Both methods are used to characterize relations between soil properties and spectra. Based on our experience, for current study (or similar studies) relations among soil properties, and between PC1 and soil properties are well characterized by Spearman, and Pearson is more relevant to the relations between spectra and soil properties.
Lines 281, 249: Why did you use Spearman's Rho correlation coefficients earlier and Pearson correlation coefficients here?