Forage yield, water use efficiency, and soil fertility response to alfalfa growing age in the semiarid Loess Plateau of China

Water availability is a major constraint for crop production in the Loess Plateau of China; thus, improving water use efficiency (WUE) is a main research target. The impact of alfalfa (Medicago sativa L.) growing age on forage yield, WUE, soil water storage, and soil fertility were examined in a 15-yr study in the western Loess Plateau of China. The results of this study indicate that long-term continuous production of alfalfa aggravated the soil water deficit and reduced soil available phosphorus (AP) and the ratio of soil organic carbon to soil total nitrogen, but increased soil organic carbon, soil total nitrogen, and the ratios of soil organic carbon to soil available phosphorus and soil total nitrogen to soil total phosphorus. After the ninth year of alfalfa production, the greatest soil water storage deficit degree was found in the 0–80 cm soil layer (63 %), followed by the 200–300 (57 %) and 80–200 cm (55 %) soil layers. Forage yield and WUE of alfalfa reached their greatest values following six- to nine-yr-old stands, and then decreased gradually. These results suggest that the yield decline with long-term stands of alfalfa may be related to reductions in soil water storage and AP, and their interaction, and the optimal alfalfa growing age should not extend beyond nine years in semiarid conditions.