Coupled Soil Moisture, Temperature And Salinity Dynamics In Salined Land During Seasonal Freezing-thawing Period

Seasonal freezing-thawing process is the main driving force for soil salinization in arid areas. It is often involved with soil moisture, temperature, spatial and temporal salinity changes. Therefore, the freezing-thawing process is an important theoretical premise for the study on the characteristics of soil salinization and its mechanism. Previous studies on the coupled relationship between soil moisture, temperature and salinity in seasonal freezing and thawing process are often focused on the role of a single or double factor. Spatial and temporal coupling between soial moisture, temperature and salinity remains to be studies. In this study, we selected salinized abandoned farmland as the test cites and collected soil samples from 2009 to 2010. To test the mechanism of salinity migration under different land cover types, we set three treatments:Bare Plot (BP), Plastic Mulching Plot (PMP) and Straw Mulching Plot (SMP). For each plot, we did three duplications. By intact soil temperature testing and physical and chemical analysis of soil samples, we used traditional statistical analysis and multi-factor correlation analysis to reveal temperature dynamics of salinized soil profile and mechanism of soil water and salinity migration. Further, the coupled relationship between soil moisture, temperature and salinity will also be explored. The main results of this study are as follows:1. Plastic film and straw mulching can retard heat transfer rate in vertical soil layers and thus reduce changes in soil temperature. Soil temperature significantly changes from 0 to 50cm. With increase in soil depth, the changing magnitude of soil temperature gets smaller. In freezing period, plastic film and straw mulching can help keep soil temperature. The depth of freezing soil under different land cover types ranks as BP>SMP>PMP. The thawing period is short and lasts for 7 days, however, for different land cover types, the thaw begins at different time. In thawing period, straw mulching reduces soil temperature and plastic film mulching increase soil temperature. Soil temperature under plastic film mulching is always higher than that under the other two land cover types. The sequence of soil temperature ranks as PMP>SMP>BP. 2. The spatial distribution patterns of soil moisture in different treatments are similar. Soil moisture changes significantly from 0 to 10cm. With increase in soil depth, the changing magnitude of soil moisture gets smaller. In the whole freezing and thawing period, soil moisture changes from high to low then back to high from the top to the deeper layer. In freezing period, with the decrease in air temperature, the surface soil begins to freeze thus soil moisture begin to increase continuously within the thicker frozen layer, and the moisture decreases in lower soil layers. In thawing period, soil moisture decreases in the upper soil layers and increased in the lower soil layers. After thawing process, soil moisture under different land cover types ranks as PMP>SMP>BP.3. The salinity in soil profiles changes from high to low then back to high from the top to the deeper layer. In the core soil layer, there is almost no accumulation of salt. The salinity in upper soil layers changes frequently and the changing magnitude is in the range of 12 ms/cm to 3 ms/cm. In freezing period, temperature gradient in soil profile makes underground water move into the frozen layer and the salinity in the water also aggregates in the layer. In thawing period, the detained water in the frozen layer and melting snow leach the salinity in the upper soil layers. Thus, the salinity in the upper layers decreases. In both freezing and thawing period, the salinity in the layer of 0-40 cm is the minimum under straw mulching. This suggests that straw mulching can best restrain salinization in the upper soil layers.4. By multi-factor correlation analysis, the significant correlation between soil moisture, temperature and salinity can only be observed in the upper soil layers. In the layer of 10-20 cm, soil temperature is significantly negative correlated with soil salinity and soil moisture is significantly positive correlated with soil salinity. In the layers lower than 20 cm, no significant correlation between soil temperature, moisture and salinity can be found.5. By Redundancy Analysis, we find that CO32- and HCO3- are significantly correlated with soil salinity. CO32- and HCO3- greater impact on the salt ions Mg2+, SO42" has little effect on the salt. Na+, K+, Ca2+, and the relevance of higher salinity, and a positive correlation. Mg2+ and SO42-have small impacts on soil salinity, however, Na+, K+ and Ca2+ have large impacts on soil salinity.Finally, coupled soil moisture, temperature and salinity dynamics during freezing and thawing period has great spatial and temporal heterogeneity. Plastic film mulching can help keep soil temperature but can make soil aggregation in the upper layers. Although staw mulching can not best keep soil temperature, it can best restrain soil salinization.