Predicting long-term dynamics of soil salinity and sodicity on a global scale

Abstract

Knowledge of spatiotemporal distribution and likelihood of (re)occurrence of salt-affected soils is crucial to our understanding of land degradation and for planning effective remediation strategies in face of future climatic uncertainties. However, conventional methods used for tracking the variability of soil salinity/sodicity are extensively localized, making predictions on a global scale difficult. Here, we employ machine-learning techniques and a comprehensive set of climatic, topographic, soil, and remote sensing data to develop models capable of making predictions of soil salinity (expressed as electrical conductivity of saturated soil extract) and sodicity (measured as soil exchangeable sodium percentage) at different longitudes, latitudes, soil depths, and time periods. Using these predictive models, we provide a global-scale quantitative and gridded dataset characterizing different spatiotemporal facets of soil salinity and sodicity variability over the past four decades at a ∼1-km resolution. Analysis of this dataset reveals that a soil area of 11.73 Mkm² located in nonfrigid zones has been salt-affected with a frequency of reoccurrence in at least three-fourths of the years between 1980 and 2018, with 0.16 Mkm² of this area being croplands. Although the net changes in soil salinity/sodicity and the total area of salt-affected soils have been geographically highly variable, the continents with the highest salt-affected areas are Asia (particularly China, Kazakhstan, and Iran), Africa, and Australia. The proposed method can also be applied for quantifying the spatiotemporal variability of other dynamic soil properties, such as soil nutrients, organic carbon content, and pH.

Keywords: global scale modeling; machine learning; soil salinity; soil salinization; soil sodicity.

Fig. 1.

Variability of different aspects of soil salinity and sodicity in the western United States. (A and D) SD of annually predicted soil salinity (EC_e) and sodicity (ESP), respectively, between 1980 and 2018. (B and E) Average of annually predicted EC_e and ESP, respectively (1980 to 2018). (C and F) Change in the likelihood (θ) of soils with an EC_e ≥4 dS⋅m⁻¹ or ESP ≥6% in the period 2000 to 2018 relative to 1981 to 1999 (the likelihood is dimensionelss, calculated by dividing the number of years with EC_e ≥4 dSm⁻¹ or ESP ≥6% by the total number of years in the studied period). Positive θ indicates that the likelihood has increased and negative shows that it has decreased.

Fig. 2.

Global distribution of salt-affected soils (excluding the frigid zones). (A and E) Likelihood of the surface soils with an EC_e ≥4 dS⋅m⁻¹ and ESP ≥6% between 1980 and 2018, respectively (the likelihood is dimensionelss, calculated by dividing the number of years with EC_e ≥4 dS⋅m⁻¹ or ESP ≥6% by the total number of studied years). The panels on the right (D and H) and below (C and G) the maps show the total area of soils with an annual predicted EC_e ≥4 dS⋅m⁻¹ and ESP ≥6%, respectively, in at least 75% of the period between 1980 and 2018 for different longitudes and latitudes at 30 arc-second resolution (∼1 km). (B and F) Total area of the soils with an annual predicted EC_e ≥4 dS⋅m⁻¹ and ESP ≥6%, respectively, in at least 75% of the period from 1980 to 2018 at the continental level.

Fig. 3.

Variations in the total area of salt-affected soils between 1980 and 2018 at the continental level. (A–F) Variations in the total area of soils with salinity of EC_e ≥4 dS⋅m⁻¹. (G–L) Variations in the total area of soils with sodicity of ESP ≥6%. Red lines show the low-pass-filtered (5-y running window) variation of the annual salt-affected areas. Mean values indicate the total area of salt-affected land on each continent averaged from 1980 to 2018.

Fig. 4.

Variations in the soil cell-level salinity/sodicity and country-level area of the salt-affected soils (P < 0.05). (A and D) Cell-level variations in EC_e and ESP between 1980 and 2018, respectively. Soil cell is any ∼1- × 1-km stretch of the soil. Maps are delimited to −55 and 55 latitudes and higher latitudes are shown only for improving the visualization of the maps. (B and C) Variations in the total area of soils with salinity of EC_e ≥4 dS⋅m⁻¹ since 1980, at the country level. (E and F) Variations in the total area of soils with sodicity of ESP ≥6% since 1980, at the country level (see SI Appendix, Tables S22 and S23 for annual gain or loss in the total area of salt-affected soils for all countries/states). Countries are sorted based on the mean annual area of soils with an EC_e ≥4 dS⋅m⁻¹ or ESP ≥6% between 1980 and 2018, largest to smallest.