Over three million hectares of salt-affected soils characterized with high salinity and sodicity caused serious land degradation in Songnen Plain, northeast China. Soil salinity–sodicity heterogeneous distribution under microtopography is usually influenced by several environmental factors. The side direction movement of soil water driven by water from depression is the key factor that aggravates the soil salinization under microtopography in dry condition. In this study, the differences in surface soil salinity–sodicity (0–10 cm) between dry year and wet year were compared, and the relationship between soil salinity–sodicity and environment factors such as ground elevation, surface ponding time, surface ponding depth, and soil moisture at four soil layers (0–10, 10–30, 30–60, and 60–100 cm) were analyzed using redundancy analysis (RDA) and simple correlation analysis (Pearson analysis) for two different hydrological years. Analyzed soil salinity–sodicity parameters include soluble ions (Na⁺, K⁺, Ca²⁺, Mg²⁺, CO₃²⁻, HCO₃⁻, Cl⁻ and SO₄²⁻), salt content (SC), electrical conductivity (EC), sodium adsorption ratio (SAR), and pH. Results showed that values of SAR, Cl⁻, and SO₄²⁻ were significantly higher in dry year than in wet year, while Ca²⁺, Mg²⁺, K⁺, and HCO₃⁻ showed the opposite results. Values of Na⁺, CO₃²⁻, and EC were significantly higher at higher ground elevation gradient (20–40 cm) in dry year than wet year. Redundancy analysis indicated that spatial distributions and variations of salinity and sodicity in surface soil layer were related with environmental factors of ponding depth, ponding time and ground elevation in wet year, and they were related with ground elevation, ponding depth, ponding time, and soil moisture at 30–60 and 60–100 cm soil layer in dry year. Ponding depth and ground elevation rank first and second as the influential factors of the spatial distribution and variation of soil salinity–sodicity in wet year. However in dry year, primary and secondary influential factors are ground elevation and soil moisture at 60–100 cm soil layer.

The objectives of this study were to investigate the degree of spatial variability and variance structure of salinization parameters using classical and geostatistical method in Songnen Plain of China, which is one of largest saline-sodic areas in the World, and to analyze the relationship between salinization parameters, including soil salinity content (SC), electrical conductivity (EC), sodium adsorption ratio (SAR), and pH, and seven environmental factors by Pearson and stepwise regression analysis. The environmental factors were ground elevation, surface ponding time, surface ponding depth, and soil moistures at four layers (0–10 cm, 10–30 cm, 30–60 cm, and 60–100 cm). The results indicated that SC, EC, and SAR showed great variations, whereas pH exhibited low variations. Four salinization parameters showed strongly spatial autocorrelation resulting from the compound impact of structural factors. The empirical semivariograms in the four parameters could be simulated by spherical and exponential models. The spatial distributions of SC, EC, SAR and pH showed similar patterns, with the coexistence of high salinity and sodicity in the areas with high ground elevation. By Pearson analysis, the soil salinization parameters showed a significant positive relationship with ground elevation, but a negative correlation with surface ponding time, surface ponding depth, and soil moistures. Both correlation and stepwise regression analysis showed that ground elevation is the most important environmental factor for spatial variation of soil sanilization. The results from this research can provide some useful information for explaining mechanism of salinization process and utilization of saline-sodic soils in the Western Songnen Plain.

Soil sodification in Northeast China is controlled by many other factors besides groundwater level. This research aims to explore the mechanism of sodium buildup in sodic soil based on in-situ observation and laboratory analysis. Results indicate that water and sodium redistribution process on the soil surface at local scale was an additional factor leading to the rapid soil sodification development in Songnen Plain in recent decades. A field observation was conducted in a 35 × 35 m² area. Mollisols were distributed in depressions and free of natric horizon, while Halaquepts were found on micro slopes and high flats. A natric horizon was found at a depth of 15–30 cm in high flats, with an exchangeable sodium percent (ESP) value of 70. Fluctuation of groundwater table in the freezing period was related, to some extent, to the formation and development of frozen soil. During the freezing period, groundwater table was gradually lowered as a result of the groundwater moving into the frozen layer, leading to higher moisture content therein. During the thawing period, soil moisture content in the surface layer of Mollisols showed a significant enrichment (p < 0.05) from 0.27 to 0.69 m³ m⁻³(over-saturated). Salt in the surface of Halaquepts enriched quickly, especially in the micro slope, by about 80%. In comparison, insignificant change was observed in Mollisols. Sodic soil should be leveled fully and adequate drainage should be utilized before farming in order to prevent soil sodification. Blocking the capillary pore channels between topsoil and subsoil by plowing before freezing season would help soil desodification in high-latitude arid and semiarid regions.