Mechanism of Soil Sodification at the Local Scale in Songnen Plain, Northeast China, as Affected by Shallow Groundwater Table

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Abstract
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.

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