Species interactions in a grassland mixture under low nitrogen fertilization and two cutting frequencies: I. dry-matter yield and dynamics of species composition

Four-species mixtures and pure stands of perennial ryegrass, tall fescue, white clover and red clover were grown in three-cut and five-cut systems at Ås, southern Norway, at a low fertilization rate (100 kg N ha−1 year−1). Over a three-year experiment, we found strong positive effects of species diversity on annual dry-matter yield and yield stability under both cutting frequencies. The overyielding in mixtures relative to pure stands was highest in the five-cut system and in the second year. Among the possible pairwise species interaction effects contributing to the diversity effect, the grass–grass interaction was the strongest, being significant in both cutting systems and in all years. The grass–legume interactions were sometimes significant, but no significant legume–legume interaction could be detected. Competitive relationships between species varied from year to year and also between cutting systems. Estimations based on species identity effects and pair-specific interactions suggested that the optimal proportions of red clover, white clover, perennial ryegrass and tall fescue in seed mixtures would have been around 0·1, 0·2, 0·4 and 0·3 in the three-cut system, and 0·1, 0·3, 0·3 and 0·3 in the five-cut system