Abstract
Irrigation-induced salinization is an important land degradation process that affects crop yield in the Brazilian semi-arid region, and gypsum has been used as a corrective measure for saline soils. Fluvent soil samples (180) were treated with increasing levels of salinization of NaCl, MgCl2 and CaCl2. The salinity was gauged using electrical conductivity (EC). Gypsum was added to one split of these samples before they were treated by the saline solutions. Laboratory reflectance spectra were measured at nadir under a controlled environment using a FieldSpec spectrometer, a 250-W halogen lamp and a Spectralon panel. Variations in spectral reflectance and brightness were evaluated using principal component analysis, as well as the continuum-removed absorption depths of major features at 1450, 1950, 1750 and 2200 nm for both the gypsum-treated (TG) and non-treated (NTG) air-dried soil samples as a function of EC. Pearson’s correlation coefficients of reflectance and the band depth with EC were also obtained to establish the relationships with salinity. Results showed that NTG samples presented a decrease in reflectance and brightness with increasing CaCl2 and MgCl2 salinization. The reverse was observed for NaCl. Gypsum increased the spectral reflectance of the soil. The best negative correlations between reflectance and EC were observed in the 1500–2400 nm range for CaCl2 and MgCl2, probably because these wavelengths are most affected by water absorption, as Ca and Mg are much more hygroscopic than Na. These decreased after chemical treatment with gypsum. The most prominent features were observed at 1450, 1950 and 1750 nm in salinized-soil spectra. The 2200-nm clay mineral absorption band depth was inversely correlated with salt concentration. From these features, only the 1750 and 2200 nm ones are within atmospheric absorption windows and can be more easily measured using hyperspectral sensors.
Highlights
Soil salinization is one of the most relevant and important processes affecting the environmental degradation of land, especially in more arid regions [1]
We focused our analysis on the spectral effects of gypsum on local saline soils using principal components analysis and the continuum-removal techniques to observe variations in the spectral reflectance, brightness and depth of the major absorption bands
Data analysis included two steps: one to characterize the variations in spectral reflectance and brightness and the other to evaluate the modifications in depth of the major absorption bands with increasing concentration of salts in the soil for the non-treated chemically with gypsum (NTG) and TG samples
Summary
Soil salinization is one of the most relevant and important processes affecting the environmental degradation of land, especially in more arid regions [1]. Howari et al [12] examined the spectral reflectance of soils treated with saline solutions in the laboratory They observed that the crusts from different salts had diagnostic absorption bands at different wavelengths, whose position did not change with the size of the crystals or the salt concentration. The use of agricultural gypsum as a corrective measure for saline-sodic soils is due to the good solubility of this salt, which reduces aluminum saturation and adds calcium [17] It is essential in the replacement of sodium, which precipitates as a sulfate and is leached with the drainage of water [15]. Adopting the general strategy of the experiments by Howari et al [12] and Farifteh et al [18], a laboratory study was carried out to examine variations in spectral reflectance and the depth of the major absorption bands in the salinized-soil spectra. The implications of the results to mapping and monitoring soil salinization using hyperspectral remote sensing in the region are discussed, in which gypsum may be an indirect indicator of the salinization process, which may affect locally irrigated rice yield
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