils with different cultivation ages, particularly in citrus orchards, remains inadequate.
Dissolved organic matter (DOM) constitutes the largest mobile pool of organic carbon on Earth and has emerged as a research hotspot due to its critical role in controlling the environmental behavior of pesticides through adsorption, complexation, and ion exchange processes [10,11,12]. Notably, polarity is an essential physicochemical property of DOM, and the presence of various functional groups determines the interactions between organic pollutants and DOM [13,14]. Consequently, understanding DOM structural changes becomes imperative for predicting contaminant fate; yet, the specific mechanisms by which DOM chemical properties regulate imidacloprid adsorption remain unresolved, especially in age-varying citrus orchard soils.
As the planting cycle of orchards extends, the content of DOM in the soil generally increases proportionally with the age of the forest [15,16,17,18]. In orchard and forest ecosystems, the main sources of DOM are the activities of soil microorganisms and the decomposition of plant residues [17,19]. Importantly, DOM provides multiple binding sites for pesticides capable of interaction, such as thiacloprid, atrazine, and imidacloprid [12,20,21,22]. Despite the critical role of DOM in soil ecosystems, few studies have investigated how orchard age influences soil DOM composition. Furthermore, studies on the adsorption of imidacloprid by soil DOM are scarce [22], particularly lacking in research on the DOM from citrus orchard soils with different planting ages in red soil regions, as well as the structural changes in DOM during the adsorption process.
Advanced research techniques, such as two-dimensional correlation spectroscopy (2D-COS) and three-dimensional excitation–emission matrix fluorescence spectroscopy (3D-EEMs), have been employed to elucidate the interactions between DOM and imidacloprid [22]. Studies have found that imidacloprid can significantly alter the carboxyl, hydroxyl, and aliphatic C-H structures within DOM, and that the binding sequence of imidacloprid with protein-like and humic-like components of DOM differs [22]. 3D-EEMs, a commonly used method for studying DOM, can simultaneously alter excitation and emission wavelengths, providing a more comprehensive insight into the fluorescence characteristics of DOM. Coupled with parallel factor analysis (PARAFAC), 3D-EEMs can infer the sources of DOM and identify the dominant factors affecting its chemical composition [23]. The synchronous fluorescence spectra processed by 2D-COS effectively assess the impact of imidacloprid on the conformation of DOM, avoiding the issue of fluorescence peak overlap, and simultaneously reveal the conformational changes in protein-like substances within DOM upon interaction with imidacloprid [24]. 2D-COS aids in identifying the components of DOM that preferentially react with imidacloprid, while Fourier transform infrared spectroscopy (FTIR) is used to reveal the changes in DOM functional groups during the interaction with imidacloprid [25,26]. These research techniques are crucial for a deeper understanding of the interaction mechanisms between DOM and imidacloprid. They unveil the structural changes in DOM and the specific binding sites of imidacloprid within DOM, providing key information for deciphering the role of DOM in the environmental behavior of pesticides.
This study utilized 3D-EEMs, 2D-COS, and FTIR techniques to investigate the interactions between soil DOM and imidacloprid in citrus orchards with different cultivation ages (10, 30, and 50 years). We elucidated the temporal changes in DOM composition and analyzed the correlations between imidacloprid and various chemical components. By calculating three-dimensional fluorescence indices, we examined the source and property characteristics of soil DOM in citrus orchards across different cultivation ages. This study explored the interaction mechanisms between imidacloprid and DOM at the micro-level, as well as the effects of DOM content and structural changes on imidacloprid sorption, deepening our understanding of imidacloprid environmental behavior and providing theoretical support for the environmental risk management of neonicotinoid pesticides. It also contributes to assessing the ecological risks associated with DOM-mediated migration of imidacloprid in soil.

2. Materials and Methods

2.1. Soil Sample Collection

Soil samples were collected from citrus orchards located in Yijiahe New Village, Zhelin Town, Yongxiu County, Jiujiang City, Jiangxi Province (115°53′ E; 29°19′ N), with cultivation ages of 10, 30, and 50 years. The orchards had a planting density of approximately 450 trees per hectare and shared similar slope orientations and gradients. The soil type was a typical red soil developed from argillaceous shale. During the sampling process, a serpentine sampling method was employed. Five randomly selected citrus trees were sampled, avoiding fertilization points. Two sampling points were arranged vertically below the edge of the tree canopy in a diagonal direction, and surface soil samples (0 to 20 cm deep) were collected from these points. Ten replicate samples were collected from each orchard with different cultivation ages, which were then mixed to obtain a composite sample. Subsequently, the composite samples were quartered to obtain the final samples. These samples were brought back to the laboratory for air-drying, grinding, and subsequent analytical measurements.

2.2. Soil Analysis

Soil pH was measured using the potentiometric method. Total phosphorus content was determined by acid digestion followed by inductively coupled plasma optical emission spectrometry (ICP-OES) using an iCAP PRO DUo instrument (Thermo Fisher Scientific, Waltham, Germany). Total nitrogen content was analyzed with an elemental analyzer (Elementar Vario Macro Cube, Elementar, Germany). Organic matter content was assessed by the oil-bath-heated potassium dichromate oxidation–colorimetric method. Available potassium content was measured using the ammonium acetate extraction-flame photometry method. Air-dried soil passed through a 2 mm sieve was mixed with deionized water at a ratio of 1:5 and shaken at 25 °C and 160 rpm for 8 h. After centrifugation and filtration to remove impurities, the supernatant was analyzed for dissolved organic carbon concentration using a total organic carbon analyzer (liquid TOC Ⅱ, Elementar, Germany). Particle size distribution was determined by the pipette method, with the particle size classes defined according to the American system (sand: 2~0.05 mm, silt: 0.050~0.002 mm, clay: <0.002 mm).

2.3. Extraction and Purification of Soil DOM

Five grams of an air-dried soil sample were taken and added to a brown bottle pre-filled with 50 mL of ultrapure water and 0.01 mol/L CaCl2, maintaining a soil-to-water ratio of 1:10. The bottle was then sealed. The bottle was placed in a temperature-controlled shaker and agitated in the dark at 25 °C and 200 rpm for 24 h. After the shaking process, the supernatant was carefully decanted and centrifuged at 4000 r/min for 10 min. The centrifuged supernatant was filtered through a 0.45 µm filter membrane and subsequently dialyzed using a 3500 Da dialysis bag in a dark environment at 4 °C for 24 h, with ultrapure water changes every 8 h to remove inorganic salts and small organic molecules. Following dialysis, the DOM sample was extracted and purified and then stored in a freezer at −20 °C for subsequent use.

2.4. Fourier Transform Infrared Spectroscopy Measurements

Imidacloprid (>98%) was sourced from TCI Development Co., Ltd. (Shanghai, China). The DOM samples, the DOM-imidacloprid complexes (with imidacloprid concentrations of 2.4 × 10−4 M and 4.8 × 10−4 M), and the pure imidacloprid chemical sample were processed by freeze-drying. Following this, Fourier transform infrared (FTIR) spectroscopy was conducted using an Invenio-s spectrometer (Bruker Corporation, Ettlingen, Germany) with the KBr pellet technique. For the analysis, the spectral scanning range was set from 4000 to 400 cm−1, and each sample was scanned 64 times to ensure the acquisition of accurate spectral data.

2.5. Fluorescence Spectroscopy Scanning

The imidacloprid standard was prepared as a stock solution using dimethylphenol. Aliquots of 2.7 mL of DOM stock solution from different planting years were transferred into 5 mL brown centrifuge tubes, to which eight concentration gradients of the imidacloprid stock solution were added. The samples were then diluted to a final volume of 3 mL with ultra-pure water, resulting in imidacloprid concentrations of 0.6, 1.2, 1.8, 2.4, 3.0, 3.6, 4.2, and 4.8 × 10−4 M. The prepared samples were mixed at 150 rpm and incubated at 25 °C for 48 h to ensure complete reaction. Fluorescence spectroscopy, including 2D-COS and 3D-EEMs, was performed using an Edinburgh-FS5 fluorimeter (Edinburgh Instruments, Edinburgh, UK). The parameters for 2D-COS scanning were as follows: excitation wavelength (Ex) range of 220–600 nm with a 2 nm interval; slit width of Ex = 10 nm and emission (Em) = 10 nm; wavelength difference Δλ = Em − Ex = 60 nm. To accurately determine the synchronous fluorescence data of the samples, the fluorescence signal of imidacloprid alone was scanned separately and subtracted from the sample data. The parameters for 3D-EEMs scanning were: Ex range of 200–450 nm with a 5 nm interval; Em range of 250–600 nm with a 2 nm interval. The slit width was maintained at Ex = 10 nm and Em = 10 nm. Additionally, the three-dimensional fluorescence spectrum of ultra-pure water was scanned to subtract background fluorescence values from the sample results.

2.6. Data Processing

After converting the synchronous fluorescence spectroscopy data into matrices, the 2D correlation Spectroscopy Analysis plugin in Origin 2020 was utilized to obtain the synchronous and asynchronous correlation spectra of the two-dimensional correlation spectroscopy. The synchronous–asynchronous two-dimensional correlation spectra were analyzed according to Noda’s rule. The PARAFAC of the 3D-EEMs matrix was conducted using the DOM Fluor toolbox in MATLAB 2015a software. The correction for scattering (Rayleigh and Raman scattering) was applied to remove outliers, and the split-half analysis, along with residual analysis, was performed to validate the effectiveness of the PARAFAC model.

3. Results and Discussions

3.1. Changes in DOM Quality and Chemodiversity in Citrus Soils with Planting Years

3.1.1. Soil Quality and DOM Quantity

Table 1 presents the physicochemical properties of orchard soils across different planting durations. As the planting duration increased, the soil pH decreased, while the organic matter content rose annually. The orchard soils with 50a planting had the highest organic matter content at 29.68 g·kg−1, which is 57.95% higher than that of the 10a orchard soils. The changes in available potassium and total nutrients are in line with the organic matter content, indicating that orchard planting contributes to the enhancement of soil fertility. However, prolonged cultivation was associated with a progressive decline in soil pH, likely attributable to fertilizer application [27]. This acidification phenomenon may be attributed to the use of ammonium-based or organic nitrogen fertilizers, which promote H+ ion release via nitrification. Furthermore, tillage practices accelerated humus mineralization, thereby releasing additional protons into the soil system [28]. Additionally, the clay content in the soil increases with the extension of planting duration, while the sand content decreases, resulting in soil more suitable for cultivation [29]. The longer the planting duration, the higher the DOM content, with a 30.84% increase in the 50a orchard soils compared to the 10a orchard soils (Table 1), which was caused by long-term fertilization and management practices. As reported in a previous study, fertilization enhanced the accumulation of microbial necromass [30].

3.1.2. DOM Chemodiversity

3D-EEMs provide comprehensive fluorescence “fingerprint” information and are widely utilized to characterize DOM, offering insights into its fluorescence properties [31]. Figure 1 presents the 3D-EEM spectra of DOM from different planting years. Based on the distinct peak regions, three primary fluorescence peaks can be identified: Peak A (Ex/Em = 250/448–462 nm) is classified as the humic-like DOM; Peak B (Ex/Em = 325/394–404 nm) could be considered as aromatic protein; Peak C (Ex/Em = 330–340/448 nm) could be regarded as soluble microbial by-product-like DOM [32,33]. These fluorescence peaks are related to carbonyl and carboxyl groups within the humic structure, typically resulting from the complex molecular structures of plant residue decomposition and degradation products [34,35].
The spectral plots of DOM from different planting years exhibited variations in fluorescence center positions and intensities. As shown in Table 2, with the extension of planting years, the intensity of Peak B gradually decreases, but its peak area increases, and a new Peak C emerges in the DOM from soil planted for 50 years, indicating an increase in the content of HA-like substances in the DOM and the appearance of new organic components, which may be associated with more complex degradation or transformation processes of organic matter. In the soil planted for 10 years, the excitation wavelength of Peak A is less than 250 nm, whereas in the other two samples, the emission wavelength is close to 250 nm. This phenomenon suggests that with increasing planting years, a noticeable red shift occurs in Peaks A and B of the soil DOM, meaning that the corresponding wavelengths of the humic substance fluorescence peaks increase, reflecting a trend of increased aromaticity, molecular weight, and polydispersity of the humic substances [36,37]. Consequently, significant differences in the structure and composition of humic substances are observed in the DOM from soils with different planting years.

3.2. Changes in DOM Functional Groups Before and After the Reaction with Imidacloprid

Infrared spectroscopy played a pivotal role in elucidating the structural and functional characteristics of DOM before and after the reaction. This analysis enabled us to determine the relative contents of functional groups such as carboxyl, amino, benzene rings, protein-like, and aromatic compounds, which have significant binding capacities for organic pollutants [23]. Figure 2 presents the FTIR spectra of DOM with different planting years before and after reaction with imidacloprid, aiming to explore the binding mechanism between DOM and imidacloprid. The absorption peak at 3413 cm−1 is attributed to the stretching vibrations of O-H bonds in phenols, carboxyl groups, and N-H bonds in amines, and may additionally indicate the stretching vibration of hydroxyl groups involved in hydrogen bonding, which is common in carbohydrates such as cellulose, sugars, starch, as well as in phenols and alcohols [36,38]. The peak at 2909 cm−1 corresponds to the asymmetric stretching vibration of C-H bonds in aromatics. The absorption peak at 1641 cm−1 may originate from the vibration of C=C bonds in aromatic rings and could also be related to the C=O stretching vibration in peptide bonds of protein-like substances, indicating the presence of protein-like materials [39]. The peak at 1201 cm−1 is associated with the deformation vibration of C-H bonds in -CH2 and -CH3 groups; the absorption at 1092 cm−1 is related to the C-O stretching of polysaccharides and the carboxyl and aromatic groups; and the peak at 608 cm−1 corresponds to the out-of-plane deformation vibration of C-H bonds on the benzene ring, confirming the presence of aromatic compounds. By comparing the FTIR spectra of DOM from different planting years before and after the addition of imidacloprid, we observed that the number of peaks in the DOM was observed to decrease with the addition of imidacloprid, suggesting that these reduced peak positions serve as binding sites involved in the interaction between DOM and imidacloprid. Throughout the process, functional groups such as C-H, C=C, C-O, carboxyl, hydroxyl, and amino groups underwent changes, participating in the reaction between the two substances.
The DOM characteristic spectra from different planting years exhibited certain similarities, with the primary differences lying in the absorption ratios of the characteristic peaks. Prior to the reaction, the absorption intensity of the DOM characteristic peaks increases with the extension of planting years, indicating that a longer planting duration favors the formation of carbohydrates, phenols, alcohols, and aromatic compounds. Compared to the treatment with only imidacloprid, the absorption in the aforementioned bands decreases after the addition of imidacloprid, suggesting that the functional groups in DOM interact with imidacloprid, leading to the fluorescence quenching of DOM and confirming the existence of a static quenching mechanism. In Figure 2, we can observe that the amplitude of the DOM-imidacloprid complex is relatively lower after 50 years of planting. This phenomenon may be attributed to the cumulative effects of biodegradation and chemical transformation that soil DOM undergoes during long-term planting. Over time, the composition of DOM becomes more complex, containing more functional groups and higher molecular weights [8]. In particular, the hydrophobic neutral and acid-insoluble matter components of DOM, which are associated with humin-like substances of high aromaticity and large molecular weight, are the primary factors inhibiting adsorption [14]. Additionally, the complication of DOM may be accompanied by a decrease in active groups. For instance, the interaction between the polar groups in DOM (such as hydroxyl groups) and the corresponding functional groups in the imidacloprid molecule alters the vibration modes, thereby affecting the spectral representation. Therefore, the study results indicated that the binding affinity of citrus orchard soil organic matter (SOM) to imidacloprid shows a decreasing trend with the increase in planting years.

3.3. Complexation Behavior of DOM to Imidacloprid in Citrus Soils with Different Planting Years

The spectral results as mentioned above suggested that management practices, such as fertilization and tillage, changed the DOM chemodiversity in citrus soils (Figure 2). The results further indicated that management practices might have significant impacts on the complexation behavior of citrus soil DOM, thereby altering the fate and risk of agrochemicals.

3.3.1. Fluorescence Quenching Behavior for Diverse DOM Fluorescent Components

Figure 3 presents the three-dimensional fluorescence contour maps of soil DOM with different planting years in the presence of imidacloprid. After the addition of imidacloprid at a concentration of 4.8 × 10−4 M to the soil DOM, a marked disappearance of Peak A is observed in comparison to the original DOM fluorescence spectrum, indicating a strong binding of the FA-like substances within the DOM to imidacloprid. The significant alteration of the FA-like fluorescence peak, particularly its near-complete loss following the reaction, is primarily due to the smaller relative molecular mass of these substances and the carboxyl groups that can easily dissociate protons to become positively charged during the reaction, facilitating hydrophobic interactions with the pyridyl and aromatic groups (e.g., benzene rings, fused benzene rings, heterocycles) of the imidacloprid molecule, as well as electrostatic interactions with the nitro group [40,41]. Moreover, the degree of fluorescence intensity decrease for the HA-like substances in the DOM from soils with various planting years varies (Table 2). As the planting years increase, the fluorescence intensity of Peak B gradually diminishes and is completely lost in the 50-year DOM-imidacloprid reaction system, while the fluorescence intensity of the newly emerged Peak C also shows a declining trend. This is mainly because, although the content of soil organic matter increases with planting years, the organic carbon normalized adsorption coefficient (Koc, defined as the adsorption capacity per unit mass of OM) for imidacloprid exhibits a decreasing trend [8]. Concurrently, a noticeable red shift occurs in the position of Peak B, suggesting changes in the conformation and rigidity of the fluorescent groups during the binding process. These findings indicate that, compared to FA-like substances, the HA-like materials in the DOM from citrus orchard soils with different planting years have a lower degree of binding tightness with imidacloprid.
Synchronous fluorescence spectroscopy, as an analytical tool, can reveal the impact of imidacloprid on the conformation of fluorescent functional groups in DOM, thereby providing significant insights into the interactions between pesticides and SOM [24]. Figure 4 presents the synchronous fluorescence spectra of DOM samples with various planting years under the action of different concentrations of imidacloprid. The synchronous fluorescence spectra of DOM from different planting years exhibit distinct fluorescence bands, which can be divided into three primary regions: the protein-like region (PLR, 200–300 nm), the FA-like region (FLR, 300–371 nm), and the HA-like region (HLR, 371–500 nm) [42,43]. The FLR is predominantly composed of polycyclic aromatic hydrocarbons with 3 to 4 benzene rings and unsaturated fatty chains with 2 to 3 conjugated structures, while the HLR corresponds to polycyclic aromatic hydrocarbons with 5 to 7 benzene rings [44]. As observed in Figure 4, the DOM from different planting years all display a sharp peak, Peak Ⅰ (Ex = 336 nm), attributed to the FLR, as well as a shoulder peak, Peak Ⅱ (Ex = 380 nm), associated with the HLR. The fluorescence intensity of Peak Ⅰ is notably higher than that of Peak Ⅱ, indicating that FA-like substances constitute the predominant fluorescent emission groups in the DOM.
By comparing the relative area ratios of the three peak regions, we can obtain qualitative analytical results. Table 3 presents a comparison of the synchronous fluorescence peak area regions for DOM with different planting years after reaction with imidacloprid. The data indicate that the area ratio of the PLR ranges from 0.01 to 0.04 after the reaction of DOM from various planting years with imidacloprid, suggesting that there is minimal variation in the protein-like region across different planting years, and within the same planting year, no significant changes in the protein-like region are observed with increasing imidacloprid concentrations. The contents of FA-like substances are the highest, with area ratios ranging between 0.55~0.74, further confirming that FA-like substances are the predominant fluorescent emission groups in the DOM. With the extension of planting years, the content of FA-like substances gradually decreases, and it also decreases with increasing imidacloprid concentrations, indicating that imidacloprid primarily interacts with the FA-like substances in the DOM. This may be attributed to the fact that fulvic acid, compared to humic acid and humin, can more effectively facilitate electron transfer, enhancing the oxidation state and adsorption capacity of DOM [45]. The area ratio of the HLR ranges from 0.22 to 0.44, showing an opposite trend to that of the FLR [17]. As the planting years increase, the FLR/HLR ratio decreases, suggesting that DOM from shorter planting years has a relatively simpler structure concerning imidacloprid, while the structure becomes more complex with longer planting years. Within the same planting year, as the concentration of imidacloprid increases, the structure of the DOM becomes more complex, further confirming the formation of complexes between imidacloprid and DOM during the reaction process.

3.3.2. Sequential Changes in the Functional Groups of Citrus Soil DOM with the Addition Imidacloprid: Two-Dimensional Correlation Spectroscopy Analysis

2D-COS, as a spectral analysis technique, can provide more detailed information on the changes in fluorescence component peaks based on three-dimensional fluorescence spectroscopy, thereby more clearly revealing the changes in chemical groups during the binding process of imidacloprid with DOM [46,47]. Examination of the synchronous spectra in Figure 5a,c,e reveals two prominent self-peaks along the diagonal, corresponding to Ex = 336 nm (Peak I) and Ex = 380 nm (Peak II). The intensity of the self-peak at Ex = 336 nm is greater than that at Ex = 380 nm, indicating that imidacloprid binds more readily to FA-like substances than to HA-like substances during the quenching process, which aligns with the quenching degree observed in synchronous fluorescence spectra across different imidacloprid concentration systems. Additionally, the cross-peaks appearing in the 330~400 nm region indicate that the intensity of the DOM fluorescent groups gradually decreases with the increasing concentration of imidacloprid. This reduction in fluorescence intensity may be attributed to energy transfer or electron transfer occurring between imidacloprid and the fluorescent groups in DOM, resulting in fluorescence quenching [48]. According to Noda’s rule, the synchronous spectrum reflects the sensitivity of DOM fluorescence components to external environmental perturbations, with higher intensity indicating greater sensitivity [49]. In this experiment, the external perturbation factor is imidacloprid concentration, and the results indicate that FA-like substances exhibit higher sensitivity to changes in imidacloprid concentration compared to HA-like substances.
Asynchronous spectra elucidate the sequential or continuous changes in peak intensity caused by external factors, such as the addition of imidacloprid at different concentrations to DOM [50]. Following Noda’s rule, analysis of the asynchronous spectra in Figure 5b,d,f reveals that the changes in positive cross-peaks are more pronounced than those in negative cross-peaks. Below the diagonal of the asynchronous spectra, a negative region exists, where the negative cross-peak Ψ (336,380) correlates with Peak I (336 nm) and Peak II (380 nm) in the synchronous spectrum. This suggests that the FA-like fluorescence groups at 336 nm have priority in the complexation process with imidacloprid, followed by the HA-like fluorescence groups. These results indicate that the binding of imidacloprid to the protein-like fraction of DOM in citrus orchard soils with different planting durations occurs after the humic-like fraction. Furthermore, the binding capacity of DOM fractions in citrus orchard soils with different planting ages to imidacloprid also exhibits variations, which is related to the chemical composition and structure of DOM [51]. For instance, the content of humic substances in soil may increase with increasing planting age, leading to enhanced binding capacity between DOM and imidacloprid [52]. The aforementioned findings complement previous reports on the interaction between DOM and imidacloprid from a temporal perspective [22], facilitating a deeper insight into the behavioral characteristics of imidacloprid in the environment. The present study demonstrates that 2D-COS can elucidate the dynamic process of imidacloprid binding to soil DOM, indicating that FA-like substances exhibit higher sensitivity during the binding process and have a preferential binding over HA-like substances, thereby offering a novel perspective on understanding the migration and transformation behaviors of imidacloprid in the environment.

4. Conclusions

This study systematically investigated the temporal evolution of dissolved organic matter (DOM) characteristics in citrus orchard soils (10-, 30-, and 50-year cultivation) and their molecular-level interactions with imidacloprid. The findings revealed significant temporal shifts in DOM chemodiversity, characterized by a 57.95% increase in organic matter content and the emergence of microbial byproduct-like components (Peak C) in 50-year soils compared to 10-year soils, reflecting enhanced organic matter humification and microbial reprocessing under extended cultivation. Imidacloprid exhibited component-specific binding preferences, with fulvic acid-like substances serving as primary targets due to their carboxyl-rich structures and electron transfer capacity, while humic acid-like components showed weaker affinity. Notably, protein-like fractions bound imidacloprid only after humic-like interactions, a sequential behavior first documented in agricultural soils. Despite a 30.84% increase in DOM content with orchard age, the organic carbon-normalized adsorption coefficient (Koc) decreased in 50-year soils, highlighting a “quantity-quality trade-off” effect where aging-induced DOM complexity reduced adsorption efficacy. These mechanistic insights suggest higher imidacloprid mobility risks in long-term orchards, necessitating age-adjusted pesticide management strategies. Specifically, optimizing fertilization to regulate DOM aromaticity (e.g., reducing labile carbon inputs) could enhance contaminant retention in erosion-prone red soils. While this laboratory-based study on acidic red soils (pH 4.08–4.83) provides molecular-scale evidence, future work should prioritize field validation under realistic conditions, multi-pesticide assessments to generalize binding hierarchies, and isotopic tracing (e.g., δ13C-DOM) to disentangle management versus natural aging effects. Collectively, this work establishes a cultivation age-DOM functionality–pesticide fate nexus, offering a predictive framework for agrochemical optimization in age-stratified orchard systems.

Author Contributions

Conceptualization, T.Z. and J.C.; methodology, T.Z.; software, J.C., Y.Z., Y.G. and K.J.; validation, J.C., Y.Z. and Y.G.; formal analysis, J.C., Y.Z. and Y.G.; investigation, J.C., Y.Z., Y.G., K.J. and D.L.; resources, T.Z.; data curation, J.C. and D.L.; writing—original draft preparation, J.C. and Y.Z.; writing—review and editing, T.Z.; supervision, T.Z.; project administration, T.Z.; and funding acquisition, T.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (grant number 42167047) and Jiangxi Ganpo Elite Talent Support Program—Young Academic Leaders Initiative (grant number QN2023017). And The APC was funded by the National Natural Science Foundation of China (grant number 42167047). Information regarding the funder and the funding number should be provided. Please check the accuracy of funding data and any other information carefully.

Institutional Review Board Statement

This study did not require ethical approval.

Data Availability Statement

The data that support the findings of this study are available upon reasonable request from the authors.

Conflicts of Interest

The authors declare no conflicts of interest.

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Agriculture 15 00997 g001
Figure 1. Three-dimensional fluorescence spectra of DOM at different planting years ((a) 50 years; (b) 30 years; (c) 10 years).
Figure 1. Three-dimensional fluorescence spectra of DOM at different planting years ((a) 50 years; (b) 30 years; (c) 10 years).
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Agriculture 15 00997 g002
Figure 2. FTIR before and after DOM reaction with imidacloprid with different cultivation ages (imidacloprid concentration of 2.4 × 10−4 M and 4.8 × 10−4 M; (a) 50 years; (b) 30 years; (c) 10 years).
Figure 2. FTIR before and after DOM reaction with imidacloprid with different cultivation ages (imidacloprid concentration of 2.4 × 10−4 M and 4.8 × 10−4 M; (a) 50 years; (b) 30 years; (c) 10 years).
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Figure 3. Three-dimensional fluorescence spectra of DOM-imidacloprid system at different cropping years (imidacloprid concentration of 4.8 × 10−4 M; (a) 50 years; (b) 30 years; (c) 10 years).
Figure 3. Three-dimensional fluorescence spectra of DOM-imidacloprid system at different cropping years (imidacloprid concentration of 4.8 × 10−4 M; (a) 50 years; (b) 30 years; (c) 10 years).
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Figure 4. Simultaneous fluorescence spectra of different concentrations of imidacloprid in DOM samples with different cultivation ages ((a) 50 years; (b) 30 years; (c) 10 years).
Figure 4. Simultaneous fluorescence spectra of different concentrations of imidacloprid in DOM samples with different cultivation ages ((a) 50 years; (b) 30 years; (c) 10 years).
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Figure 5. 2D-COS synchronous and asynchronous spectra of DOM-Imidacloprid system with different cultivation ages (Synchronous spectra (a) 50 years; (c) 30 years; (e) 10 years; asynchronous spectra (b) 50 years; (d) 30 years; (f) 10 years).
Figure 5. 2D-COS synchronous and asynchronous spectra of DOM-Imidacloprid system with different cultivation ages (Synchronous spectra (a) 50 years; (c) 30 years; (e) 10 years; asynchronous spectra (b) 50 years; (d) 30 years; (f) 10 years).
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Table 1. Soil physicochemical properties of different cultivation ages.
Table 1. Soil physicochemical properties of different cultivation ages.
Cultivation Ages pHTN/g·kg−1TP/g·kg−1AK/mg·kg−1SOM/g·kg−1DOC/mg·L−1Mechanical Composition
Clay/%
<0.002 mm		Silt/%
0.002~0.050 mm		Sand/%
>0.050 mm
10a4.83 ± 0.121.25 ± 0.150.78 ± 0.61341.38 ± 167.3412.48 ± 1.4820.23 ± 0.4128.87 ± 1.3742.70 ± 2.9828.44 ± 1.60
30a4.68 ± 0.711.60 ± 0.250.98 ± 0.16545.64 ± 56.4315.94 ± 2.5222.06 ± 0.5242.33 ± 2.8538.85 ± 0.0418.83 ± 2.81
50a4.08 ± 0.232.97 ± 0.453.07 ± 0.21507.48 ± 49.2029.68 ± 4.5026.47 ± 0.9741.98 ± 2.3541.79 ± 0.5916.24 ± 1.77
Note: TN: total nitrogen; TP: total phosphorus; AK: available potassium; SOM: soil organic matter; DOC: dissolved organic carbon. The different lowercase letters indicate significant differences between different planting years (p < 0.05, n = 6).
Table 2. Fluorescence position and intensity of peaks before and after the reaction of DOM with imidacloprid at different planting years.
Table 2. Fluorescence position and intensity of peaks before and after the reaction of DOM with imidacloprid at different planting years.
Cultivation AgesSystemPeaksPeaks Position
Ex/Em (nm/nm)		Intensity
10aOnly DOMPeak A250/44837,817
Peak B325/39486,500
DOM−imidaclopridPeak A
Peak B340/40048,023
30aOnly DOMPeak A250/45453,966
Peak B325/39855,556
DOM−imidaclopridPeak A
Peak B340/45030,733
50aOnly DOMPeak A250/46238,495
Peak B325/40436,791
Peak C330/44838,708
DOM−imidaclopridPeak A
Peak B
Peak C340/44827,323
Table 3. Comparison of DOM-Imidacloprid synchronous fluorescence peak area with different cultivation ages.
Table 3. Comparison of DOM-Imidacloprid synchronous fluorescence peak area with different cultivation ages.
Cultivation AgesIC (mol/L)PLR (%)FLR (%)HLR (%)FLR/HLR
10a0.6 × 10−40.040.740.223.36
1.2 × 10−40.020.740.243.08
1.8 × 10−40.020.730.252.92
2.4 × 10−40.020.720.262.77
3.0 × 10−40.020.710.272.63
3.6 × 10−40.020.710.272.63
4.2 × 10−40.020.700.282.50
4.8 × 10−40.020.690.292.38
30a0.6 × 10−40.020.610.371.65
1.2 × 10−40.010.600.391.54
1.8 × 10−40.010.590.401.48
2.4 × 10−40.010.580.411.41
3.0 × 10−40.010.570.431.33
3.6 × 10−40.010.570.431.33
4.2 × 10−40.010.550.441.25
4.8 × 10−40.010.550.441.25
50a0.6 × 10−40.030.610.361.69
1.2 × 10−40.020.610.371.65
1.8 × 10−40.020.600.381.58
2.4 × 10−40.020.590.391.51
3.0 × 10−40.020.570.411.39
3.6 × 10−40.020.570.411.39
4.2 × 10−40.020.560.421.33
4.8 × 10−40.030.550.421.31
Note: IC: imidacloprid concentration; PLR: protein; FLR: fulvic acid; HLR: humic acid.
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Chen, J.; Zhang, Y.; Guo, Y.; Jiang, K.; Li, D.; Zheng, T. Differential Molecular Interactions of Imidacloprid with Dissolved Organic Matter in Citrus Soils with Diverse Planting Ages. Agriculture 2025, 15, 997. https://doi.org/10.3390/agriculture15090997

AMA Style

Chen J, Zhang Y, Guo Y, Jiang K, Li D, Zheng T. Differential Molecular Interactions of Imidacloprid with Dissolved Organic Matter in Citrus Soils with Diverse Planting Ages. Agriculture. 2025; 15(9):997. https://doi.org/10.3390/agriculture15090997

Chicago/Turabian Style

Chen, Junquan, Yawen Zhang, Yanqi Guo, Kai Jiang, Duo Li, and Taihui Zheng. 2025. "Differential Molecular Interactions of Imidacloprid with Dissolved Organic Matter in Citrus Soils with Diverse Planting Ages" Agriculture 15, no. 9: 997. https://doi.org/10.3390/agriculture15090997

APA Style

Chen, J., Zhang, Y., Guo, Y., Jiang, K., Li, D., & Zheng, T. (2025). Differential Molecular Interactions of Imidacloprid with Dissolved Organic Matter in Citrus Soils with Diverse Planting Ages. Agriculture, 15(9), 997. https://doi.org/10.3390/agriculture15090997

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