Laboratory Vis–NIR spectroscopy as an alternative method for estimating the soil aggregate stability indexes of Mediterranean soils

Abstract

The characterization of soil aggregate stability is an important step for assessing the susceptibility of soil to water erosion. A normalized international method has recently been proposed for estimating soil aggregate stability indexes (ISO/FDIS 10930, 2012), but extensive measurements of the soil aggregate stability for mapping erosion risks on the regional scale remain a fastidious enterprise. This paper explores three different approaches as an alternative to the normalized international measurements. The first, called the PTFs Approach, estimates aggregate stability indexes via elementary soil properties using multiple linear regression. The second one, called the RS Approach, estimates aggregate stability indexes via Laboratory (Lab) Visible–Near Infrared (Vis–NIR) spectra using multivariate linear regressions. The third, called the RS+PTFs Approach first used the Lab Vis–NIR spectra to predict the elementary soil properties using multivariate linear regressions, which are then used to predict the soil aggregate stability indexes using multiple linear regression. These tests were performed on a collection of 113 soil samples from the Mediterranean region of Northern Tunisia. Four soil aggregate stability indexes were considered: three indexes calculated using three disruptive tests that correspond to various wetting conditions and energies, and the fourth index was the mean of the first three. This study shows that Lab Vis–NIR spectroscopy may be used directly in multivariate regression models to estimate two soil aggregate stability indexes (R2val between 0.52 and 0.57, RPD between 1.47 and 1.61) with accuracy comparable to the multiple linear models in the PTFs Approach. Further investigations on various soil types, especially those for which the soil aggregate stability indexes were strongly correlated to organic carbon content, are encouraged to extend the utility and applicability of Lab Vis–NIR spectroscopy as an alternative method for soil aggregate stability indexes estimates.