The impact of benzoxazinoids on agroecological plant-soil feedbacks

Plants modulate their growth environment by changing their root surrounding soil, which in turn modifies the performance of the next plant growing in that soil. How such plant-soil feedbacks are affected by root exuded secondary metabolites is not well understood. In particular, we know very little about how secondary metabolite-mediated plant-soil feedbacks affect agricultural productivity and food quality in crop rotations, and how secondary metabolites could help to alleviate negative agroecological plant-soil feedbacks. In this thesis, I aim to assess the potential of benzoxazinoids, an important class of secondary metabolites that are produced by cereals, to improve crop rotations through plant-soil feedbacks. First, in a two-year field experiment, I demonstrated that maize benzoxazinoid soil conditioning improved the performance of three subsequently growing wheat varieties without compromising food quality. Cereal leaf beetle infestation was reduced in response to benzoxazinoid soil conditioning and wheat yield was increased by more than 4%, mostly caused by enhanced emergence and tillering. Second, in another two-year field experiment, I found that such benzoxazinoid-dependent plant-soil feedbacks depend on local soil parameters. Soil chemistry was closely associated with soil benzoxazinoid concentrations and rhizosphere microbial community composition. Soil chemistry also explained the magnitude and direction of the feedbacks on plant performance, resistance, and kernel quality. Further, in a climate chamber and an incubation experiment I elucidated how benzoxazinoid degradation, but not exudation, was influenced by soil chemistry. In both field experiments, benzoxazinoid soil conditioning modified soil benzoxazinoid concentrations and the community compositions of root-associated microbes. The differences in rhizosphere microbial communities were only transient, while the chemical fingerprint of benzoxazinoid degradation products persisted to the next crop. Third, in climate chamber experiments, I demonstrated that three out of five tested preceding crops suppressed growth of maize through negative plant-soil feedbacks, and that benzoxazinoid exudation reduced this growth suppression. This resistance to growth suppression was, at least partially, dependent on soil biota. Overall, the results of this thesis reveal several new facets of secondary metabolites in agroecological plant-soil feedbacks. Exuded secondary metabolites can enhance crop rotation productivity and confer resistance to negative plant-soil feedbacks, thus making them a promising breeding target to improve crop productivity in a sustainable manner.

2022

dissertation

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