A plant’s deepest secret: translating root exudate profiles into their effects on soil microbial respiration in grasslands

Plant roots release a great diversity of root exudates into soil, creating hotspots of biochemical activity. Species-level differences in root exudate chemistry may modulate these dynamics, with consequences for soil biogeochemical cycles at larger spatial scales. Therefore, the aim of this research was to investigate the link between species variation in root exudates and soil microbial respiration in temperate grasslands. We hypothesized that sugars would stimulate and phenolics would suppress soil microbial respiration across species. Second, we hypothesized that species variation in sugars and phenolics would be oppositely associated with root traits indicating fast vs. slow growth and their degree of collaboration with mycorrhizal fungi, allowing for these results to be generalized across grassland species.

To test these hypotheses, we conducted a greenhouse study with 53 plant species (grasses, forbs, legumes) common to managed and semi-natural grasslands on sandy soil in the Netherlands. We collected root exudates during peak vegetative growth using a hybrid soil-hydroponic collection method. Root exudates were analysed for total organic carbon, phenolic and sugar content, as well as individual metabolites using untargeted LC-MS analyses. Collected root exudates were freeze-dried and applied at the same carbon concentration to soil using an incubation setup in order to test their effects on soil microbial respiration. Root morphological traits and mycorrhizal colonization of plant species were also measured and aboveground growth was monitored during the study. Preliminary results suggest that species variation in root exudate chemical classes does not relate to core root traits representing nutrient use and acquisition strategies. Results on more detailed metabolite analysis and species-specific root exudate effects on soil microbial respiration will be presented at the conference.