Deep soil water deficit and recovery in alfalfa fields of the Loess Plateau of China

Alfalfa (Medicago sativa L.) is the most widely planted forage crop on the semiarid Loess Plateau of China. It has a deep rooting system and high evapotranspiration rate, and how water content in deep soil is affected by continuous alfalfa planting and how soil water recovery responds to the conversion of alfalfa to annual crops are not fully understood. In this study, we conducted a meta‐analysis of soil water content data taken from 45 published studies conducted at 21 sites of the Loess Plateau to evaluate the effects of continuous alfalfa planting and the conversion of alfalfa to annual crops on soil water conditions in the 0−1500 cm profile. Response size (RS), the relative change in soil water content after initiating and stopping alfalfa compared to the long-term cropland, was calculated. The results show that the RS values in all soil layers from 100 to 1400 cm were significantly lower than zero (P < 0.05). RS values in soil layers below 200 cm linearly increased with increasing local rainfall (P < 0.05). The RS decreased with increasing alfalfa crop age, with average values of -0.143, -0.270, and -0.356 found for 0–5-, 5–10- and >10-year-old alfalfa fields, respectively. After converting alfalfa into annual crops for 1–28 years, the soil water content in the 0−300 cm layer was recovered, but that in the 300−1000 cm layer was still significantly lower than that in the reference field (P < 0.05) with an average RS value of -0.28. The relationship between RS and recovery years can be fit to linear equations (P < 0.001), and equations show that 18 and 23 years were needed to fully recover soil water depletion in the 200−500 and 500−1000 cm layers, respectively. The results of this study indicate that the long-term cultivation of alfalfa can severely reduce soil water content to a depth of 1500 cm and that the water deficit is difficult to recover. Short-term alfalfa and annual crop rotation is recommended to be applied to reduce the risk of deep soil desiccation.