Optimising grass-legume mixtures based on growth strategies for high N-yield and low N-loss in fertilised grasslands

Aims: Managed grasslands are important agro-ecosystems, consisting of grass monocultures with high nitrogen (N) fertiliser inputs. This management results in low N use efficiency and high N losses to the environment. Growing mixtures of plant species with diverse N acquisition strategies can reduce N losses and maintain high grassland productivity, yet determining the best mixture remains a challenge. The aim of this study was to investigate how grass-legume mixtures with contrasting growth strategies affect plant productivity, N use efficiency, N uptake, and soil mineral N, and how these effects depend on the N-fertilisation level. Methods: Two complementing field experiments were established: the first determined how monocultures and mixtures (two and four grass-legume mixtures) with contrasting growth strategies (fast- vs. slow-growing) affect productivity and N-cycling. The second determined the effect of fertilisation level on productivity and N-cycling in monocultures and two-species mixtures. Results: We found that productivity and N uptake of the four-species mixture was as high as the most productive monoculture and two-species mixtures. This was associated with an increase in legume N-fixation and high N use efficiency of the plant community. Fast-growing grass and legume combination increased productivity and reduced soil mineral N, thereby the risk of N loss for both N-fertilisation levels, while combining a fast-growing grass with a slow-growing legume promoted high legume N-fixation under low N-fertilisation. Conclusions: This study shows that productivity and N-cycling decreases via complementarity effects when growing mixtures of fast- and slow-growing grasses and a fast-growing legume at moderate level of N-fertilisation.