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Tree symbioses sustain nitrogen fixation despite excess nitrogen supply

May 25, 2023

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 versus other resources. SNF that is obligate or exhibits incomplete downregulation 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 of 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 year−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 year−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.

Publication Year 2023
Title Tree symbioses sustain nitrogen fixation despite excess nitrogen supply
DOI 10.1002/ecm.1562
Authors Duncan N.L. Menge, Amelia A. Wolf, Jennifer L. Funk, Steven Perakis, Palani R. Akana, Rachel Arkebauer, Thomas A. Bytnerowicz, K. A. Carreras Pereira, Alexandra M. Huddell, Sian Kou-Giesbrecht, Sarah K. Ortiz
Publication Type Article
Publication Subtype Journal Article
Series Title Ecological Monographs
Index ID 70243931
Record Source USGS Publications Warehouse
USGS Organization Forest and Rangeland Ecosystem Science Center