Nonlinear response of lucerne (Medicago sativa) biomass and biological nitrogen fixation to different irrigations and sowing modes

It remains unclear how the biomass and biological nitrogen (N) fixation by lucerne (Medicago sativa) respond to different irrigations and sowing modes in arid and semi-arid regions. To clarify the question, a field 15N isotope labeling experiment was conducted to evaluate the biological N fixation in pure and mixed lucerne (lucerne growing alone or lucerne growing with Elymus nutans (EN), Agropyron cristatum (AC) or Bromus inermis (BI)) under four water conditions: a control (CK) and three levels of irrigation (low (LI), medium (MI), and high (HI)) by the 15N dilution technique. The biomass of lucerne in pure and mixed lucerne conditions responded nonlinearly to different irrigations, and in mixed lucerne, the biomass varied with the specific coexisting plant species, with the maximum in CK for lucerne-EN, in LI for lucerne-AC, and in HI for lucerne-BI. The N derived from the atmosphere (%Ndfa) in mixed lucerne (90.8%) was higher than that in pure lucerne (88.3%). Biological N fixation by lucerne in mixed lucerne ranged from 2.86 to 15.05 g N m−2, and that in pure lucerne was approximately 10.99 g N m−2. A positive relation was observed between the competitive response and N fixation by lucerne, which showed that interspecific competition or facilitation in mixed lucerne altered N fixation by lucerne. These findings indicate that an appropriate legume-grass mixture with moderate irrigation can promote the biomass of forages and biological N fixation by legumes and further improve soil quality in artificial grasslands.