Spectral Soil Quality Index (SSQI) for Characterizing Soil Function in Transformed Land-Use Areas.

During the last decades, a large proportion of the planet's terrestrial surface has transformed from natural ecosystems to human dominated systems.  These land-use dynamics affects soil quality and health.  The current study was conducted at the desert fringe of the northern Negev, Israel, and strived to assess and compare soil quality in three land-uses (afforestation, traditional grazing, and agro-pastoral) that were altered from managed to unmanaged or vice versa.  The overall aim of this research is twofold: (1) to evaluate the ability of reflectance spectroscopy to detect changes in 14 soil physical, biological, and chemical properties and their derived Soil Quality Index (SQI) across the transformed land-uses; and (2) to develop a Spectral Soil Quality Index (SSQI) toward applying the reflectance spectroscopy technique as a diagnostic tool of soil quality.  To achieve these objectives several mathematical/statistical procedures consist of a series of operations were used including a principal component analysis (PCA), partial least squares – regression (PLS-R), and partial least squares – discriminate analysis (PLS-DA).  The PLS-R most suitable models successfully predicted soil properties (R² ≥ 0.80; ratio of performance to deviation (RPD) ≥ 2.0) including sand-silt-clay content, NH₄, NO₃, and pH. Moderately-well predicted soil properties (0.50 ≤ R² < 0.80; RPD ≥ 2) were residual water, soil organic matter, electric conductivity, and potassium.  Poor validation (R² < 0.50; RPD < 2) results were obtained in the potential active carbon, phosphorus, and hydraulic conductivity.  In addition, the PLS-R model predicted the SQI in the transformed land-uses range between 0.650 ≤ R² < 0.80 with RPD ≥ 2.  The PLS-DA model was used to develop the SSQI model.  The correlation between the SSQI and the SQI was moderately-well (0.63 < R² < 0.72). This study underscores the potential application of reflectance spectroscopy as a reliable diagnostic screening tool for assessing soil quality.   The results suggest that the SSQI can be used to assess hot-spots of change in transformed land-use and to identify soil degradation areas. 

Keyword: land-use transformation, semi-arid, reflectance spectroscopy, spectral soil quality index, Soil Quality Index.