Humid tropical forests are often characterized by
large nitrogen (N) pools, and are known to have
large potential N losses. Although rarely measured,
tropical forests likely maintain considerable biological
N fixation (BNF) to balance N losses. We
estimated inputs of N via BNF by free-living microbes
for two tropical forests in Puerto Rico, and
assessed the response to increased N availability
using an on-going N fertilization experiment.
Nitrogenase activity was measured across forest
strata, including the soil, forest floor, mosses, canopy
epiphylls, and lichens using acetylene (C2H2)
reduction assays. BNF varied significantly among
ecosystem compartments in both forests. Mosses
had the highest rates of nitrogenase activity per
gram of sample, with 11 ± 6 nmol C2H2 reduced/g
dry weight/h (mean ± SE) in a lower elevation
forest, and 6 ± 1 nmol C2H2/g/h in an upper elevation
forest. We calculated potential N fluxes via
BNF to each forest compartment using surveys of
standing stocks. Soils and mosses provided the
largest potential inputs of N via BNF to these ecosystems.
Summing all components, total background
BNF inputs were 120 ± 29 lg N/m2/h in
the lower elevation forest, and 95 ± 15 lg N/m2/h
in the upper elevation forest, with added N significantly
suppressing BNF in soils and forest floor.
Moisture content was significantly positively correlated
with BNF rates for soils and the forest floor.
We conclude that BNF is an active biological process
across forest strata for these tropical forests,
and is likely to be sensitive to increases in N
deposition in tropical regions.