Soil microorganisms mediate many nutrient transformations that are central in terrestrial cycling of carbon and nitrogen. However, uptake of organic nutrients by microorganisms is difficult to study in natural systems. We assessed quantum dots (fluorescent nanoscale semiconductors) as a new tool to observe uptake and translocation of organic nitrogen by fungi and plants. We conjugated quantum dots to the amino groups of glycine, arginine, and chitosan and incubated them with Penicillium fungi (a saprotroph) and annual bluegrass (Poa annua) inoculated with arbuscular mycorrhizal fungi. As experimental controls, we incubated fungi and bluegrass samples with substrate-free quantum dots as well as unbound quantum dot substrate mixtures. Penicillium fungi, annual bluegrass, and arbuscular mycorrhizal fungi all showed uptake and translocation of quantum dot-labeled organic nitrogen, but no uptake of quantum dot controls. Additionally, we observed quantum dot-labeled organic nitrogen within soil hyphae, plant roots, and plant shoots using field imaging techniques. This experiment is one of the first to demonstrate direct uptake of organic nitrogen by arbuscular mycorrhizal fungi.
Our study contributes to the growing body of literature demonstrating that AMF may have a more significant effect on ON dynamics than previously believed (Na¨ sholm et al. 1998, Hawkins et al. 2000, Hodge et al. 2001, Rains and Bledsoe 2007). The QD technique provides a simple, inexpensive, and nonde- structive measure of mycorrhizal fungi and root uptake in natural systems. By using this technique to trace the uptake of ON molecules, researchers could improve our knowledge of nutrient acquisition by organisms in general and the role of AMF in plant ON uptake in particular.