Effect of sowing ratio on interspecific relationships between species and grassland productivity in legume-grass mixtures

In this study, four common legumes and grasses, Dactylis glomerata, Lolium perenne, Trifolium repens, and T. pratense, were selected to establish legume-grass mixture with their monocultures i.e., DG, LP, BP, and TP, respectively, and three sowing ratios 5 ꞉ 5 (L₅G₅), 4 ꞉ 6 (L₄G₆), 3 ꞉ 7 (L₃G₇); forming a total of seven treatments. ¹⁵N was used as a tracer to determine the effect of AM fungi on nutrient uptake from shallow (3 cm) and deep (30 cm) soil layers in both mixtures and monoculture treatments. The relative yield total, competitive intensity, land equivalent ratio, overyielding, complementarity, and selection effects, plant nitrogen and phosphorus content, the amount and rate of nitrogen fixation were determined to assess the production performance and vertical niche nutrient uptake strategy of legume-grass mixtures, and reveal the maintenance mechanism of high-yield and stable legume-grass mixture. The results showed that, compared with monocultures, the four plant species in legume-grass mixtures had significant overyielding advantage, and the L₅G₅ treatment had the greatest shoot biomass, relative yield total, land equivalent ratio, overyielding effect, and plant total phosphorus content. The higher productivity in the legume-grass mixtures was mainly affected by the complementarity effect. Compared to monocultures, the ¹⁵N uptake of plants and the amount of nitrogen fixation in the legume-grass mixtures were significantly increased (P < 0.05). The nitrogen absorption and fixation amount from the shallow 3 cm soil were significantly higher than those from the deep 30 cm soil ( P < 0.05). The results of this study provide a theoretical basis and technical support for the productivity improvement of perennial mixed grassland in southwest China.