Red and white clover sustain high and stable yields in diversified perennial grasslands under low N input

Increasing species richness and functional diversity in managed grasslands can enhance ecosystem services, particularly when legumes are included. However, it remains unclear whether higher species richness consistently improves productivity and stability in intensively managed grasslands under low nitrogen (N) inputs, or if species identity plays a more dominant role. This study evaluated how legume identity, non-legume identity, and species richness influence dry matter (DM) yield, N yield, and biological N₂ fixation (BNF) in grass–legume–forb mixtures managed with low fertilizer inputs. In a 3-year field experiment, we tested (i) 2-species mixtures of white clover ( Trifolium repens , Tr) or red clover ( Trifolium pratense , Tp) with perennial ryegrass ( Lolium perenne , Lp), tall fescue ( Festuca arundinacea , Fa), chicory ( Cichorium intybus , Ci) or ribwort plantain ( Plantago lanceolata , Pl); and (ii) multi-species mixtures with 6 and 18 species. Additional 2-species mixtures of ryegrass with six clover genotypes were included. Mixtures received 75 kg N ha⁻¹ yr⁻¹ and were compared with ryegrass monocultures fertilized with 0–450 kg N ha⁻¹ yr⁻¹ . BNF was quantified using the ¹ ⁵N isotope dilution method. Species identity, particularly the presence of red clover, was the primary driver of productivity, outweighing the effect of species richness. Red clover mixtures produced 14–21 t DM ha⁻¹ yr⁻¹ across three years, matching ryegrass fertilized with 450 kg N ha⁻¹ yr⁻¹ , while white clover 2-species mixtures yielded less (8–16 t DM ha⁻¹ yr⁻¹) but gained over time relative to red clover mixtures. N yields in red clover mixtures equaled or exceeded those of highly fertilized ryegrass, whereas white clover mixtures matched ryegrass at 300 kg N ha⁻¹ yr⁻¹ . BNF was greatest in red clover mixtures due to higher legume biomass, although the proportion of N derived from the atmosphere (%Ndfa) was similar or higher in white clover mixtures. Increasing species richness beyond two species did not increase yield when red clover was present, but the 18-species mixture maintained more stable legume–non-legume proportions and consistent yields across years. Red clover drives high productivity in low-input grassland mixtures, whereas higher species richness primarily contributes to temporal yield stability rather than increased yield. Red clover-based mixtures can replace high fertilizer N inputs while sustaining yields equivalent to intensively fertilized monocultures, offering substantial gains in N-use efficiency and reduced fertilizer-related emissions. Incorporating higher species richness may enhance long-term stability and resilience of managed grasslands, supporting more sustainable and climate-smart forage production systems. • Legume–based grasslands matched high N-input grass monoculture yield over 3 years. • Red clover identity, not species richness, drove high mixed grassland productivity. • White clover enhanced non-legume growth and long-term mixture balance. • High species richness maintained high yield and facilitated better species balance. • Red and white clover cultivar choice mattered for optimal community productivity.