Effects of nitrogen fertilizer and sowing ratio of oat-pea mixed planting grassland on forage production performance and soil nutrients in Horqin Sandy Land

To determine the nitrogen fertilizer application rate and sowing ratio of oat-pea mixed planting grassland suitable for Horqin Sandy Land, four nitrogen fertilizer (pure N) application levels at 0, 70, 140, and 210 kg·hm⁻² (represented by N₀, N₇₀, N₁₄₀, and N₂₁₀, respectively) were used. Avena sativa L. and Pisum sativa L. were mixed at 100 ꞉ 0, 80 ꞉ 20, 70 ꞉ 30, 60 ꞉ 40, 50 ꞉ 50, and 0 ꞉ 100 (represented by W₀, W₂₀, W₃₀, W₄₀, W₅₀, and W₁₀₀, respectively), and six single and mixed planting ratios were used in the field in a two-factor randomized block experiment. The effects of nitrogen fertilizer application rate, planting ratio, and their interaction on forage production performance and soil physicochemical properties of oat-pea mixed planting grassland were analyzed. The results showed that forage yield, crude protein (CP) content, and relative feeding value (RFV) showed an increasing trend with the increase of nitrogen fertilizer application. The N₂₁₀ treatment increased by 48.24%, 0.76%, and 17.48%, respectively compared to N₀; neutral detergent fiber (NDF) and acid detergent fiber (ADF) showed a decreasing trend as the nitrogen fertilizer application increased, with N₂₁₀ decreasing by 3.62% and 9.16%, respectively compared to N0. As the mixed sowing ratio of forage peas increased, the forage yield first increased and then decreased. CP and RFV showed an upward trend and reached their maximum values under the W50 mixed sowing ratio, with significant differences (P < 0.05) compared to the W0 treatment. NDF and ADF showed a decreasing trend with the increase of mixed sowing ratio. Forage yield was highly significantly different (P < 0.01) when there was an interaction between nitrogen fertilizer and mixed planting ratio treatment. With the increase of nitrogen fertilizer application, the soil organic matter and total nitrogen contents showed an upward trend. Under N₂₁₀ treatment, the organic matter and total nitrogen content increased by 92.98% and 24.08% compared to the control, respectively, whereas the available nutrients and total phosphorus and potassium contents first increased and then decreased. The organic matter, total nitrogen, available potassium, and total potassium content of mixed grassland first increased and then decreased as the mixed sowing ratio of forage peas increased. The contents of organic matter, total nitrogen, and available potassium in W40 treatment were significantly higher than those in single sowing treatments (P < 0.05). In summary, it is suggested that the appropriate planting ratio of oats and fodder peas in the Horqin Sandy Land is 60:40, and the nitrogen application rate should be 210 kg·hm⁻².