Impacts of long-term abandonment of alfalfa plantations on soil physicochemical properties and plant diversity in an agricultural pastoral ecotone

Background and aims The abandonment of alfalfa fields is increasing in a semi-arid region. To determine the change of soil physicochemical properties and plant diversity in abandoned alfalfa fields for a long-term is essential for vegetation restoration. This study surveyed alfalfa fields of different planting ages in 2003 (1–30 years), 2014 (12–41 years), and 2022 (20–49 years) to explore the change in various indicators using a space-for-time method. Results The results suggested that abandoned alfalfa fields converted into natural grasslands after 15 years, where the abundances of Gramineae and Asteraceae (59.1–84.0%) increased, whereas those of legumes decreased (16.5–9.4%). As the plant composition changed, the soil water content (SWC) decreased by 19.0% 1–15 years after abandonment, but increased by 54.5% 15–49 years after abandonment. The plant diversity (Margalef, Pielou, Simpson, and Shannon–Wiener indices), and soil organic carbon (SOC), and total nitrogen (TN) contents increased by 0.5% to 51.5% with the number of years alfalfa planting. Furthermore, the plant composition significantly affected the SWC through the soil physicochemical characteristics and plant diversity. Conclusions This study demonstrated that the longest time of alfalfa planting was 15 years. Furthermore, the plant composition changed from alfalfa to local grasses due to the natural succession of abandoned alfalfa fields, with increases in SWC, SOC, TN, and plant diversity in the semi-arid region.