In collaboration with Satish Myneni (link) in the Geoscience department, we are investigating the chemical speciation of Mo and V in soils using synchrotron-based X-ray spectroscopy. Our present work deals with Mo, whose concentrations in soils are extremely low, around 1 to 2 ppm, making it is the least abundant of all biologically essential trace metals in terrestrial systems by at least an order of magnitude.  How Mo availability varies with soil characteristics, such as content of organic matter and pH depends on its chemical form. Because of the low affinity of molybdate for organic ligands, Mo speciation in soils is usually believed to be controlled by adsorption to particles such as iron oxides or clays at low pH.  Using X-ray techniques, we are currently investigating the relative importance of adsorption and NOM complexation of molybdate in soils, which is critical to understand the response of Mo availability to pH changes in soils.