Data from: Light regulates tropical symbiotic nitrogen fixation more strongly than soil nitrogen

Nitrogen limits primary production in almost every biome on Earth1,2.
Symbiotic nitrogen fixation, conducted by certain angiosperms and their
endosymbiotic bacteria, is the largest potential natural source of new
nitrogen into the biosphere3, influencing global primary production,
carbon sequestration and element cycling. Because symbiotic nitrogen
fixation represents an alternative to soil nitrogen uptake, much of the
work on symbiotic nitrogen fixation regulation has focused on soil
nitrogen availability4,5,6,7,8. However, because symbiotic nitrogen
fixation is an energetically expensive process9, light availability to the
plant may also regulate symbiotic nitrogen fixation rates10,11. Despite
the importance of symbiotic nitrogen fixation to biosphere functioning,
the environmental factors that most strongly regulate this process remain
unresolved. Here we show that light regulates symbiotic nitrogen fixation
more strongly than does soil nitrogen and that light mediates the response
of symbiotic nitrogen fixation to soil nitrogen availability. In a
shadehouse experiment, low light levels (comparable with forest
understories) completely shut down symbiotic nitrogen fixation, whereas
soil nitrogen levels that far exceeded plant demand did not fully
downregulate symbiotic nitrogen fixation at high light. For in situ forest
seedlings, light was a notable predictor of symbiotic nitrogen fixation
activity, but soil-extractable nitrogen was not. Light as a primary
regulator of symbiotic nitrogen fixation is a departure from decades of
focus on soil nitrogen availability. This shift in our understanding of
symbiotic nitrogen fixation regulation can resolve a long-standing
biogeochemical paradox12, and it will improve our ability to predict how
symbiotic nitrogen fixation will fuel the global forest carbon sink and
respond to human alteration of the global nitrogen cycle.