Interaction effects of plant density and nitrogen rate on the growth and physiological traits of alfalfa in saline alkali soils

Field experiments were conducted to study the effect of nitrogen rate and planting density on the growth and physiological traits of alfalfa WL919 grown in saline soil. The effects of three planting densities (D₁ = 15.0 kg·ha⁻¹, D₂ = 30.0 kg·ha⁻¹, and D₃ = 45.0 kg·ha⁻¹) and three nitrogen rates (N₁ = 150.0 kg·ha⁻¹, N₂ = 225.0 kg·ha⁻¹, and N₃ = 300.0 kg·ha⁻¹) were investigated in this study to provide technical support for high-yield cultivation of alfalfa in saline-alkali soils. The results showed that 1) the plant height and dry biomass yield of alfalfa increased at first and then decreased with the increase in planting density. They reached their highest at medium density (D₂) and then decreased, but they always increased with the increase in nitrogen application rate. 2) The results for the interactions between planting density and nitrogen application rate showed that alfalfa plant height and hay yield were both optimal under the medium-density and medium-nitrogen (D₂N₂) treatment. 3) Alfalfa is at the budding stage and the initial flowering stage at 120 days after sowing. At this time, the forage quality of alfalfa is optimal and the hay yield is 11 057.2 kg·ha⁻¹, which means that it is the most suitable harvest time. 4) The malondialdehyde content was the lowest at 60 d after sowing. At this time, the nitrogen application rate had no significant effect on the malondialdehyde content (P > 0.05). At 120 d after sowing, the D₃N₂ combination had the lowest malondialdehyde content. 5) The proline content first increased and then decreased as the planting density and nitrogen application rate increased. In general, superoxide dismutase activity increased as the planting density and nitrogen application rate rose, and the maximum value was showed under D₂N₂ treatment. Peroxidase activity was higher under the medium density (D₂) treatment and under the D₂N₂ treatment. Oxidase activity was optimal, whereas catalase activity increased at first and then decreased with the increase in nitrogen application at the D₂ and D₃ densities. Catalase reached its optimal activity under medium nitrogen (N₂). The results for the interaction between planting density and nitrogen application rate showed that the growth and physiological characteristics of alfalfa in saline soil under the medium density and medium nitrogen (D₂N₂) treatment were optimal.