Soil chemical and microbial gradients determine accumulation of root exuded secondary metabolites and plant-soil feedbacks in the field

Abstract IntroductionHarnessing positive plant-soil feedbacks via crop rotations is a promising strategy for sustainable agriculture. Plants can influence soil properties including microbes by exuding specialized metabolites. However, the effects are often context dependent and variable. If and how local soil heterogeneity may explain this variation is unknown. Benzoxazinoids are specialized metabolites that are released in high quantities by cereals such as wheat and maize. Benzoxazinoids can alter rhizosphere microbiota and the performance of plants subsequently growing in the exposed soils and are thus an excellent model to study agriculturally relevant plant-soil feedbacks in the field, and to assess how soil factors affect their outcome. Materials & methodsTo understand the importance of local variation in soil properties on benzoxazinoid-mediated plant-soil feedbacks, we conditioned plots with wild-type maize and benzoxazinoid-deficient bx1 mutant plants in a grid pattern across an arable field. We then grew winter wheat across the entire field in the following season. We determined accumulation of benzoxazinoids, root-associated microbial communities, abiotic soil properties and wheat performance in each plot. We also determined benzoxazinoid conversion dynamics in a labelling experiment under controlled conditions, and then assessed associations between soil chemical variation and benzoxazinoid-mediated plant-soil feedbacks. ResultsAcross the field, we detected a marked gradient in soil chemical and microbial community composition. This gradient resulted in significant differences in benzoxazinoid accumulation. These differences were explained by differential benzoxazinoid degradation rather than exudation. Benzoxazinoid exudation modulated alpha diversity of root and rhizosphere bacteria and fungi during maize growth, but not during subsequent wheat growth, while the chemical fingerprint of benzoxazinoid accumulation persisted. Averaged across the field, we detected no significant feedback effects of benzoxazinoid conditioning on wheat performance and defence, apart from a transient decrease in biomass during vegetative growth. Closer analysis however, revealed pronounced feedback effects along the chemical and microbial gradient of the field, with effects gradually changing from negative to positive along the gradient. ConclusionOverall, this study revealed that plant-soil feedbacks differ in strength and direction within a field, and that this variation can be explained by standing chemical and microbial gradients, which strongly affect benzoxazinoid accumulation in the soil. Understanding within-field soil heterogeneity is crucial for the future exploitation of plant-soil feedbacks in sustainable precision agriculture.