Abstract
Soil salinization is a major soil health issue globally. Over the past 40 years, extreme weather and increasing human activity have profoundly changed the spatial distribution of land use and water resources across seven oases in southern Xinjiang, China. However, knowledge of the spatial distribution of soil salinization in this region has not been updated since a land survey in the 1970s to 1980s (the harmonized world soil database, HWSD) due to scarce observational data. Now, given the uncertainty raised by near future climate change, it is important to develop quick, reliable and accurate estimates of soil salinity at larger scales for a better manage strategy to the local fragile ecosystem that with limited land and water resources. This study collected electromagnetic induction (EMI) readings near surface soil to update on the spatial distribution and changes of water and salt in the region and to map apparent electrical conductivity (ECa, mS·m−1), in four coil configurations: vertical dipole in 1.50 m (ECav01) and 0.75 m (ECav05), so as the horizontal dipole in 0.75 m (ECah01) and 0.37 m (ECah05), then all the ECa coil configurations were modeled with random forest algorithm. The validation results showed an R2 range of 0.77–0.84 and an RMSE range of 115.17–142.76 mS·m−1. The validation accuracy of deep ECa dipole (ECah01, ECav05, and ECav01) was greater than that of shallow ECa (ECah05), as the former integrated a thicker portion of the subsurface. The range of EC spatial variability that can be explained by ECa is 0.19–0.36 (farmland, mean value is 0.28), grassland is 0.16–0.49 (shrub/grassland, mean value is 0.34), and bare land is 0.28–0.70 (bare land, mean value is 0.56). Among them, ECav01 has the best predictive ability. As the depth increased, the influence of soil-related variables decreased, and the contribution of climate-related variables increased. The main factor affecting ECa variation was climate-related variables, followed by vegetation-related variables and soil-related variables. Scatter plot show ECa was significantly correlated with ECe_HWSD_030 (0–30 cm, r = 0.482, p < 0.01) and ECe_HWSD_30100 (30–100 cm, r = 0.556, p < 0.01). The predicted spatial ECa maps were similar to the ECe values from HWSD, but also implies that the distribution of soil water and salt has undergone tremendous changes since 1980s. The study demonstrates that EMI data provide a reliable and cost-effective tool for obtaining high-resolution soil maps that can be used for better land evaluation and soil improvement at larger scales.
Highlights
One billion hectares of saline soil are distributed throughout more than 100 countries and regions [1], most commonly in China [2], India, Pakistan, Iran [3], Australia [4] and the United States [5]
As the increasingly food demanding in the future, properly managed saline-alkali soil is still attractive and profitable to food production for local stakeholders and policy makers at this arid region, high- quality and spatial resolution soil salinity maps is in urgent need, especially new dynamic in climate change and fast developed technology are both meet the accurate assessments requirement of local existing soil and water resources
The coefficient of variation (CV) of ECa gradually decreased, the maximum value appeared in ECah05, and the minimum value appeared in ECav01
Summary
One billion hectares of saline soil are distributed throughout more than 100 countries and regions [1], most commonly in China [2], India, Pakistan, Iran [3], Australia [4] and the United States [5]. Soil salinization causes an annual global agricultural loss of ~$12.7–27.3 billion, reducing agricultural output by up to 97% in some regions [9]. Soil salinization has caused a 50% reduction in agricultural production so far in the 21st century. The United Nations Food and Agriculture Organization (FAO) has estimated that the world’s population is expected to grow to 10 billion by 2050; with moderate economic growth, the demand for agricultural production will increase by ~50% compared to 2013 [10]. As the increasingly food demanding in the future, properly managed saline-alkali soil is still attractive and profitable to food production for local stakeholders and policy makers at this arid region, high- quality and spatial resolution soil salinity maps is in urgent need, especially new dynamic in climate change and fast developed technology are both meet the accurate assessments requirement of local existing soil and water resources
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