Improving the Prediction of Soil Organic Matter Using Visible and near Infrared Spectroscopy of Moist Samples

Soil moisture reduces the accuracy of in situ measurements of soil properties using visible and near infrared reflectance spectroscopy and limits the application scope of existing soil spectral libraries built from air-dried samples. The preprocessing method of external parameter orthogonalisation (EPO) has been successfully used to improve the prediction of soil organic matter (SOM) in moist samples. However, the traditional strategy of EPO development (EPOI) requires a complex experimental design. In this study, we proposed a new EPO strategy (EPOII) that only uses a single sample but containing various soil moisture content (SMC) levels. Reflectance spectra (350– 2500 nm) of 130 samples with different SMC levels were measured in the laboratory. We built calibration models using air-dried samples and partial least squares regression before and after EPOI and EPOII, which were validated using air-dried samples and moist samples, respectively. For the validation of moist samples, we first classified the SMC into four groups (A: all SMC levels; B: SMC < 0.1 g g−1; C: 0.1 < SMC < 0.2 g g−1; and D: SMC > 0.2 g g−1) and then used a Monte Carlo method to simulate SMC distributions in the field with a result of 500 cases for each SMC group. Before EPO, an apparent decrease in the accuracy of the SOM predictions for moist samples occurred in each SMC group when using the calibration model derived from air-dried samples, compared with that of air-dried samples. Both EPOI and EPOII improved the SOM prediction of moist samples for most scenarios of SMC variations (groups A–C), but did not apply to the scenario with both large SMC variation and high SMC (group D). When considering EPOII, it could be a feasible strategy of EPO implementation for removing moisture effects on SOM prediction using visible and near infrared spectroscopy, although the degree of improvement was less than that from EPOI. Also, EPOII will pave the way as a standard pretreatment especially for soil spectra measured in the field when projection matrices of EPOII have been derived and incorporated in spectroscopy-related software.