Faba Bean Genotype, Mixed Cropping and Soil Fauna Jointly Influence Plant Roots and Rhizosphere Microbial Communities

Zhi-Yuan Wang; Shu Li; Jing-Zhong Lu; Amandine Erktan; Li-Bo Fu; Wei-Dong Cao; Pan-Lei Wang; Stefan Scheu

Abstract

Integrating belowground biodiversity into agricultural systems is key to enhancing agricultural sustainability, yet the role of soil decomposer animals in mediating genotype-specific root and rhizosphere responses remains largely unknown. Using a full-factorial microcosm experiment, we examined how two faba bean genotypes with contrasting root strategies (resource-acquisitive S-62 vs. resource-conservative S-69), mixed cropping with wheat and soil decomposer fauna (earthworms, collembola) jointly shape plant root characteristics and rhizosphere interactions. We quantified root architecture, microbial phospholipid fatty acids (PLFAs), litter carbon incorporation by microorganisms and soil fauna and fauna abundance/biomass. Bean root characteristics differed between genotypes, with higher specific root length and lower root mass density in genotype S-62 than in genotype S-69, reflecting their different resource strategies. Bean genotype S-69 increased fungal/bacterial and Gram⁺/Gram⁻ PLFA ratios, indicating stronger microbial resource limitation. This was accompanied by reduced earthworm biomass, which shifted their resource use toward litter-derived carbon. Mixed cropping uniformly decreased bean root mass density, but increased microbial basal respiration and biomass in beans while elevating fungal dominance in wheat. Collembola enhanced bean specific root length and increased the fungal/bacterial PLFA ratio and microbial metabolic quotient in the bean rhizosphere, suggesting grazing-induced fungal dominance and stress. Earthworms increased wheat root mass density, while collembola reduced wheat root diameter, and in combination they increased microbial activity and reinforced fungal carbon use in wheat. Further, earthworms reduced the saturated/monoenoic fatty acid ratio in bulk soil and increased the use of litter carbon by Gram⁻ bacteria, indicating microbial stress mitigation. Overall, the results document that plant genotypes with different root strategies fundamentally drive the structure of rhizosphere microbial communities, their interactions with decomposer animals and plant litter dynamics. These findings highlight the importance of considering root phenotypic variation and belowground biodiversity when designing sustainable legume–cereal intercropping systems.

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