Abstract: Background, aim, and scope Soil salinization is one of the most important types of land degradation, which receives highly attention by scientists and governments around the world. During the past 50 years, China has achieved great success in controlling soil salinization, while it is still a serious problem of land degradation in many areas of China, e.g. the Yellow River Basin. The Chinese Loess Plateau (CLP) locates the middle reaches of the Yellow River, where is an area prone to soil salinization with low precipitation, high evaporation, and alkaline soil. The valley area on the CLP is the place where has high risk of soil salinization being high groundwater table and poor drainage system. However, the valley on the CLP has a large area of farmlands, e.g. the dammed farmlands formed through human and natural factors in the hilly-gully regions of the Northern Shaanxi Province. Currently, we lack a comprehensive understanding of the situation, mechanisms and prevention measures of soil salinization in these dammed farmlands in the valley of the loess hilly-gully regions, which has substantially constrained the knowledge demands of agricultural practice on the CLP. Materials and methods In this study, three kinds of dammed farmlands in the valley area of the Northern Shaanxi Province were selected to examine the situation and impact factors of soil salinization. The dammed farmlands included the man-made farmlands in the valley of the Gutun watershed, Yan’an City (formed through gully land consolidation), the semi-natural farmlands behind the fulfilled check dams in the valley of the Majiawan watershed, Yanchuan County, and the natural farmlands formed through the ancient landslide-dammed lake in the valley of the Huangtuwa watershed, Zizhou County. Soil samples were collected in the three kinds of farmlands, and soil electric conductivity (EC), pH value, soil moisture and soil texture were measured. Moreover, based on the measured soil moisture content in the profile, groundwater levels in the three kinds of farmlands were estimated through an empirical equation. Results According to the determined soil physical and chemical parameters and the estimated groundwater levels, the situation and impact factors of soil salinization in the three kinds of farmlands were evaluated. Results showed that: (1) soil salinization both occurred in the Gutun and Majiawan watersheds, and Majiawan showed the most serious situation of soil salinization among the three valley sites; (2) in the Guntun watershed, soil salinization occurred in the upper reaches of the valley; while in the Majiawan watershed, soil salinization extended to the middle reaches of the valley; (3) in the Huangtuwa watershed, no soil salinization was found in the valley farmlands. Discussion Majiawan watershed showed the most serious situation of soil salinization due to a shallow water table, where profile soil moisture demonstrated the highest average content (28.4%) among the three kinds of farmlands. Moreover, the estimated average water table depth was 1.23 m, which demonstrated a very shallow groundwater level. In the Guntun watershed, the average groundwater level was 2.85 m and the middle and lower reaches of the valley showed a relatively deep groundwater table; while in the upper reaches of the valley, the groundwater table was shallow and many farmlands showed a groundwater table less than 1 m . Therefore, obvious soil salinization occurred in the upper reaches of the valley in the Gutun watershed. The ancient landslide-dammed farmlands in the valley of the Huangtuwa watershed demonstrated no salinization mostly being a deep groundwater table. The Huangtuwa watershed locates in a high platform, and the estimated groundwater level was 4.77 m. Moreover, the profile soil moisture content showed a moderate level and demonstrated a decreasing trend with the increase of soil depth, which indicates that soil water is well drained at the bottom of the farmland. Therefore, no soil salinization occurred in the valley farmlands. Conclusions Shallow groundwater table was the critical reason of soil salinization in the dammed farmlands in the valley of the loess hilly-gully regions of the Northern Shaanxi Province. Recommendations and perspectives Strengthening the drainage system and suppressing the groundwater table are the core issues in deal with the problems of soil salinization in the dammed farmland in the valley of the Northern Shaanxi Province. The results of this study can provide scientific basis for soil salinization control on the CLP.
Keywords: gully land consolidation; check dam; ancient landslide-dammed lake; groundwater table; soil salinization; Chinese Loess Plateau