Abstract: The underlying mechanisms related to the impact of nitrogen fertilizer on alfalfa (Medicago sativa) growth at different soil water contents remain unclear. A pot experiment with three soil water levels (30%, 50% and 70% of the field water-holding capacity; W₁, W₂ and W₃), and four nitrogen application rates (0, 0.02, 0.04 and 0.06 g N·kg^-1; N₁, N₂, N₃ and N₄) was conducted to study the utilization of nitrogen fertilizer by alfalfa under different soil water regimes, and the relationships with nitrogen accumulation and root nitrogenase activity. The results indicated that: 1) for W₁, alfalfa biomass increased significantly with increase in nitrogen application rate, while for W₂ and W₃, the highest biomass values were observed in N₃ treatment; 2) for W₁ and W₂, the highest nitrogen accumulation was obtained in the N₄ treatment, while for W₃, the N₃ treatment had the highest nitrogen accumulation. A positive relationship between the amount of nitrogen applied and alfalfa biomass was therefore concluded; 3) the root nitrogenase activity was increased with increase in soil water availability, but decreased with increase in nitrogen rate. No significant difference in root nitrogenase activity was observed between W₃ and W₄ with N₄ application. Therefore, it was concluded from these results that under lower soil water content, nitrogen application was essential for meeting nitrogen demand and would promote alfalfa growth, and under higher soil water content, moderate nitrogen fertilizer application was advantageous for alfalfa growth, while higher rates of nitrogen fertilizer application inhibited root nitrogenase activity, and therefore no significant differences in nitrogen accumulation and biomass were found. The results of the present study provide a theoretical basis for determining water and nitrogen management in alfalfa production systems.

Key words: alfalfa, nitrogen application, soil water, nitrogenase activity, nitrogen accumulation