Differences in nutrient content between heterospecific plant neighbours affect respiration rates of rhizosphere microbiota

Abstract Aims Plant-soil interactions play a pivotal role in governing ecosystem dynamics. Plants directly interact with rhizosphere microorganisms, providing them with labile carbon in exchange for mineral nutrients that are the product of decomposition. Such processes are modulated by ecological interactions between plant species in ways that are not fully understood. We assessed whether rhizosphere respiration rates, as a proxy for decomposition, are influenced by i) heterospecific versus conspecific plant interactions, and whether these effects are positive or negative, and ii) how these effects are linked to the identity and the below- and aboveground functional traits of the interacting plant species. Methods We conducted a greenhouse experiment with 300 pairwise hetero- and conspecific combinations of ten Mediterranean herbs and shrubs species, covering a range of functional distances calculated based on 33 traits. In addition, we quantified heterotrophic respiration in the rhizosphere as a proxy of decomposition. Results Plant neighbour identity was the main factor explaining changes on respiration rates. Respiration increased along with the functional distance between heterospecific pairs of interacting plants when considering aboveground or nutritional traits. Morphological and belowground traits were not significant predictors of changes on respiration rates. Conclusions Interspecific plant-plant interactions lead to faster respiration rates in the rhizosphere as functional distance between neighbours increases. This study provides experimental support that functional trait dissimilarities between heterospecific neighbouring plants promote the rates of organic matter decomposition, showing cascading effects of aboveground interactions on belowground ecosystem processes.