Response of soil moisture to the age of alfalfa standing and subsequent spring wheat in the rainfed Loess Plateau

Spring wheat (Triticum aestivum) is the main cereal crop in the rainfed agricultural area of the Loess Plateau. Alfalfa (Medicago sativa) is also widely cultivated in this area. Cropping systems are dominated by continuous spring wheat in the north-western Loess Plateau of China, and the inclusion of alfalfa in cereal-based cropping systems is a traditional practice as well. Continuous production of alfalfa can form a dry soil layer and have a serious impact on the growth of subsequent crops. The aim of this study was to discuss the soil moisture characteristics of the 0 – 300 cm soil layer on land with different standing years of alfalfa and the alfalfa-spring wheat rotation system on the western Loess Plateau and to determine the soil moisture response to alfalfa standing years and succession spring wheat over an 8-year (2012 – 2019) field experiment on a 9-year-old alfalfa pasture. The results showed that the soil water content in the 50 – 110 cm soil layer was 14.17% in 1-year-old alfalfa and began to present light desiccation. The soil moisture in the 110 – 200 cm soil layer was 12.39% after 3-year-old alfalfa and showed light desiccation. Furthermore, the soil moisture in the 200 – 300 cm soil layer was 13.50% in the third year of alfalfa planting, which was lower than that under withering humidity and was severely desiccated. After the 9-year-old alfalfa was removed, the moisture of the desiccated soil layers recharged significantly over the rotation year. The soil moisture in the 50 – 110 cm soil layer fully recharged after five years of rotation, and the soil moisture in the 110 – 200 cm soil layer was fully recovered after six years of rotation. The soil moisture in the 200 – 300 cm soil layer after seven years of spring wheat rotation was only 15.11%, which could not be fully recovered. In conclusion, the soil profile was very dry after long-term continuous alfalfa production, especially the soil layer below 200 cm which was severely desiccated. Therefore, to maintain high land productivity and improve soil water storage conditions, crop rotation is needed after long-term continuous alfalfa production in the semiarid Loess Plateau of China.