Pedological assessments along an arid and semi-arid transect using soil spectral behavior analysis

Abstract

Reflectance spectroscopy as an efficient proximal sensing method, can be employed in soil genesis, monitoring, and management studies. However, limited data about the performance of this technique in different pedoenvironmental conditions is available. This study was conducted to investigate the capability of VIS-NIR-SWIR in detection of soil variations along a transect, monitoring the vertical changes of soil properties in relation to soil-forming factors and processes, and determination of the relationship between soil variables and their effects on the spectral features. Nine representative pedons were selected on different geomorphic positions including alluvial fan, piedmont plain, flood plain, and lowland. Soil samples were collected from different diagnostic horizons and analyzed for routine physico-chemical properties, iron forms, clay mineralogy, and VIS-NIR-SWIR spectroscopy. The results of the spectral interpretations revealed that the differences between soil horizons, clay mineralogy and the variations in key soil properties can be detected regarding the reflectance intensity, shape and absorption features. Smectite, chlorite, illite, kaolinite, and palygorskite were identified in the studied soils based on X-ray diffraction (XRD) analyses and their detection ranges were presented using spectral library and the second-derivative spectrum analyses. OC, CEC, clay, Fed and ᵡlf were negatively correlated with the reflectance wavelengths, while CCE, EC, sand, silt, and the evolution index (Feo/Fed) showed positive correlations. The most important factors affecting spectral behavior of the studied soils were found as particle size distribution, clay minerals, iron oxides, organic carbon, gypsum, CaCO3, soluble salts, and soil drainage conditions. Moreover, quantitative estimation of soil properties using partial least squares regression (PLSR) model showed VIS-NIR-SWIR to be accurate for estimation of CEC (RPD > 2), and reliable for OC, clay, Fed and ᵡlf (1.4 < RPD < 2). Overall, the results highlighted the significant role of VIS-NIR-SWIR on pedological assessments through soil characterization and differentiation of the horizons.