Effects of different mixed sowing ratios on soil nitrogen substitution potential in mixed-sowing grassland with Vicia sativa + Avena sativa

We aimed to determine how different sowing ratios of Vicia sativa can reduce the need for nitrogen application on Avena sativa grassland. As test factors, we used a one-way randomized block design of pots and fields with nine different mixed bean and grass planting ratios and five different nitrogen administration amounts. To correctly evaluate the nitrogen substitution effect of different mixed planting ratios of arrow pea on the yield of oat grassland, we investigated the nitrogen fixation and transfer capability of the mixed bean-grass planting system. The findings revealed the following: 1) With increasing mix ratio, both the nitrogen transfer rate (P_Nt) and amount of nitrogen transfer (W_Nt) of Vicia sativa first decreased, then increased, and finally decreased. 2) The nitrogen fixation, transfer, and transfer rate were all positively correlated (P < 0.05) with the nitrogenase activity of arrow pea, which showed a substantially positive link with the severity of arbuscular mycorrhizal fungal infection. 3) In the plot test, more than 70% of the applied amount of nitrogen fertilizer was replaced; In the field experiment, more than 60% of the applied amount of nitrogen fertilizer was replaced. 4) The nitrogen substitution potential was significantly higher when the mixed sowing ratios of arrow pea and oat were 12.5 ꞉ 87.5, 25 ꞉ 75, and 50 ꞉ 50 than when the ratios of 62.5 ꞉ 37.5, 75 ꞉ 25, and 87.5 ꞉ 12.5 were used. According to the results of the comprehensive analysis, a mixing ratio of 12.5 ꞉ 87.5 was optimal in the mixed soybean and grass cropping system, which maximized the nitrogen substitution potential. The mixed planting of arrow pea and oat could replace 40.17% to 104.86% of the need for nitrogen fertilizer, so has a high nitrogen substitution potential to achieve nitrogen application reduction and increase the efficiency of mixed bean-grass sowing on grassland.