Response of soil compaction to the seasonal freezing-thawing process and the key controlling factors

The seasonal freezing-thawing process can alleviate saline farmland soil compaction, and improve soil aeration and soil water retention. However, the response of soil compaction to the seasonal freezing-thawing process and the key factors affecting soil compaction are still unclear. Therefore, in this study, we conducted a regional monitoring of soil compaction and other soil physical and chemical properties before and after the 2016-2017 seasonal freezing-thawing period in Yongji Irrigation Area, Hetao Irrigation District, North China. We used a combination of genetic algorithm-projection pursuit evaluation, grey relational analysis and stepwise regression to analyze the measured soil physical and chemical properties and their variations during the freezing-thawing period. Using this method combination, we obtained one projection value to represent the whole-profile soil compaction, and found the most important influencing soil physical and chemical properties-soil water content, groundwater level and total inorganic nitrogen. Ultimately, we developed a regression model to predict soil compaction given the values of the predictive variables (soil water content, groundwater level and total inorganic nitrogen) at the same depth and time. The importance of autumn irrigation and planting structure, as well as the spatial heterogeneity of field management regime on the relationship between freezing-thawing processes and soil compaction were highlighted. Our findings provide solid foundations for regional field management policies, especially irrigation and drainage policies. Future studies are recommended to focus on the effect of different irrigation regimes, different drainage regimes and different nitrogen fertilizer application regimes on the relationship between the seasonal freezing-thawing processes and saline farmland soil compaction.