Forage yield, soil water depletion, shoot nitrogen and phosphorus uptake and concentration, of young and old stands of alfalfa in response to nitrogen and phosphorus fertilisation in a semiarid environment

•N and P fertilisation increased forage yield and water use efficiency of alfalfa by more than 100% for both young and old stands in the semiarid region of the Loess Plateau, China.•P fertilisation increased shoot P concentration in both young and old alfalfa stands but N fertilisation had no effect on N concentration in shoots.•Shoot N:P ratios of more than 17 were associated with low forage yield and indicates that P application is required to boost yield.