Estimation of aggregate stability indices in Mediterranean soils by diffuse reflectance spectroscopy

Abstract

The proportion of water stable aggregates larger than 0.25 mm (WSA), mean weighted aggregate diameter (MWD) and water dispersible clay (WDC) are three good indicators of the risk of surface sealing, runoff generation and soil erosion by water. These aggregation indices are influenced by the contents in clay, iron oxides, calcium carbonate and organic matter, among other soil properties. Because these soil components possess specific spectral characteristics, one can use reflectance measurements to estimate the previous aggregation indices. To this end, we sampled the surface layer of 80 soils developed on granodiorites, quartzites, sandstones, shales and marls in southwestern Spain. Diffuse reflectance spectra were recorded with an Ultraviolet–Visible–Near Infrared (UV–Vis–NIR) spectrophotometer equipped with a diffuse reflectance sphere for the 300–2500 nm range, a Vis–NIR spectrophotometer with a spinning sample module for the 400–2500 nm range, and a mid infrared (MIR) spectrophotometer with a diffuse reflectance hemisphere accessory for the 4000–500 cm − 1 (2500–20,000 nm) range. Partial least-squares regression analysis based on log (1 / Reflectance) for various spectral ranges provided good estimates of the concentrations of calcium carbonate ( R 2 = 0.95; standard error of validation (SEV) = 56 g kg − 1 ), clay ( R 2 = 0.84; SEV = 54 g kg − 1 ), free iron oxides ( R 2 = 0.83; SEV = 3.3 g kg − 1 ), and acceptable estimates for pH ( R 2 = 0.69; SEV = 0.6), organic matter ( R 2 = 0.41; SEV = 5.2 g kg − 1 ), WSA ( R 2 = 0.60; SEV = 67 g kg − 1 ), MWD ( R 2 = 0.62; SEV = 0.4 mm), and WDC ( R 2 = 0.66; SEV = 29 g kg − 1 ). The instrument/wavelength range combination providing the highest R 2 values and accuracy (lowest SEV) was found to depend on the particular soil property. The goodness of prediction from measurements of the fine (< 2 mm) earth and ground (< 0.1 mm) samples was similar. Overall, our results suggest that diffuse reflectance spectroscopy is useful for (i) categorising soil zones according to aggregate stability (and hence susceptibility to water erosion); and (ii) as a screening tool for deciding whether any further laboratory analyses are needed, among other purposes.