Atmospheric dust and sea-salt aerosol spray as sources of molybdenum, a control on tropical forest function?

Background/Question/Methods

Nitrogen fixation is the primary source of new nitrogen to tropical forests and is exclusively performed by bacteria and archaea.  While these N-fixing microorganisms are very diverse, they all share the nitrogenase enzyme, which almost always requires molybdenum (Mo). Increasing evidence in recent years suggests that Mo availability may be low in highly weathered soils and can constrain N-fixation rates.  Mo is generally either present in a highly soluble form (MoO₄^2-) that is susceptible to leaching or tightly bound in minerals unavailable for biological uptake.  How then is there sufficient Mo in highly weathered tropical systems to support N-fixation? Several mechanisms may help systems retain Mo, such as complexation with organic matter.  Alternatively, atmospheric transport through sea-salt aerosol spray and dust may be an important source of new Mo to weathered soils. Atmospheric transport of Mo has not previously been explicitly measured or modeled.  Here, we use a global atmospheric transport model computed from modeled meteorological fields, inputting extrapolated sea-salt aerosol and dust Mo concentrations to calculate global distribution of sea-salt aerosol and dust deposition of Mo.

Results/Conclusions

Using the known seawater Mo concentration, the model predicts Mo inputs from sea-salt aerosols in coastal regions up to 0.002 mg m^-2yr^-1.  Significant sea-salt deposition occurs up to 300 km inland.  In addition, dust deposition occurs across the entirety of some tropical forests.  We focused analysis on the world’s largest tropical forest in the Amazon Basin.  The model indicates that the Amazon Basin receives substantial inputs of dust, especially the entire northern Amazon Basin, while the southern half receives less.  Most of the dust reaching the Amazon originates from the Sahara Desert, and about half of this dust originates from one part of the Sahara, the Bodélé Depression.  We are making measurements of Mo in soils and dust from the Bodélé Depression as well as other areas in the deserts of northern Africa, and will use this information to calibrate the Mo dust deposition to the Amazon Basin.