Effects of mixing forage species contrasting in vertical and temporal nutrient capture on nutrient yields and fertilizer recovery in productive grasslands

Mixing grasses with N-2-fixing legumes benefits grassland yields. We examined if grass/forb/legume mixtures combining species with complementary vertical and temporal nutrient capture further promote nutrient yields and improve fertilizer N recovery. Four species differing in N-2-fixing ability and in vertical and temporal nutrient capture were grown in monoculture and two- and four-species mixtures. N-15 was used to examine vertical complementarity in nutrient capture and fertilizer N recovery. Asynchrony was quantified from the N yield distribution over the year. The positive mixture effects on N and P yields (P < 0.001) between species contrasting in vertical and temporal nutrient capture (with same N-2-fixing ability) were of a similar magnitude (+45 kg N) as those between N-2-fixing and non-fixing species with shallow roots (+52 kg N). Mixture benefits were maximized up to +99 kg N when constituent species differed in N-2-fixing ability and vertical and temporal nutrient capture. Species modified their vertical N capture when grown in mixture (niche shift). Remarkably, mixing Trifolium pratense and Cichorium intybus, which were similar in both vertical and temporal nutrient capture, generated negative mixture effects. The 4-species mixture captured as much fertilizer N as the grass monoculture, despite significant abundance of legumes. Combining species differing in multiple characteristics for nutrient capture optimizes nutrient capture of non-legume/legume-mixtures.