Research on the Effects of Drying Temperature for the Detection of Soil Nitrogen by Near-Infrared Spectroscopy

Abstract

Nitrogen nitrates play a significant role in the soil’s nutrient cycle, and near-infrared spectroscopy can efficiently and accurately detect the content of nitrate–nitrogen in the soil. Accordingly, it can provide a scientific basis for soil improvement and agricultural productivity by deeply examining the cycle and transformation pattern of nutrients in the soil. To investigate the impact of drying temperature on NIR soil nitrogen detection, soil samples with different N concentrations were dried at temperatures of 50 °C, 65 °C, 80 °C, and 95 °C, respectively. Additionally, soil samples naturally air-dried at room temperature (25 °C) were used as a control group. Different drying times were modified based on the drying temperature to completely eliminate the impact of moisture. Following data collection with an NIR spectrometer, the best preprocessing method was chosen to handle the raw data. Based on the feature bands chosen by the RFFS, CARS, and SPA methods, two linear models, PLSR and SVM, and a nonlinear ANN model were then established for analysis and comparison. It was found that the drying temperature had a great effect on the detection of soil nitrogen by near-infrared spectroscopy. In the meantime, the SPA-ANN model simultaneously yielded the best and most stable accuracy, with Rc2 = 0.998, Rp2 = 0.989, RMSEC = 0.178 g/kg, and RMSEP = 0.257 g/kg. The results showed that NIR spectroscopy had the least effect and the highest accuracy in detecting nitrogen at 80 °C soil drying temperature. This work provides a theoretical foundation for agricultural production in the future.

Prediction of nitrogen, active carbon, and organic carbon‐to‐clay ratio in agricultural soils by in‐situ spectroscopy

“…Additionally, an unsuccessful prediction of NH 4 + (RPIQ 1.16) means that the use of vis–NIR for Nmin prediction remains unattainable. Our results, although based on a relatively low number of soil samples, seem in line with other works reporting a possible prediction of nitrate using vis–NIR spectroscopy from sieved and dried soil samples (Amirul et al, 2020; Ehsani et al, 1999; Zhou et al, 2023), whereas ammonium was only reliably predicted by vis–NIR in soil pore‐water samples (Yupiter et al, 2023). Further studies are necessary to better clarify the possibility to predict nitrate content in agricultural soils based on in‐situ vis–NIR spectroscopy.…”