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

Background The 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. Scope This 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. Conclusion This 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.

Effects of Different Planting Patterns on the Growth and Yield of Maize and Soybean in Northwest China

Aboveground and belowground interactions are crucial in the over-yielding of intercropping systems. However, the relative effects of aboveground and belowground interactions on yields in maize (Zea mays L.) and soybean (Glycine max) intercropping systems are still unclear. Field experiments, including measurements of plant height, soil-plant analysis development (SPAD) value, photosynthetically active radiation (PAR), root length density (RLD), root volume density (RVD), and grain yield, were conducted in 2018-2019 to analyze the advantages and effects of above-ground and belowground inter-species interactions. This study adopted three different planting patterns: mono-cropping maize (MM), mono-cropping soybeans (MS), and maize-soybean intercropping (IM and IS). This study showed that intercropping promotes the growth of maize and makes maize have a better photosynthetic environment, while the growth of intercropping soybeans is inhibited and the photosynthetic environment becomes worse. In the upper layer (0-40 cm) and close to the plants, the root growth and distribution of intercropped maize increased, resulting in greater root length density and volume density, while the root growth and distribution of intercropped soybean decreased, resulting in lower root length density and volume density. The intercropping increased the maize yield by 18.52-19.8%, and reduced the soybean yield by 55.87-57.44%. The results indicated that intercropping improves the competitiveness of maize and reduces the competitiveness of soybeans. The increase in maize yield made up for the loss of soybean yield and led to an overall significant advantage in the maize-soybean intercropping system.

Plant Hosts Modify Belowground Microbial Community Response to Extreme Drought

ABSTRACT Drought stress negatively impacts microbial activity, but the magnitude of stress responses is likely dependent on a diversity of belowground interactions. Populus trichocarpa individuals and no-plant bulk soils were exposed to extended drought (∼0.03% gravimetric water content [GWC] after 12 days), rewet, and a 12-day “recovery” period to determine the effects of plant presence in mediating soil microbiome stability to water stress. Plant metabolomic analyses indicated that drought exposure increased host investment in C and N metabolic pathways (amino acids, fatty acids, phenolic glycosides) regardless of recovery. Several metabolites positively correlated with root-associated microbial alpha-diversity, but not those of soil communities. Soil bacterial community composition shifted with P. trichocarpa presence and with drought relative to irrigated controls, whereas soil fungal composition shifted only with plant presence. However, root fungal communities strongly shifted with drought, whereas root bacterial communities changed to a lesser degree. The proportion of bacterial water-stress opportunistic operational taxonomic units (OTUs) (enriched counts in drought) was high (∼11%) at the end of drying phases and maintained after rewet and recovery phases in bulk soils, but it declined over time in soils with plants present. For root fungi, opportunistic OTUs were high at the end of recovery in drought treatments (∼17% abundance), although relatively not responsive in soils, particularly planted soils (<0.5% abundance for sensitive or opportunistic). These data indicate that plants modulate soil and root-associated microbial drought responses via tight plant-microbe linkages during extreme drought scenarios, but trajectories after extreme drought vary with plant habitat and microbial functional groups. IMPORTANCE Climate change causes significant alterations in precipitation and temperature regimes that are predicted to become more extreme throughout the next century. Microorganisms are important members within ecosystems, and how they respond to these changing abiotic stressors has large implications for the functioning of ecosystems, the recycling of nutrients, and the health of the aboveground plant community. Drought stress negatively impacts microbial activity, but the magnitude of this stress response may be dependent on above- and belowground interactions. This study demonstrates that beneficial associations between plants and microbes can enhance tolerance to abiotic stress.