Data for: Tree symbioses sustain nitrogen fixation despite excess nitrogen supply

Symbiotic nitrogen fixation (SNF) is a key ecological process whose impact
depends on the strategy of SNF regulation – the degree to which rates of
SNF change in response to limitation by N vs. other resources. SNF that is
obligate or exhibits incomplete down-regulation can result in excess N
fixation, whereas a facultative SNF strategy does not. We hypothesized
that tree-based SNF strategies differed by latitude (tropical vs
temperate) and symbiotic type (actinorhizal vs rhizobial). Specifically,
we expected tropical rhizobial symbioses to display strongly facultative
SNF as an explanation for their success in low-latitude forests. In this
study, we used 15N isotope dilution field experiments in New York, Oregon,
and Hawaii to determine SNF strategies in six N-fixing tree symbioses.
Nitrogen fertilization with +10 and +15 g N m-2 y-1 for 4-5 years
alleviated N limitation in all taxa, paving the way to determine SNF
strategies. Contrary to our hypothesis, all six of the symbioses we
studied sustained SNF even at high N. Robinia pseudoacacia (temperate
rhizobial) fixed 91% of its N (%Ndfa) in controls, compared to 64% and 59%
in the +10 and +15 g N m-2 y-1 treatments. For Alnus rubra (temperate
actinorhizal), %Ndfa was 95%, 70%, and 60%. For the tropical species,
%Ndfa was 86%, 80%, and 82% for Gliricidia sepium (rhizobial); 79%, 69%,
and 67% for Casuarina equisetifolia (actinorhizal); 91%, 42%, and 67% for
Acacia koa (rhizobial), and 60%, 51%, and 19% for Morella faya
(actinorhizal). Fertilization with phosphorus did not stimulate tree
growth or SNF. These results suggest that the latitudinal abundance
distribution of N-fixing trees is not caused by a shift in SNF strategy.
They also help explain the excess N in many forests where N-fixers are
common. In addition to influencing community dynamics and rates of
nutrient cycling, excess N accumulation and export from sustained SNF may
exacerbate water pollution and greenhouse gas fluxes.