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Article

Nitrate Nitrogen Quantification via Ultraviolet Absorbance: A Case Study in Agricultural and Horticultural Regions in Central China

by
Yiheng Zang
1,†,
Jing Chen
1,†,
Muhammad Awais
1,
Mukhtar Iderawumi Abdulraheem
1,
Moshood Abiodun Yusuff
1,
Kuan Geng
1,
Yongqi Chen
1,
Yani Xiong
1,
Linze Li
1,
Yanyan Zhang
1,
Vijaya Raghavan
2,
Jiandong Hu
1,3,*,
Junfeng Wu
1 and
Guoqing Zhao
4,*
1
Henan International Joint Laboratory of Laser Technology in Agriculture Sciences, College of Mechanical & Electrical Engineering, Henan Agricultural University, Zhengzhou 450002, China
2
Department of Bioresource Engineering, Faculty of Agriculture and Environmental Studies, McGill University, Sainte-Anne-de-Bellevue, QC H9X 3V9, Canada
3
State Key Laboratory of Wheat and Maize Crop Science, Zhengzhou 450002, China
4
School of Management, Swansea University, Swansea SA2 8PP, UK
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Agriculture 2025, 15(11), 1131; https://doi.org/10.3390/agriculture15111131
Submission received: 2 April 2025 / Revised: 22 May 2025 / Accepted: 22 May 2025 / Published: 23 May 2025
(This article belongs to the Section Agricultural Soils)
Browse Figures
Versions Notes

Abstract

:
Soil nitrate nitrogen (NO3−-N) is a key indicator of agricultural non-point source pollution. The ultraviolet (UV) dual-wavelength method is widely used for NO3−-N detection, but interference from complex soil organic matter affects its accuracy. This study investigated how organic matter influences NO3−-N detection by optimizing UV dual-wavelength combinations. Density functional theory (DFT) calculations showed slight spectral broadening of fulvic and humic acids in the presence of NO3−-N under UV spectrum. Standard solutions and soil samples were used to compare the detection performance of different wavelength pairs. The findings indicated that the dual-wavelength combination of 235 nm/275 nm is optimal rather than 220 nm/275 nm for measuring soil samples at NO3−-N concentrations exceeding 5 mg·L−1. The 235/275 nm method gave an average calibration coefficient of 1.57. Compared to the national standard and flow analysis methods, the average relative errors were 19.7% and 22.3% (p < 0.001), respectively, indicating its suitability for practical soil applications. These results demonstrate the method’s potential for rapid and accurate NO3−-N detection in real soil samples, supporting its application in environmental monitoring and agricultural management.

1. Introduction

Growing concerns about the environmental impact of excessive fertilizer use, particularly excess nutrient deposits in soils, and understanding NO3−-N levels have become critical to sustainable agricultural practices [1,2]. Elevated NO3−-N concentrations present a significant risk to the ecological environment, potentially resulting in groundwater contamination and eutrophication, which can adversely affect soil, water quality, and human health [3,4]. However, due to its high solubility and fluidity in water, NO3−-N is easily biodegraded, so its measurement results are easily interfered with by many environmental factors.
For example, changes in frequency and mode of farmland irrigation will affect the migration and transformation of NO3−-N in soil [5,6]. The key factor that influences measurement accuracy is the content of soil organic matter (SOM), which plays an important role in the storage and stability of NO3−-N by reacting with or adsorbing NO3−-N ions [7,8]. Overlapping spectral absorption from NO3−-N and organic matter can result in substantial measurement errors when employing the Ultraviolet (UV) dual-wavelength method. Numerous studies have illustrated the intricacy of this relationship, underscoring the necessity for calibration methods that consider SOM variations to enhance the accuracy of NO3−-N measurements in the field [9,10]. Therefore, considering the impact of organic matter in NO3−-N measurements is crucial to ensure accuracy, especially in soils with high organic content [11,12].
Traditionally, the determination of NO3−-N in soil mainly depended on laboratory methods, such as the ion-selective electrode method, ion chromatography, which commonly requires the use of chemical reagents to accurately determine the concentration of NO3−-N in soil [13,14]. Thus, these laboratory methods, while excellent in accuracy, are cumbersome, time-consuming, and not cost-effective. On the other hand, dangerous chemical reagents, such as strong acids and alkali, used in experiments not only have high technical requirements for operators but also pose certain safety risks [15]. In modern agricultural practices, precision agriculture technology has gradually become an important measure to improve crop productivity by helping farmers recommend fertilizer more rationally, and reduce environmental pollution through real-time monitoring of soil nutrient status [16,17]. Nonetheless, traditional laboratory methods struggle to fulfill this demand as they are unable to rapidly deliver soil data on site. This delay may result in excessive fertilization or the failure to apply fertilizers at the ideal time, subsequently impacting crop yields and the health of the environment [18,19].
In recent years, sensor-based real-time monitoring technology has gradually emerged as a possible alternative, such as spectral analysis, electrochemical sensors, and the Internet of Things, which can make rapid and non-destructive determination of NO3−-N in soil [20,21,22]. These approaches not only improve inspection efficiency but also enable remote monitoring and decision making through data networking, providing timely and reliable data support for agricultural management [23,24]. Through the application of precise resource allocation and an intelligent management system, the efficiency of agricultural production factors has been significantly improved, and agricultural errors have been reduced [25].
Ultraviolet spectroscopy is currently very popular for detecting soil NO3−-N as a promising alternative to traditional chemical analysis [11]. In particular, the ultraviolet dual-wavelength method has shown great potential in overcoming the interference caused by SOM on NO3−-N measurements [26,27]. The method includes measuring the ultraviolet light absorption of different substances at specific wavelengths, one is sensitive to NO3−-N and organic matter (220–240 nm), and the other is sensitive to organic matter only (254–280 nm). By quantifying the difference in absorbance at two specific wavelengths, the concentration of the target sample can be accurately determined from a mixture with potentially interfering components [28,29,30]. In addition, the dual ultraviolet wavelength combination is also a key factor in the determination of the calibration coefficient. Eight soil types in different ecological regions of Inner Mongolia were selected to determine NO3−-N in soil, and the influence of wavelength selection and soil type on the calibration coefficient of ultraviolet spectrophotometry was investigated [31].
Although the NO3−-N measurement instrument has made important advances with ultraviolet spectrophotometry in the measurement of NO3−-N in soil [32], the complexity of relying on broadband ultraviolet spectrophotometers and bulky deuterium lamps as ultraviolet light sources makes the technology not yet suitable for commercial applications. With the breakthrough of AlGaN semiconductor materials with a wide bandgap [33], deep ultraviolet light-emitting diodes (UVC-LEDs) with an operating band of 235–290 nm have made a breakthrough in optical power output performance [34,35]. This progress not only promotes the technological innovation of the miniaturized spectrophotometric nitrate sensor, but also systematically improves the core performance of the sensor through the collaborative optimization and precision structural design of LEDs and photodiodes (PDs)—while increasing the sensitivity by 20% and extending the linear detection range to 0–20 mg·L−1, where long-term operational stability is achieved [36]. At present, it has been successfully applied to a water quality online monitoring system, providing intelligent solutions with high precision and low power consumption for the environmental monitoring field [37,38].
This study primarily focuses on examining the impact of NO3−-N measurements in soil that varies in organic matter content. The assessment of NO3−-N in soil using UV dual-wavelength combinations of 220 nm/275 nm and 235 nm/275 nm will be investigated with soil samples from agricultural and horticultural regions, China. The impact of NO3−-N in soil with varying organic matter contents on the calibration coefficient α of the ultraviolet dual-wavelength method is examined, followed by further optimization. The ultimate objective of this research is to create a more dependable and efficient method for measuring NO3−-N in the field, which will have substantial advantages for environmental monitoring and precision agriculture.

2. Materials and Methods

2.1. Study Area and Field Sampling

The samples originated from agricultural and horticultural regions in Henan province, a significant agricultural hub in central China recognized for its extensive crop cultivation (Figure 1a,b). These regions have been meticulously chosen to yield a representative sample of agricultural methodologies and crop varieties. In this experiment, different soil samples were collected from the Mengzhou Kiwifruit planting zone (112°47′31.5″ E, 34°54′23.5″ N) in Jiaozuo and the Yichuan Wheat planting zone (112°25′30.4″ E, 34°25′31.1″ N) in Luoyang. The sampling points in the Yichuan area were random (Figure 1c), and the kiwifruit planting zone was divided into 10 sample areas (Figure 1d). The plum blossom point method [39] was used to collect three soil samples ranging in depth from 0 to 30 cm in each plot.

2.2. Chemical Treatment and Analysis

In this study, standard NO3−-N solutions (0–25.0 mg·L−1) were prepared using analytical-grade NaNO3 (purity ≥ 99%, Sinopharm Group, Beijing, China) and deionized water. Potassium chloride (KCl, purity 99.5%, Sinopharm Group, Beijing, China) was used for soil extraction. Absorbance at 220 nm, 235 nm, and 275 nm was recorded using a UV–visible spectrophotometer (Nanjing Feller Instrument Co., Ltd, Nanjing, China) and a self-developed integrated flow injection analyzer (IRFA) to establish calibration curves.
To assess the effects of organic matter, humic acid (HA, 104 mg·L−1) and fulvic acid (FA, 63 mg·L−1) solutions were added to the NO3−-N standards, and calibration coefficients (220/275 nm and 235/275 nm absorbance ratios) were calculated.
Additionally, 24 soil samples were collected from kiwifruit and wheat plantations. After extraction with 100 mL of 1 M KCl for 1 h, the mixtures were filtered to obtain soil extracts. To assess the impact of varying organic matter content on the selection of the optimal detection wavelength combination for NO3−-N, 10 samples of soil from two regions were divided into multiple aliquots and processed as described. To simulate a range of NO3−-N concentrations, mixed extracts were spiked with known volumes of 1 g·L−1 NO3−-N standard solution and KCl, forming 7–9 concentration gradients. The final volume of each solution was adjusted to 50 mL. Dual-wavelength absorbance measurements at 220/275 nm and 235/275 nm were then performed to evaluate detection accuracy.

2.3. Absorption Spectrum Computation of Organic Matter and NO3−

Fulvic acid (FA) and humic acid (HA) are the main components of soil organic matter [40,41]. HA, a major fraction of humic substances, is widely present in soil, water, and sediments [42], while FA plays a key role in the humus structure of nutrient-rich soils and regulates plant growth. The presence of organic matter can interfere with the accurate determination of soil NO3−-N concentrations [43].
To investigate the absorption spectrum of these two organic matters and NO3−-N, density functional theory (DFT) calculations were conducted using Gaussian 16 ab initio software [44]. Structural optimizations of fulvic acid, humic acid, and NO3−-N were performed at the M06-2X/def2-TZVP theoretical level. Frequency calculations were conducted on the optimized molecules to verify that the optimized structures represent the minima on the potential energy surface. Based on the calculations, the electrostatic potential (ESP) of the pertinent molecule or ion was exported using Multiwfn software [45] and visualized with VMD software [46]. As illustrated in Figure 2, NO3− is enveloped by a robust electronegative electric field, attributable to the presence of three oxygen atoms (Figure 2a). An electro-positive electric field is present in the vicinity of the surfaces of HA and FA (Figure 2b,c). Consequently, NO3− is eventually trapped or bound by organic matter to a certain degree by electrostatic interaction. The binding free energies, ΔGbind of NO3− with FA or HA, are −8.57 kcal·mol−1 and −6.95 kcal·mol−1, respectively (Figure 2d,e), indicating a relatively strong electrostatic interaction between the organic compound and NO3−. The measurement of NO3− is readily affected by organic matter. The binding free energy ΔGbind was computed with the following Equation (1):
ΔGbind = GFA/HA+NO3− − GFA/HA − GNO3−
where the GFA/HA+NO3− denotes the Gibb’s free energy of the complex, namely, the association of the FA/HA molecule with the nitrate ion, GFA/HA signifies the Gibb’s free energy of the unbound FA/HA molecule, and GNO3− indicates the Gibb’s free energy of the free NO3−-N. The values on the right side of Equation (1) were derived using the corresponding frequency computations.
The calculated UV spectra of the relevant molecules or ions in aqueous solution were obtained using the time-dependent density functional theory (TDDFT) method, using the optimized structures at the M06-2X/6-311+G (2 d,p) theoretical level. The SMD implicit solvent model was used to replicate the aqueous environment in order to theoretically support the impact of organic matter on the absorption spectrum of NO3−-N (Figure 3). There exists a significant overlap between NO3−-N and organic matters at the wavelength below 225 nm, particularly for NO3−-N and HA, which complicates the measurement of NO3−-N in the mixed solution system when relying on adsorption at a single wavelength. Furthermore, the amalgamation of NO3−-N with organic matters solely exhibits the adsorption properties of the latter, hence diminishing the measurability of NO3−-N. Fortunately, the adsorption intensity of organic matters varies significantly upon combination, particularly for HA, enabling measurement of NO3−-N by an indirect method, such as dual wavelengths.

2.4. NO3−-N Measurement System Using Dual-Ultraviolet Wavelength Absorption

The research indicated that NO3−-N in soil exhibits a strong absorption near 220 nm, but no absorption characteristics at 275 nm [47]. The direct measurement of the NO3−-N concentration in soil samples is significantly restricted by the extensive interference of soil organic matter in the dual-ultraviolet wavelength absorption. Here, a key calibration coefficient α, the dual-wavelength spectral absorption ratio of organic matter in the soil solution, the measured A275 is multiplied by this coefficient α and then reduced by A220 to eliminate the interference. However, the composition of soil organic matter is highly diverse and complex, which complicates its spectral characteristics [48]. As a result, it is difficult to directly and accurately determine the calibration coefficient α through theoretical calculation.
Therefore, for the different soil types or the same soil type with different soil organic matter content, the calibration coefficient α should be improved before the measurement. In this work, experiments were carried out for soil types with different organic matter and NO3−-N concentration, and the calculation formula of NO3−-N concentration is shown in Equation (2):
CNO3−-N = k × (A220,235 − α × A275) + b
where CNO3−-N is the concentration of NO3−-N in the sample solution; k is the conversion coefficient, which is used to convert the corrected absorbance into the concentration of NO3−-N; A220,235 is one of the absorbances at 220 nm and 235 nm; α is the calibration coefficient, which is derived from the absorbance ratio of multiple sets of organic matter at 220 nm or 235 nm to 275 nm; and b is the error used to compensate for systematic errors or background signals in the experiment.

2.5. Statistical Analysis

All statistical analyses, including linear regression fitting and calculation of the determination coefficients (R2), were performed using Origin 2022 (OriginLab Corporation, Northampton, MA, USA) based on the least square method. To evaluate the significance of the regression models, p-values were calculated by Origin 2022. Specifically, the overall significance of each linear regression was assessed using an F-test, and the associated p-values (Prob > F) were reported. Graphs were also generated using Origin 2022.

3. Results

3.1. Absorbance of NO3−-N Solutions at 220 nm,235 nm, and 275 nm

The absorbance analysis curve shown in Figure 4 illustrates the characteristics of the ultraviolet wavelength method for determining NO3−-N concentrations. Within the low concentration range of 0–5 mg·L−1, the absorbance at 220 nm increases sharply with NO3−-N concentration, indicating that this wavelength is particularly sensitive to NO3−-N in this interval. The high determination coefficient (R2 = 0.9983, p < 0.001) supports its reliability for detecting low concentrations of NO3−-N. For concentrations between 0 and 20 mg·L−1, absorbance at 235 nm shows minimal variation but maintains a notable linear relationship with a determination coefficient (R2) of 0.9978 (p < 0.001). This makes 235 nm a preferable wavelength for quantifying higher levels of NO3−-N, especially when avoiding saturation effects at lower wavelengths. In contrast, absorbance at 275 nm exhibits a relatively small linear slope, consistent with its role as a background correction wavelength.

3.2. Analysis of Calibration Coefficients on Samples with Organic Matter

Preliminary experimental results indicate that for FA samples of varying concentrations, absorbance exhibits an increasing trend at several wavelengths (220 nm, 235 nm, 275 nm) as concentration rises (Figure 5). The rate of rise in absorbance at 220 nm is notably rapid, suggesting that within the examined concentration range, the sensitivity of the 220 nm wavelength to FA is elevated (Figure 5a). Like FA, the absorbance of HA likewise escalates with concentration across all wavelengths. In addition, the absorbance ratio of FA and HA (220 nm/275 nm and 235 nm/275 nm, denoted as calibration coefficient α) exhibits considerable variability in the low concentration range (0~5 mg·L−1) (Figure 5c,d). In comparison with the 220 nm/275 nm wavelength combination, the 235 nm/275 nm wavelength combination exhibits a more gradual variation in the absorbance ratio of FA and HA. At low concentrations, the variation in absorbance ratio was less pronounced, while at medium and high concentrations, the absorbance ratio changed smoothly and linearly between 0.95 and 1.31.

3.3. Experiment and Analysis of NO3−-N in the Samples with Adding Organic Matters

Figure 6 presents the absorbance values and corresponding absorbance ratios at 220 nm, 235 nm, and 275 nm after the addition of two types of organic matter.
The influence of NO3−-N concentration on the calibration coefficients remains relatively stable across the tested wavelengths (220 nm and 235 nm) (Figure 6a,b). When fulvic acid (FA) was added, the calibration coefficient (absorbance ratio) at 220 nm/275 nm ranged from 2.2 to 3.1, while the ratio at 235 nm/275 nm varied from 1.4 to 2.0, indicating a narrower fluctuation at the latter wavelength combination (Figure 6c). Upon the addition of humic acid (HA), the calibration coefficients showed even smaller variations: 1.15–1.26 for 220 nm/275 nm and 1.09–1.19 for 235 nm/275 nm (Figure 6d).

3.4. Experiments for Actual Soil Samples

In this experiment, a total of 18 soil samples were gathered from both the northern and southern regions of the kiwifruit planting base located in Mengzhou City, Henan Province. Additionally, 6 soil samples were collected from the Yichuan wheat planting area in Luoyang City. To ensure soil fertility and promote the healthy growth of kiwifruit or other crops, the experimental study outlined above was validated, and the actual concentrations of soil NO3−-N and organic matter content were assessed.
As shown in Figure 7, soil samples measured at this time have a noticeable range in organic matter concentrations and NO3−-N concentrations. The organic matter concentrations of kiwifruit plantation range from 15.18 to 24.03 g·kg−1, while those of Yichuan wheat plantation range from 3.33 to 5.29 g·kg−1 (Figure 7a). The NO3−-N concentrations of kiwifruit plantation range from 3.12 to 28.29 mg·kg−1, while those of Yichuan wheat plantation range from 0.91 to 4.68 mg·kg−1 (Figure 7b).
The measured NO3−-N concentrations (obtained via instrument calibration) were compared with the theoretical values calculated using Equation (3). The resulting detection errors under the 220/275 nm and 235/275 nm wavelength combinations were analyzed to assess the applicability and reliability of each dual-wavelength approach.
C1 × V1 = C2 × V2
where C1 and V1 are the concentration and volume of the initial solution. C2 and V2 are the concentration and volume after dilution or concentration.
The organic matter content in the soil of the kiwi cultivation region in Mengzhou is comparatively elevated. At low NO3−-N concentrations (<4.8 mg·L−1), the discrepancy between the calculated and theoretical concentrations for the two wavelength combinations is minimal. However, as the NO3−-N concentration rises (>4.8 mg·L−1), the variance in detection results between the 220 nm/275 nm wavelength combination becomes pronounced (Figure 8a). The Yichuan agricultural planting area exhibits a relatively low soil organic matter content, with no variation in detection results between the 220 nm and 235 nm wavelengths, particularly at low concentrations (<6 mg·L−1). Nonetheless, at elevated concentrations, the 235 nm/275 nm wavelength combination exhibits a relative detection advantage (Figure 8b).
To further validate the reliability of the 235 nm/275 nm dual-wavelength method for determining soil NO3−-N using a calibration coefficient of 1.57, a comparative analysis was performed against the widely used flow injection analysis method and the national standard dual-wavelength method (calibration coefficient α = 2.23) (Figure 9a,b). The NO3−-N concentrations measured by the three methods exhibited strong linear relationships. The determination coefficients (R2) between the proposed method and the flow analysis method, and between the proposed method and the national standard method, were 0.962 and 0.961, respectively, with both comparisons yielding statistically significant correlations (p < 0.001).

4. Discussion

4.1. Influence of Different Concentrations of NO3−-N on Its Dual-Wavelength Determination

As shown in Figure 4, within a specific concentration range, both 220 nm and 235 nm wavelengths demonstrate excellent linear correlation with NO3−-N absorption, demonstrating strong potential for dual-wavelength UV spectrophotometric analysis. The absorbance at 220 nm is notably high due to the strong molar absorptivity of NO3−-N, making it highly sensitive for low-concentration detection. This observation is consistent with previous studies, which also identified 220 nm as the optimal wavelength for detecting trace levels of nitrate in aqueous solutions [49,50].
However, when the NO3−-N concentration exceeds approximately 5 mg·L−1, the absorbance at 220 nm nears saturation (about 2.5 absorbance units), exhibiting non-linear behavior. This saturation effect has been reported in similar studies and is attributed to limitations in the optical system such as stray light interference and detector overload [49]. In contrast, the absorbance at a wavelength of 235 nm exhibits a more stable and linear response across a broader concentration range. Prior research has highlighted the suitability of 235 nm for medium to high NO3−-N concentrations, where 220 nm becomes unreliable or necessitates tedious dilution steps [51].
Our results corroborate this finding, as the absorbance analysis demonstrates a sensitivity of 220 nm for low concentrations, a linear behavior of 235 nm for higher concentrations, and a function of 275 nm as a background correction wavelength. For concentrations exceeding 5 mg·L−1, 235 nm provides a more reliable and efficient alternative, especially when organic interferences are minimal or absent. These findings not only align with previous work but also contribute a practical threshold (~5 mg·L−1) for wavelength switching between 220 nm and 235 nm. Unlike earlier studies that typically focused on fixed-wavelength calibration [52], our research proposes a concentration-dependent wavelength selection strategy.

4.2. The Influence of Different Organic Matter on Its Dual-Wavelength Calibration Coefficient

For the difference in absorbance, HA possesses a more intricate molecular structure, an elevated molecular weight, and more functional groups that can absorb ultraviolet light [53]. These variables also lead to HA exhibiting greater absorbance values in the UV region than FA (Figure 5b). Moreover, within the tested range, variations in organic matter concentration significantly influence the absorbance ratio (Figure 5c,d). The consistent and linear response at medium-to-high concentrations suggests that the 235 nm/275 nm wavelength combination is more suitable for quantitative detection under these conditions.
Given the variability of organic matter composition in soils, particularly the fluctuating HA/FA ratios, stable calibration becomes critical [54,55]. The 235 nm/275 nm wavelength combination demonstrates a more consistent calibration coefficient (α ≈ 1.57) across different organic matter types and concentrations, compared to the standard 220 nm/275 nm ratio (α = 2.23, per GB/T 32737-2016 [56]) and a more stable trend, thereby more effectively fulfilling the measurement requirements for NO3−-N in soil. In addition, the calculation formula of calibrated NO3−-N concentration is shown in Equation (4):
CNO3−-N = k × (A235 − 1.57 × A275) + b
where A235 − 1.57 × A275 is the calibrated absorbance. k is the conversion coefficient, which is used to convert the calibrated absorbance into the concentration of NO3−-N.
As shown in Figure 6a,b, the presence of HA and FA significantly increases absorbance values at both 220 nm and 235 nm, confirming that organic matter interferes with NO3−-N detection. However, at higher NO3−-N concentrations (>5 mg·L−1), the absorbance at 235 nm remains more stable, reducing risks of saturation or signal distortion. Similar results were reported by Dong et al., who found that longer UV wavelengths can effectively suppress the matrix effect of dissolved organic matter during nitrate analysis [57]. In summary, the 235 nm/275 nm combination offers enhanced robustness for NO3−-N determination in complex soil extracts, especially when organic interference is significant. Thus, it is more suitable for stable NO3−-N quantification in the presence of humic substances. This finding supports a practical, concentration, and matrix-informed wavelength selection strategy.

4.3. Influence of NO3−-N Determination in Actual Soil and Applicability of Wavelength

Determination of actual soil, when we validated the 235/275 nm dual-wavelength combination for soil NO3−-N measurement, effectively mitigates the impact of higher content of organic matter on the measurement results. Considering that the organic matter content in typical agricultural soils was from 1% to 6%, while wetland or horticultural regions may have organic matter concentrations as high as 20% or more [58,59], the determination results need to be optimized for different soil types and terrain.
This study focused on crop-growing areas in the central region of China, including various soil types and organic matter content, to support more accurate NO3−-N monitoring strategies for regional soil management. To further explore the application effect of the dual-wavelength combination on the actual soil measurement, we mixed the soil extract and re-measured the standard. Initial investigations suggest that the organic matter content influences the precise measurement of NO3−-N levels in soil (Figure 8a,b). In instances of low organic matter content (Yichuan soil sample), the influence of wavelength on detection outcomes is more contingent upon the actual NO3−-N concentrations. The variance in detection results is minimal when employing the 220 nm/275 nm dual wavelength for measuring low concentrations of NO3−-N, whereas the 235 nm/275 nm dual wavelength can precisely ascertain the NO3−-N content in soil at 10 mg·L−1. Nevertheless, for solutions with elevated organic matter content (Mengzhou soil sample), the increase in NO3−-N concentration amplifies the influence of wavelength on detection outcomes. The 220 nm/275 nm dual wavelength is unable to correctly measure 4.8 mg·L−1 of NO3−-N, likely due to UV saturation or spectral overlap with dissolved organics [60,61]. Consequently, the 235 nm/275 nm dual-wavelength combination is more suitable.
Moreover, the NO3−-N content measured by the dual-wavelength method of 235 nm/275 nm was highly correlated with the national standard method, and the relative error was also low. Comparing the flow analysis method and the national standard dual-wavelength method, the root-mean-square error range is 1.52~1.17, and the average relative error range is 19.7%~22.3%. These results demonstrate the method’s accuracy and field relevance. To further reduce the relative error and achieve rapid detection, UV-based sensor systems with fixed optical paths and UVLED sources can improve detection stability and minimize interference. Figure 10 illustrates the soil NO3−-N measurement system based on dual-ultraviolet wavelength absorption (235 nm and 275 nm UVLED). The system uses two wide-bandgap semiconductor UV photodiodes for real-time online monitoring of NO3−-N. After photoelectric conversion and signal amplification, the signals are transmitted to a microcontroller for processing. When combined with soil-specific calibration models, such sensors offer better accuracy and are more suitable for in-field NO3−-N monitoring [62,63].
Overall, this study provides a foundation for improving NO3−-N detection accuracy under varying SOM conditions. In addition, it is also important to recognize that external environmental and management factors—such as crop cultivation patterns, climate variability, and organic farming practices—can considerably influence the distribution and dynamics of soil NO3−-N, fulvic acid (FA), and humic acid (HA), especially within the top 30 cm of soil [64,65]. Different crops vary in their nitrogen uptake and root exudation profiles, which affect nitrate mobility and organic matter transformation [66]. In addition, organic farming practices—such as compost application, reduced synthetic fertilizer input, and cover cropping—can significantly increase soil organic carbon and microbial activity, leading to changes in the composition and reactivity of humic substances [67,68]. While these factors were not directly examined in this study, they represent critical areas for future research, which could help refine nitrate detection methods by accounting for variable environmental and biological influences.

5. Conclusions

This study confirms that UV dual-wavelength detection methods, particularly the 235 nm/275 nm combination, are more reliable for quantifying soil NO3−-N in the presence of organic matter. The influence of organic components was validated through both DFT simulations and laboratory experiments, revealing their impact on UV absorbance and the need for calibration. By introducing an optimized calibration coefficient, the method effectively reduces matrix interference and enhances measurement accuracy. Compared to conventional techniques, it offers a simplified approach with reduced reagent use and improved measurement reliability. While the current results are based on selected soil types from specific agricultural regions, further validation across broader soil conditions is necessary. Overall, this UV dual-wavelength approach holds strong potential for supporting environmental monitoring, precision fertilization, and routine soil testing in agriculture.

Author Contributions

Y.Z. (Yiheng Zang): Writing—review & editing, Writing—original draft, Investigation, Methodology. J.C.: Writing—review & editing, Writing—original draft, Validation. M.A.: Writing—review & editing, Investigation. M.A.Y.: Writing—review & editing. K.G.: Data curation. Y.C.: Software. Y.X.: Formal analysis. L.L.: Resources. Y.Z. (Yanyan Zhang): Investigation. M.I.A.: Validation. V.R.: Visualization. J.H.: Writing—review & editing, Supervision, Resources, Funding acquisition, Conceptualization. J.W.: Writing—review & editing, Investigation, Funding acquisition, Conceptualization. G.Z.: Writing—review & editing, Investigation, Conceptualization. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the 14th Five-Year National Key Research and Development Program (2024YFD17000802, 2021YFD1700904), the Major Science and Technology Projects of Henan Province (221111320700), and Henan Center for Outstanding Overseas Scientists (GZS2021007).

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

Acknowledgments

The authors of this manuscript highly acknowledge Henan Agricultural University, Zhengzhou, China, for providing resources to carry out this research work.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. The marked study areas. Map of China (a) and Henan province (b); The photos of the sampling locations in Yichuan (c) and Mengzhou (d).
Figure 1. The marked study areas. Map of China (a) and Henan province (b); The photos of the sampling locations in Yichuan (c) and Mengzhou (d).
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Figure 2. The electrostatic potential (ESP) of (a) NO3−, (b) FA molecule, and (c) HA molecule. (d,e) show the binding energy calculations of NO3−-N with (b) FA and (c) HA. In these pictures of ESP, the red color represents the electronegativity, the blue color represents the electropositivity, and the green color represents the electroneutrality.
Figure 2. The electrostatic potential (ESP) of (a) NO3−, (b) FA molecule, and (c) HA molecule. (d,e) show the binding energy calculations of NO3−-N with (b) FA and (c) HA. In these pictures of ESP, the red color represents the electronegativity, the blue color represents the electropositivity, and the green color represents the electroneutrality.
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Figure 3. Calculated UV spectra of FA, HA, and NO3−-N and their combination.
Figure 3. Calculated UV spectra of FA, HA, and NO3−-N and their combination.
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Figure 4. Absorbance of different NO3−-N concentrations at 220 nm, 235 nm, and 275 nm wavelengths.
Figure 4. Absorbance of different NO3−-N concentrations at 220 nm, 235 nm, and 275 nm wavelengths.
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Figure 5. The absorbance of the standard solutions of (a) fulvic acid and (b) humic acid at the wavelengths of 220 nm, 235 nm, and 275 nm; The absorbance ratios (220/275 and 235/275) of the standard solutions of (c) fulvic acid and (d) humic acid.
Figure 5. The absorbance of the standard solutions of (a) fulvic acid and (b) humic acid at the wavelengths of 220 nm, 235 nm, and 275 nm; The absorbance ratios (220/275 and 235/275) of the standard solutions of (c) fulvic acid and (d) humic acid.
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Figure 6. The absorbance of the NO3−-N standard solutions with (a) adding FA and (b) HA at the wavelengths of 220 nm, 235 nm, and 275 nm; The absorbance ratios (220/275 and 235/275) of the NO3−-N standard solutions with adding (c) FA and (d) HA.
Figure 6. The absorbance of the NO3−-N standard solutions with (a) adding FA and (b) HA at the wavelengths of 220 nm, 235 nm, and 275 nm; The absorbance ratios (220/275 and 235/275) of the NO3−-N standard solutions with adding (c) FA and (d) HA.
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Figure 7. Measured (a) organic matter content and (b) NO3−-N content in Yichuan wheat planting and horticultural kiwifruit fields.
Figure 7. Measured (a) organic matter content and (b) NO3−-N content in Yichuan wheat planting and horticultural kiwifruit fields.
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Figure 8. Measured NO3−-N content in (a) Mengzhou kiwifruit planting zone, (b) Yichuan wheat planting field.
Figure 8. Measured NO3−-N content in (a) Mengzhou kiwifruit planting zone, (b) Yichuan wheat planting field.
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Figure 9. Correlation of NO3−-N content measured by 235 nm/275 nm method and other different methods, such as (a) flow analysis method, (b) Chinese national standard dual-wavelength method (220 nm/275 nm).
Figure 9. Correlation of NO3−-N content measured by 235 nm/275 nm method and other different methods, such as (a) flow analysis method, (b) Chinese national standard dual-wavelength method (220 nm/275 nm).
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Figure 10. Schematic diagram of NO3−-N determination by ultraviolet dual-wavelength method based on UVLED.
Figure 10. Schematic diagram of NO3−-N determination by ultraviolet dual-wavelength method based on UVLED.
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MDPI and ACS Style

Zang, Y.; Chen, J.; Awais, M.; Abdulraheem, M.I.; Yusuff, M.A.; Geng, K.; Chen, Y.; Xiong, Y.; Li, L.; Zhang, Y.; et al. Nitrate Nitrogen Quantification via Ultraviolet Absorbance: A Case Study in Agricultural and Horticultural Regions in Central China. Agriculture 2025, 15, 1131. https://doi.org/10.3390/agriculture15111131

AMA Style

Zang Y, Chen J, Awais M, Abdulraheem MI, Yusuff MA, Geng K, Chen Y, Xiong Y, Li L, Zhang Y, et al. Nitrate Nitrogen Quantification via Ultraviolet Absorbance: A Case Study in Agricultural and Horticultural Regions in Central China. Agriculture. 2025; 15(11):1131. https://doi.org/10.3390/agriculture15111131

Chicago/Turabian Style

Zang, Yiheng, Jing Chen, Muhammad Awais, Mukhtar Iderawumi Abdulraheem, Moshood Abiodun Yusuff, Kuan Geng, Yongqi Chen, Yani Xiong, Linze Li, Yanyan Zhang, and et al. 2025. "Nitrate Nitrogen Quantification via Ultraviolet Absorbance: A Case Study in Agricultural and Horticultural Regions in Central China" Agriculture 15, no. 11: 1131. https://doi.org/10.3390/agriculture15111131

APA Style

Zang, Y., Chen, J., Awais, M., Abdulraheem, M. I., Yusuff, M. A., Geng, K., Chen, Y., Xiong, Y., Li, L., Zhang, Y., Raghavan, V., Hu, J., Wu, J., & Zhao, G. (2025). Nitrate Nitrogen Quantification via Ultraviolet Absorbance: A Case Study in Agricultural and Horticultural Regions in Central China. Agriculture, 15(11), 1131. https://doi.org/10.3390/agriculture15111131

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