An exudate extravaganza – how changes in root traits and exudation in response to insect-based fertilizer could elucidate differences in crop species nitrogen uptake strategy

Root exudates are a primary pathway through which plants can recruit and interact with microbial communities surrounding their roots. Yet little is known about how crop species differing in root exudate quantity and quality influence the microbial mineralization of organic nitrogen in the rhizosphere. This is a particularly urgent question considering the need for effective novel organic fertilizers, such as insect-based fertilizer from black soldier fly (flytilizer), without compromising crop yields. Therefore, the aim of this study was to examine root exudation across 20 crop species, then link this to root traits indicative of fast vs. slow-growing strategies, mineralization of nitrogen in the rhizosphere, and plant nitrogen uptake. Additionally, we wanted to examine how these patterns changed when flytilizer was added. We expected that the total organic carbon (TOC) of root exudation would be positively correlated to rhizosphere microbial nitrogen cycling enzyme activity and plant N content. We also expected higher TOC and sugar to phenolic ratio to be positively correlated to strategies where plants grow quickly. Finally, we expected that when flytilizer is added, the relationship between TOC of root exudates and rhizosphere microbial nitrogen cycling activity to be weakened. To fill this gap, we conducted a greenhouse experiment with 20 crop species from 10 families grown in sandy field soil without and with flytilizer. We ensured the plants were nitrogen limited by applying mineral fertilizer containing all essential elements for plant growth apart from nitrogen. We measured relative growth rate and, after 7 weeks, we measured a variety of root traits, root exudation, as well as microbial biomass and the activity of five nutrient-cycling enzymes in the rhizosphere. Plant productivity and plant nitrogen (N) content were also quantified and for each crop species.