Biogeochemical controls of Mo and P availability for N2-fixing trees across the Amazon Basin

Background/Question/Methods

Di-nitrogen (N₂) fixation may be critical for alleviating nitrogen limitation on the ability of tropical forests to sequester carbon, but tropical N₂-fixing plants are subject to their own constraints in a world of rising CO₂. While rising CO₂ may stimulate tropical forest growth and, in doing so, may alter and possibly exacerbate the dependence of N₂ fixers on soil phosphorus (P) and molybdenum (Mo), independently and on their combined interaction within the forest ecosystem. Our research evaluated two mechanisms that control the availability of Mo vs. P to nitrogen fixers and potentially fixer P:Mo stoichiometry: i) weathering rate and soil age controls total concentrations and ii) that soil organic matter and clay and iron-rich soil minerals control local availability and loss of Mo vs. P. To do so, we measured P and Mo concentrations (total and resin extracted) in soil profile samples across from 23 sites that varied in soil development age and soil properties (clay and organic content) as well as soil litter, leaf and root tissues from nitrogen fixing species. Lastly, we are constructing a P-Mo availability model to expand our finding to a greater diversity of soil types across the Amazon Basin.

Results/Conclusions

Our preliminary results found that weathering over time controlled the total concentrations of nutrients in the soil profile. However, we found the soil’s clay content and organic matter greatly affected the bio-availability of these nutrients, in particular the soil organic matter has a very different effect in reducing Mo availability compared to P.  We will also present model results that will help expand our empirical findings to a broader range or soil types found in the Amazon. The implications of our findings may help researchers identify areas of the Amazon where fixers are most likely to run into P and Mo limitation in the future.