Much research over the past two decades has examined the potential for plant species to take up organic forms of nitrogen (N) to supplement their nutritional needs not satisfied by inorganic sources.  This work has caused a paradigm shift in ecology, refocusing our efforts on plant-microbial interactions, and how these processes are influenced by the chemistry of the nutrient in question.  Although critically important, much of this research has not accounted for possible confounding with potentially rapid N-transformations in the soil, with short time horizons and dual-labeling techniques often applied as a remedy.  Here I present an alternative framework for examining this interaction, one that labels all atoms of a nutrient except N in a greenhouse experiment (i.e. hydrogen of ammonium [2H], oxygen of nitrate [18O], and carbon of glycine [13C]), with and without the presence of microbes, as the movement of the off-N atom(s) should yield important insight as to the exact form of uptake.  
Results/Conclusions

I find for a common prairie forb species that plants were able to effectively compete with soil microbes for both nitrate and glycine, as evidenced by enrichment of 18O and 13C within the plant biomass.  However, plants were evidently only weakly able to compete with microbes for ammonium, as evidenced by weak enrichment of 2H in the presence of microbes.  These results offer new insight on the exact process of plant-microbial interactions, and offer new techniques for measuring these complex processes which may not have been adequately resolved by earlier experimental work.