Advances and expectations of researches on saline soil reclamation by freezing saline water irrigation

The use of freezing saline water to irrigate saline lands has proven an effective method of using highly saline water and reclaiming saline lands in coastal regions. The method was based on the basic principle of desalination during melting of frozen saline water in combination with soil water and salt movement characteristics and crop growth pattern in coastal region. In winter, the saline groundwater was pumped and irrigated saline farmlands. The low air temperature forced the irrigated saline water on the top soil to freeze into saline ice. With increasing air temperature in spring, water of high salt concentration melted firstly and infiltrated into the soil, and the slightly saline melting water and freshwater infiltrated into the soil late and effectively facilitated leaching of soil salt. From the above process, freezing saline water irrigation induced soil salt leaching in spring, the period of soil salt accumulation. This, in combination with mulching in spring to control soil salt concentration and rainfall leaching in summer, lowed soil salt content to levels conducive for normal crop growth throughout the growth stages of the crops and plants. The natural characteristics of soil water and salt movement were modified by freezing saline water irrigation, which changed soil salt accumulation into soil salt leaching in spring after irrigation. Thus the remaining soil salinity in the root zone effectively decreased from 12 g.kg^-1 to 4 g.kg^-1 and the rate of salt leaching exceeded 66%. This facilitated the cultivation of crops including cotton, oil-sunflower and sugar beet in saline coastal regions of the Bohai Sea, and increased the survival rates of Tamarix ramosissima, Lycium barbarum and Fraxinus chinensis transplanted seedlings in the region. After first year freezing saline water irrigation, the yields of seed cotton, oil-sunflower and sugar beet were 3 t.hm^-2, 1.5 t.hm^-2 and 60 t.hm^-2, respectively. Cutting and transplanted seedling survival rates of halophytes and salt-tolerant plants exceeded 90%. Freezing saline water irrigation promoted saline soil exploiting, agricultural development and ecological environmental construction. Through systematic researches in recent years, the separation process of saline water and freshwater was clarified in the process of saline water freezing and thawing. The effects of freezing saline water irrigation on soil salt leaching were explicated, and the indexes system of irrigation time, irrigation amount and water quality of freezing saline water irrigation was established. Based on the above researches, this paper summarized the advances in researches on saline soil reclamation and saline water use, and introduced the freezing saline water irrigation strategy which enhanced leaching of soil salt. The paper further systematically analyzed the effect of freezing saline water irrigation on agricultural production, vegetation recovery and saline water utilization in saline soil regions, and the development trend of freezing saline water irrigation.