ANALYSIS of Soil Fourier Transform Infrared/Attenuated Total Reflection Spectral Data using Wavelet Analysis to Determine Soil Nitrate Content

Experiments were conducted involving adding nitrate to various soil types both in the laboratory and field. Soil pastes from ten different soils, including sandy loam and clay soils, were analyzed for soil nitrate content using the Fourier Transform Infrared (FTIR) Attenuated Total Reflection (ATR) technique. Three of the soil types were known to be calcareous, thus containing large carbonate amounts. Nitrate concentrations for the laboratory experiments varied from approximately 0 to 1000 ppm NO3-N while concentrations for the field experiments varied from approximately 0 to 100 ppm NO3-N. Wavelet analysis was applied to the spectra obtained from the soil pastes in order to allow for calibration equations to be developed to predict nitrate concentrations. Three-dimensional plots were created by graphing the wavelet deconvoluted values for each sample. From each plot, the volume of the nitrate peak was determined. Calibrations equations were developed by correlating the volume of these peaks to nitrate concentrations. High correlation values were found for all the soil types. Results of the laboratory experiments indicated R2-values as high as 0.99 and standard errors as low as 24 ppm NO3-N. Results of the field experiments gave R2-values as high as 0.98 and standard errors as low as 5 ppm NO3-N. In both cases, the slopes of the calibration equations depended on the soil types, indicating site specific calibration may be needed. Another technique that involved correlating absorbances at a fixed number of wavenumbers to nitrate concentrations was used to develop calibration equations. This method led to calibration equations that were soil independent and gave superior results to those obtained based on correlating wavelet deconvoluted volumes to nitrate concentrations. Also, these calibration equations allowed for the calcareous soils to be pooled with the noncalcareous soils for predicting nitrate concentrations.