Abstract
Soil salinity is one of the most damaging environmental problems worldwide, especially in arid and semiarid regions. The objectives of this study were to improve the inversion accuracy of soil salt content (SSC) in soils with spectral heterogeneity by using optimized spectral indices. Soil samples at a 0–20 cm depth were taken from Keriya Oasis (98 soil samples), Ugan-Kuqa Oasis (49 soil samples), and Ebinur Lake Basin (57 soil samples). SSC and spectral reflectance (SR) of all the 204 soil samples were determined. To comprehensively analyze the field-collected hyperspectral data, various band combinations were used to calculate a normalized difference spectral index (NDSI) and ratio spectral index (RSI). Then, the relationships between the indices and SSC were examined, and the most robust relationships were demonstrated. The partial least squares regression (PLSR) method was utilized to develop a predictive model of SSC, and the variable importance in the projection (VIP) method was used during modeling. The results revealed that (i) the salinized soils in different regions had apparent differences in both reflectance and spectral curve morphology, but the optimized spectral indices method effectively overcame the regional heterogeneity of salinized soil hyperspectral characteristics, and the correlation with SSC was always kind, with correlation coefficients up to 0.748 at 0.001 level of significance; (ii) the VIP filtering method effectively selected the optimal independent model, and the modeling accuracy was better than the single optimization index (R2Pre = 0.83 and RMSEPre = 2.31 g·kg−1) by using the combination of two optimal indices; (iii) although the global modeling accuracy was significantly lower than the local modeling accuracy due to the inconsistent salt sensitivity bands of salinized soils in different regions, combined with cross-validation analysis, the global model had the ability to predict soil salinization accurately (R2Pre = 0.69 and RMSEPre = 8.45 g·kg−1). The methods developed in this study can be applied in other arid and semiarid areas. Besides, the study also provides examples for aerospace hyperspectral remote sensing of cross-regional soil salinization.
Highlights
Soil salinization can lead to decline in soil fertility, decrease in crop productivity, and deterioration of the ecological environment, which are the main factors restricting the sustainable development of agricultural production and the ecological environment [1,2,3]. e rational development, improvement, and treatment of salinized soil require rapid, accurate, and dynamic information acquisition about salinized soil
The samples were sorted from low to high according to the soil salt content (SSC), and based on the equidistance method, the calibration set and validation set were chosen. e characteristics of the descriptive statistics (Table 2) show that the calibration and validation sets of Ebinur Lake Basin corresponded to the following: the maximum values of SSC were 56.4 g·kg− 1 and 27.7 g·kg− 1, respectively; the minimum values were 0 g·kg− 1 and 1.1 g·kg− 1, respectively; and the mean values were 8.33 g·kg− 1 and 8.21 g·kg− 1, respectively. e coefficients of Coefficient of determination Root mean square error
SSC had variation coefficients greater than 36% in the three regions. e results showed that SSC had the greatest dispersion in the study area, which was largely impacted by human activities
Summary
Soil salinization can lead to decline in soil fertility, decrease in crop productivity, and deterioration of the ecological environment, which are the main factors restricting the sustainable development of agricultural production and the ecological environment [1,2,3]. e rational development, improvement, and treatment of salinized soil require rapid, accurate, and dynamic information acquisition about salinized soil. E rational development, improvement, and treatment of salinized soil require rapid, accurate, and dynamic information acquisition about salinized soil. Traditional soil salinization monitoring adopts the field fixed-point survey method, which is time-consuming, laborious, and destructive, with few measuring points and poor representativeness [4, 5]. It is difficult to obtain salt information across a large salinization area quickly and dynamically. Hyperspectral remote sensing has overcome the shortcomings of traditional artificial ground monitoring methods and has become an advanced method in the field of soil salinization monitoring due to its substantial advantages of multiple bands and continuous, abundant information and high quantitative inversion accuracy. Construction of a soil salinity hyperspectral quantitative inversion model is one of the essential aspects of soil salinization hyperspectral remote sensing monitoring.
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