The Role Of Molybdenum In The Nitrogen Cycle Of Soils
Dr. Ann Kraepiel
University of Louis Pasteur
Strasbourg

Many ecosystems on Earth are limited by nitrogen availability. Concomitantly, the intensive use of nitrogen fertilizers in agriculture often results in high nitrate concentrations that are responsible for the eutrophication of surface waters and may be toxic in drinking water. The pool of bioavailable nitrogen in soils depends, among other factors, on the relative rates of nitrogen fixation and denitrification. It has long been known that both reactions are inhibited in acidic soils, although the mechanism for the inhibition is unclear. Nitrogenase (the enzyme responsible for nitrogen fixation) and nitrate reductase (the enzyme that catalyzes the first step of denitrification) require molybdenum at their active center. Since (in the absence of reduced sulfur) molybdenum in soils is in the form of the oxyanion molybdate which adsorbs strongly on mineral particles at low pH, we hypothesize that reduced molybdenum bioavailability may be partly responsible for the low rates of nitrogen fixation and denitrification observed in acidic soils.

To test this hypothesis, we are pursuing a research projects in three parts: i) the development of buffers to control molybdate availability in chemically defined media; ii) laboratory studies of the molybdate requirements of a variety of nitrogen fixers and denitrifiers in cultures; and iii) field studies of Mo availability in soils. We have investigated the possible use of the bis-catecholate azotochelin to use as a complexing buffer for molybdate and other oxyanions in laboratory cultures. This ligand can serve a role for oxyanions equivalent to that of EDTA for divalent metals and should be a powerful tool for the systematic investigation of the oxyanion requirements of a variety of microorganisms. We are now embarking on a systematic study of Mo requirements in Azotobacter vinelandii. We are also starting field studies to measure bioavailable concentrations of Mo and relate them to nitrogen fixation rates. Our objective is to relate the molybdenum requirements of nitrogen fixers and denitrifiers with bioavailable molybdenum levels in soils, with the ultimate goal of optimizing the treatment of high nitrate water and managing intelligently the nitrogen cycle in soils.

Print page