Mid-infrared Spectroscopic Determination of Soil Nitrate Content

Mid-infrared (mid-IR) spectroscopy experiments were conducted to detect added nitrate in various soil types both in the laboratory and field. Soil pastes from ten different soils, including sandy loam, clay, and peat soils, were analysed for soil nitrate contents using the Fourier transform infrared (FTIR) attenuated total reflectance (ATR) technique. Nitrate concentrations for the laboratory experiments varied from approximately 0–1000 ppm. NO3-N while concentrations for the field experiments varied from approximately 0–140 ppm. NO3-N. Three-dimensional plots were created by graphing the wavelet deconvoluted values at 32 scales for each sample. From each plot, the volume of the nitrate peak was determined and correlated to nitrate concentrations. Results of the laboratory experiments indicated values for the coefficient of determination R2 as high as 0·99 and standard errors as low as 24 ppm. NO3-N for soil-specific calibrations. Results of the field experiments gave values for R2 as high as 0·98 and standard errors as low as 5 ppm NO3-N for soil-specific calibrations. An alternative technique to determine nitrate content was developed in which wavelet analysis was used to identify a few wavenumbers at which interferences from other ions were minimal. This method produced calibration equations that were soil independent and gave superior results to those obtained based on correlating wavelet deconvoluted volumes to nitrate concentrations.