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 (%N_dfa) in controls, compared to 64% and 59% in the +10 and +15 g N m^-2 y^-1 treatments. For Alnus rubra (temperate actinorhizal), %N_dfa was 95%, 70%, and 60%. For the tropical species, %N_dfa 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.

See paper.

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The data files are analyzed using code files.

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