Assessment of laboratory VIS-NIR-SWIR setups with different spectroscopy accessories for characterisation of soils from wildfire burns

Thousands of hectares of Mediterranean forests are burned every year. Fires affect all the landscape components. They trigger erosion processes, which can have catastrophic consequences. Thus, detection and post-fire monitoring of soil properties is of great importance. Changes in soil caused by the fire can be detected by proximal soil sensing. In this context, the study evaluates the applicability of laboratory experimental setups for spectral analysis of burnt soils. Three setups of Analytical Spectral Device (ASD) FieldSpec®4 spectroradiometer with different spectroscopy accessories (external integrating sphere, illuminator lamp and contact probe) were used for measurement of reflectance spectra and evaluation of soil organic matter (SOM) in 82 soil samples from wildfire burns in Aragon, Northern Spain. No statistically significant differences were detected between values obtained by different setups. Lower reflectances registered with integrating sphere are probably due to the fact that the internal cavity of the device is not perfectly spherical because of the existence of multiple port windows. Measurements with Illuminator lamp and contact probe were more stable and corresponding calibration models for SOM built using partial least square regression combined with step-down variable selection algorithm (SA-PLSR) demonstrated acceptable predictive ability (0.75 ≤ R2V ≤ 0.81; 2.00 ≤ RPD ≤ 2.55). The coefficients are ∼10% higher than those obtained with the integrating sphere. The study demonstrated feasibility of using Visible - Near InfraRed - Short Wave InfraRed (VIS-NIR-SWIR) spectroscopy for monitoring post-fire evolution of burnt soils and showed that the choice of the appropriate accessory (e.g., Illuminator lamp) improves the reliability of SOM estimations.