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

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.