Close Encounters

Improving overyielding in legume-based mixtures

A major challenge when supporting the development of intercropping systems remains the design of efficient species mixtures. The ecological processes that sustain overyielding of legume-based mixtures compared to pure crops are well known, but their links to plant traits remain to be unravelled. A common assumption is that enhancing trait divergence among species for resource acquisition when assembling plant mixtures should increase species complementarity and improve community performance.

Intercropping coconut and Tagetes erecta flowers in Kerala, India. Image:Ezhuttukari / Wikipedia

Taking Faverjon et al. (2019a) as the starting point, Gaëtan Louarn and colleagues used a modelling approach to investigate the determinants of overyielding in legume-based forage mixtures. The first aim was to identify the principal plant parameters (i.e. plant traits) involved in carbon (C) and nitrogen (N) economy that control total above-ground biomass production and OY in such binary mixtures (which major traits, and do they differ from those in sole crops?). The second aim was to determine whether ‘trait divergence’ for resource acquisition (C and N) was indeed a relevant proxy to maximize species complementarity and overyielding (which trait variations and which combinations between legume and non-legume species?). To achieve this, the ‘Virtual Grassland’ model was used to simulate virtual competition experiments in theoretical binary mixtures with and without legumes, under three N management scenarios. The model considered plant competition for light and mineral N, and the facilitation effects caused by legumes through N released into the soil, all previously identified as being critical processes that affect the dynamics of legume-based mixtures. Model outputs relative to forage production, overyielding and relative species abundance were analysed with respect to the different scenarios and parameter combinations tested.

Maximal overyielding was achieved in cases where trait values were divergent for the physiological functions controlling N acquisition and temporal development but convergent for light interception. It was also found that trait divergence should not affect competitive abilities of legume and non-legumes at random. However, no simple assembly rule based on trait divergence could be confirmed. Plant models able to infer plant–plant interactions can be helpful for the identification of major interaction traits and the definition of ideotypes adapted to a targeted intercropping system.

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