Purpose 
A growing perspective in plant ecology highlights the ways that microbial interactions can promote or hinder plant coexistence. Towards this aim, recently-proposed approaches provide ways to empirically quantify how microbes affect the outcome of competition between plants. One such approach experimentally measures competition coefficients by comparing biomass performance of plants growing individually or with competitors, then quantifies microbial effects by comparing with a sterilization treatment. Nonetheless, the complexity of plant-microbe interactions presents general challenges when linking observations of microbial effects to underlying models.

Results 
We show how the implementation of a density gradient design can help quantify potential nonlinearities and facilitative interactions in plant-soil microbe interactions, which are properties once thought to hinder the quantification of the microbial impact of plant competitive outcome. We provide guidelines for setting up the experiment and accurately interpreting the results.

Conclusion 
We argue that future studies should aim to parameterize suitable demographic models to characterize the contribution of soil microbes to plant coexistence. 

Building on insights from modern coexistence theory, we propose a comprehensive density gradient design to quantify the combined impact of plant-plant competition and plant-soil microbe interactions on plant coexistence patterns.