Helgadóttir et al. grew monocultures and mixtures of two grass and two clover species under different N-fertilizer levels in extreme growing conditions for five years. They find that mixtures yielded consistently more than their component species in monoculture, irrespective of species proportion in the mixture.
The results show that even in the less productive agricultural systems in the cool maritime regions grass–legume mixtures can contribute substantially and persistently to a more sustainable agriculture. Positive grass–legume interactions suggest that symbiotic N2 fixation is maintained even under these marginal conditions, provided that adapted species and cultivars are used.
However, it’s not just a case of ‘add clover to marginal land and everything’s fine’. Writing in Annals of Botany, the authors add: “…[M]aintaining legumes in the sward still remains a challenge in these environments. Suitable sward management strategies need to be found to stabilize the legume proportion, which may at first occur through adjusted N fertilizer application rates and harvesting regimes. It is also important to increase the persist- ence of legumes in the sward through breeding, and thus further improve the robustness of the system.”