Effects of irrigation quantity and nitrogen application rate on Elymus nutans biomass and its components

The interaction between irrigation quantity and nitrogen application rate is one of the important ways of improving forage biomass and the efficiency of natural resources use. A pot experiment was performed to investigate the effects of different nitrogen application rates (0, 200, 400 mg·kg^–1) and irrigation quantities (35%～40%, 60%～65%, 75%～80% of saturated soil moisture) on Elymus nutans biomass and its components. This study showed that plant height, tillers per plant, and the ratio of aboveground biomass to underground biomass increased significantly with the increasing nitrogen application rate (P < 0.05), whereas aboveground biomass first increased and then decreased. However, nitrogen application rate had no significant effect on root volume or root biomass(P > 0.05). Plant height, tillers per plant, aboveground biomass, root volume, and root biomass of E. nutans also increased significantly and the ratio of aboveground biomass to underground biomass first increased and then decreased, as irrigation quantity increased (P < 0.05). The interaction between irrigation quantity and nitrogen application rate increased and then decreased plant height, tiller number, aboveground biomass and the ratio of aboveground biomass to underground biomass, as indicated by 3D response surface plots, in which the aforementioned parameters reached their highest values when the nitrogen application rate was 200 mg·kg^–1 and the irrigation quantity was 60%～65%. These results suggested that the interaction between irrigation quantity and nitrogen application rate had a synergistically positive effect on E. nutans biomass and its composition, with an optimal theoretical combination pattern.