Nitrogen (N) is often the limiting nutrient controlling the fertility of terrestrial ecosystems.  The response of some of these ecosystems to increased CO2 in the atmosphere (the so-called “CO2 fertilization effect”) may be ultimately limited by N availability. The only natural source of new nitrogen to ecosystems is nitrogen fixation, which reduces atmospheric N2 into available ammonia. The identification of the key factors controlling nitrogen fixation is thus critical to understanding the fertility of ecosystems and to predict whether or not they will provide a negative feedback to higher atmospheric CO2 concentrations in the long term.  Nitrogen fixation is mediated in nature by a few N2-fixing bacteria that express the enzyme nitrogenase, of which the most common and efficient form requires Mo as a cofactor.  Mo is in very short supply in soils, and we are interested in understanding whether and how Mo availability controls nitrogen fixation in terrestrial ecosystems.  Vanadium is another transition metal of interest because it is more abundant than Mo in soils, and can also be used to fix nitrogen in the alternative V-nitrogenase which is expressed by some microorganisms when Mo is unavailable.  Both Mo and V form oxoanions in solution (molybdate and vanadate) and their environmental chemistry has been little studied compared to trace metals such as iron, zinc or copper which are present as cations in the environment.

We have approached the question of the role of Mo and V in the terrestrial nitrogen cycle through three interrelated sub-projects investigating: i) the mechanisms of Mo and V acquisition by N2-fixing bacteria; ii) the chemical speciation and bioavailability of Mo and V in soils; and iii) the parameters controlling N2 fixation rates in the field.

Projects

Acquisition of molybdenum and vanadium by nitrogen-fixing bacteria (link)

Chemical speciation and bioavailability of molybdenum and vanadium (link)

Field studies of N2 (link)