Data and metadata files for: Wong, M.Y., Neill, C., Marino, R., Silvério, D.V., and Howarth, R.W. Molybdenum, phosphorus, and pH do not control free-living nitrogen fixation in a tropical forest in the southeastern Amazon. Ecology.

Abstract:
High rates of biological nitrogen fixation (BNF) are
commonly reported for tropical forests, but most studies have been conducted in
regions that receive substantial inputs of molybdenum (Mo) from atmospheric
dust and sea-salt aerosols. Even in these regions, the low availability of Mo
can constrain free-living BNF catalyzed by heterotropic bacteria and archaea.
We hypothesized that in regions where atmospheric inputs of Mo are low and
soils are highly weathered, such as the southeastern Amazon, Mo would constrain
BNF. We also hypothesized that the high soil acidity, characteristic of the
Amazon Basin, would further constrain Mo availability and therefore soil BNF.
We conducted two field experiments across the wet and dry seasons, adding Mo,
phosphorus (P), and lime alone and in combination to the forest floor in the
southeastern Amazon. We sampled soils and litter immediately, and then weeks
and months after the applications, and measured Mo and P availability through
resin extractions and measured BNF by the acetylene reduction assay. The
experimental additions of Mo and P increased their availability and the lime
increased soil pH. While the combination of Mo and P increased BNF at some time
points, BNF rates did not increase strongly or consistently across the study as
a whole, suggesting that Mo, P, and soil pH are not the dominant controls over
BNF. In a separate short-term laboratory experiment, BNF did not respond
strongly to Mo and P even when labile carbon was added. We postulate that high
nitrogen (N) availability in this area of the Amazon, as indicated by the
stoichiometry of soils and vegetation and the high nitrate soil stocks, likely
suppresses BNF at this site. These patterns may also extend across highly
weathered soils with high N availability in other topographically stable
regions of the tropics.