Impact of plant nutrient use strategies on investment in root exudation, and consequences on the diversity and functioning of rhizospheric microbiota

Plant strategies for soil nutrient uptake have the potential to strongly influence plant– microbiota interactions, due to the competition between plants and microorganisms for soil nutrient acquisition and/or conservation. In the present study, we proposed root exudation as plant functional trait and investigate whether plant strategies could influence rhizosphere microbial activities via root exudation quality and quantity, and contribute to the microbiota diversification of active bacterial communities colonizing the root-adhering soil (RAS) and inhabiting the root tissues. We applied a DNA-based stable isotope probing (DNA-SIP) approach to six grass species distributed along a gradient of plant nutrient resource strategies, from conservative species, characterized by low nitrogen (N) uptake, a long lifespans and low root exudation level, to exploitative species, characterized by high rates of photosynthesis, rapid rates of N uptake and high root exudation level. We analyzed their (i) root exudation level and quality and (ii) their associated microbiota composition involved in root exudate assimilation and soil organic matter (SOM) degradation by 16S- rRNA-based metabarcoding. (iii) We determine the impact of root exudation level on microbial activities (denitrification and respiration) by gas chromatography. Measurement of microbial activities revealed an increase in denitrification and respiration activities for microbial communities colonizing the RAS of exploitative species. This increase of microbial activities results probably from a higher exudation rate and more diverse metabolites by exploitative plant species. We present evidence demonstrating that plant nutrient use strategies shape active microbiota involved in root exudate assimilation and SOM degradation via root exudation.