One of the first steps is to identify and determine flux or concentration of exudates.  In recent years we have made great strides in identifying root exudates via mass spectrometry.  Mass spectrometry coupled with labelling is also providing insights into the bi-directional fluxes across roots (i.e. efflux and uptake).  Some recent papers of mine (attached) details some of these methods and you'll find some other papers on mass spectrometry of exudates in the literature.

Article Specific influence of white clover on the rhizosphere microb...

Article Simultaneous efflux and uptake of metabolites by roots of wheat

Thank you Dr. warren for uploading some interesting papers. The other recent techniques for rhizosphere studies are: molecular technique like qPCR, a modified root study container (RSC) technique and push-pull technique for measuring denitrification in the rhizosphere.

Dear Mara Cloutier

please check the resources given below which are related to your topic

regards

link.springer.com/article/10.1023/A:1004264830936

www.cell.com/trends/plant-science/fulltext/S1360-1385(15)00311-8

popups.ulg.ac.be/1780-4507/index.php?id=7578

aem.asm.org/content/67/10/4742.abstract

http://onlinelibrary.wiley.com/doi/10.1111/j.1574-6941.2009.00654.x/full

Thank you all for the papers and helpful suggestions/comments. I certainly have a lot to think about!