Direct estimation of nitrate, total and fractionated water extractable organic carbon (WEOC) in an agricultural soil using direct UV absorbance deconvolution

In this study, a multiwavelength UV spectral deconvolution (UVSD) procedure is proposed as a robust and simple procedure for direct estimation of carbon and nitrate contents in soil water extracts. Soil samples were collected from an open-air field cultivated with maize at 3 different depths, 30 cm each, between 0 and 90 cm of soil surface during a period of 7 months. Fractionation of water extractable organic carbon (WEOC) into hydrophobic (Hpo-WEOC), transphilic (Tpi-WEOC), and hydrophilic (Hpi-WEOC) fractions is performed using XAD-8 and XAD-4 resins connected in series. In order to perform UVSD, 3 representative reference spectra of WEOC fractions were selected automatically, in addition to a 4th spectrum representative of NO3− selected manually in order to compose the deconvolution basis. The restitution of UV spectra was made in the range 235–350 nm. Through exploitation of soil water extract UV spectra, it was possible in a single-step deconvolution procedure to determine the organic carbon (mg C l−1) and NO3− (mg l−1) concentrations and to differentiate and to quantitatively estimate carbon content of WEOC fractions. Statistical tests indicated satisfactory correlations between values estimated using UVSD and those determined by conventional reference methods for each parameter determined. The ranges of concentrations of carbon and NO3− in the soil water extracts studied are between 3.00 and 15.00 mg C l−1 and 60–300 mg l−1, respectively. The limits of quantification (LQ) and of detection (LD) of WEOC and NO3− were found to be 0.10 and 0.05 mg C l−1, and 0.10 and 0.03 mg l−1, respectively.