Root traits with team benefits: understanding belowground interactions in intercropping systems

Z Homulle,A J Karley,T S George

doi:10.1007/s11104-021-05165-8

Abstract

BackgroundThe potential benefits of intercropping are manifold and have been repeatedly demonstrated. Intercropping has the potential to create more productive and resilient agroecosystems, by improving land utilisation, yield and yield stability, soil quality, and pest, disease and weed suppression. Despite these potential benefits, significant gaps remain in the understanding of ecological mechanisms that govern the outcomes when crop species are grown together. A major part of plant-plant interactions takes place belowground and these are often overlooked.ScopeThis review synthesises current evidence for belowground plant-plant interactions of competition, niche differentiation and facilitation, with the aim of identifying root traits that influence the processes contributing to enhanced performance of intercrops compared with monocultures. We identify a suite of potentially complementary root traits for maximising the benefits of intercropping. These traits underpin improved soil exploration, more efficient resource use, and suppression of soil-borne pathogens and pests in intercrops.ConclusionThis review brings together understanding of the mechanisms underpinning interactions between intercropped roots, and how root traits and their plasticity can promote positive outcomes. Root trait ‘ideotypes’ for intercropped partners are identified that could be selected for crop improvement. We highlight the importance of examining belowground interactions and consider both spatial and temporal distribution of roots and rhizosphere mechanisms that aid complementarity through niche differentiation and facilitation. Breeding of crop ideotypes with specific beneficial root traits, combined with considerations for optimal spatio-temporal arrangement and ratios of component crops, are essential next steps to promote the adoption of intercropping as a sustainable farming practice.

Highlights

Prior to the “fossilization” of agriculture with chemical fertilizers and pesticides, cultivation of crop mixtures was a common agricultural practice (Mikić et al 2015)
We focus on identifying root traits that promote better use of available niche space and/or facilitative processes that enhance resource capture as the desirable goals for intercrop improvement, but we highlight studies revealing root traits that might drive selection effects in high yielding intercrops
Root interactions between rapeseed (Brassica napus) and Chinese milk vetch (Astragalus sinicus) facilitated P uptake (c. 30% increase) and root growth (c. 25% increase) in rapeseed grown with three plants of Chinese milk vetch (Zhang et al 2019a)

Summary

Introduction

Prior to the “fossilization” of agriculture with chemical fertilizers and pesticides, cultivation of crop mixtures was a common agricultural practice (Mikić et al 2015). Intercropping, referred to as mixed cropping, companion cropping or polyculture, is the agroecological practice of cultivating two or more crops simultaneously. In rural sub-Saharan Africa, intercropping is a common practice aimed at minimizing risks associated with monocultures, with the predominant crop combinations being maize, bean/cowpea and pumpkin (Bedoussac et al 2018). Intercropping has the potential to create more productive and resilient agroecosystems, by improving land utilisation, yield and yield stability, soil quality, and pest, disease and weed suppression. Despite these potential benefits, significant gaps remain in the understanding of ecological mechanisms that govern the outcomes when crop species are grown together. A major part of plant-plant interactions takes place belowground and these are often overlooked.

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Conclusion