Crop yield and soil water restoration on 9-year-old alfalfa pasture in the semiarid Loess Plateau of China

The alfalfa pastureland in the semiarid Loess Plateau region of Northwest China usually has dry soil layers. A field experiment was conducted from October 2000 to October 2004 to examine soil water recovery and crop productivity on a 9-year-old alfalfa pasture. This experiment included six treatments: alfalfa pasture for 10–14 years, a conventional farming system without prior alfalfa planting, and four alfalfa–crop rotation treatments. For the rotation treatments, after 9 years of alfalfa selected crops were planted from 2001 to 2004 in the following sequence: (1) millet, spring wheat, potatoes, peas; (2) millet, corn, corn, spring wheat; (3) millet, potatoes, spring wheat, corn; (4) millet, fallow, peas, potatoes. The results showed that dry soil layers occurred in alfalfa pasture. We then plowed the alfalfa pasture and planted different crops. The soil water gradually increased during crop growth in the experimental period. The degree of soil water recovery in the four alfalfa–crop rotation treatments was derived from comparison with the soil water in the conventional system. After 4 years, the soil water recovery from the alfalfa–crop rotation systems at 0–500 cm soil depth was 90.5%, 89.8%, 92.2% and 96.7%, respectively. Soil total N content and soil respiration rate were high in the alfalfa–crop rotation systems. The yields of spring wheat in 2002, peas in 2003 and potatoes in 2004 in the alfalfa–crop rotation systems were not significantly different from yields in the conventional system. In the alfalfa–crop rotation systems, the yields of spring wheat and peas were greatly influenced by rainfall and were lowest in the dry year of 2004; the yields of corn and potatoes had a direct relationship with water use and were lowest in 2003. In summary, soil water in dry soil layers can recover, and crop yields in the alfalfa–crop systems were equal to those of the conventional system.