Seasonal climate modulates water and nitrogen use efficiency and biodiversity contributions to yield stability in cereal/legume forage intercropping

Increasing climatic variability and water scarcity threaten forage yield stability, yet how cereal/legume intercropping stabilizes yields under contrasting seasonal conditions remains unclear. A 4-year-long field experiment (2022–2025) was conducted with five cropping systems, including oat ( Avena sativa L.) monoculture (SO), common vetch ( Vicia sativa L.) monoculture (SC), oat/common vetch intercropping (OC), hairy vetch ( Vicia villosa L.) monoculture (SH), and oat/hairy vetch intercropping (OH), under spring and summer sowing seasons. Forage productivity and yield stability were evaluated, and the relative contributions of water productivity (WP), nitrogen use efficiency (NUE), and biodiversity effects to yield stability were quantified. Compared with monocultures, intercropping improved forage productivity and resource use efficiency while reducing CV by 23.6%, indicating greater yield stability. OC showed the strongest overall performance, increasing DMY by 87.4% and CPY by 51.1% relative to the mean of the corresponding monocultures, while achieving the highest WP DMY (32.9 kg ha −1 mm −1 ) and nitrogen uptake (179.3 kg ha −1 ). Summer sowing further enhanced productivity and stability, particularly in intercropping systems. The dominant drivers of yield stability shifted from water productivity under spring sowing to biodiversity effects under summer sowing, with complementarity and selection effects becoming increasingly important. Variance partitioning revealed that shared effects among complementarity effects, selection effects, water productivity, and NUE accounted for more than 80% of the explained variation in yield stability. Overall, the findings support the proposed hypotheses that intercropping can enhance forage yield stability and that the relative importance of its underlying mechanisms varies with sowing season conditions.