Investigation of soil surface organic and inorganic carbon contents in a low-intensity farming system using laboratory visible and near-infrared spectroscopy

In marginal rural areas, intensification and mechanization of agriculture have determined changes in farming systems, which can increase soil erosion and reduce soil quality and carbon stock. Low input farming systems are alternative to conventional ones. The study aims to gain some insights in the composition of TOC in calcaric soils (Leptic Calcisols) and (Luvic Phaezem) using Vis-NIR reflectance spectroscopy in combination with partial least squares regression (PLSR). Secondary objectives were defining the most appropriate pre-processing techniques of Vis-NIR reflectance spectra, and predicting TOC in calcaric soils. The study area was located at Montecorvino Rovella (Salerno, southwestern Italy), characterized by an olive grove. Univariate PLSR analyses were performed using as response variables either TOC or total carbonate content and as predictors pH, electrical conductivity (EC) and absorbance spectra. The best results were obtained using as spectral data pre-treatments standard normal variate and Savitzky-Golay polynomials with the first derivative. Variable Importance for Prediction (VIP) statistics showed to be a good tool to gain insights in TOC composition also when its content is low and influenced by carbonate.