Biological nitrogen fixation is critical for the nitrogen cycle of tropical forests, yet we know little about the factors that control the microbial nitrogen fixers that colonize the microbiome of leaves and branches that make up a forest canopy. Forest canopies are especially prone to nutrient limitation because they are (1) disconnected from soil nutrient pools and (2) often subject to leaching. Earlier studies have suggested a role of phosphorus and molybdenum in controlling biological N-fixation rates, but experimental confirmation has hitherto been unavailable. Here we present the results of a manipulation of canopy nutrient availability. Our findings demonstrate a primary role of phosphorus in constraining overall N fixation by canopy cyanobacteria, but also a secondary role of molybdenum in determining per-cell fixation rates. A conservative evaluation suggests that canopy fixation can contribute to significant N fluxes at the ecosystem level, especially as bursts following atmospheric inputs of nutrient-rich dust.
Bibliographical noteFunding Information:
D. Stanton and L. Hedin designed the study, D. Stanton set up experiment and all authors collected data. D. Stanton wrote the first draft of the manuscript, and all authors contributed to revisions. D. Stanton was supported by a Princeton University President's Award. S. A. Batterman acknowledges funding from a Carbon Mitigation Initiative young investigator grant (with funding from BP), a University of Leeds University Academic Fellowship, the Natural Environment Research Council (NE/M019497/1; NE/N012542/1) and the British Council (award 275556724). We would like to thank J. Wright, N. Salazar, O. Acevedo, H. Castaneda, M. Samarniego, G. Mendieta, K. Wagner, and E. Sanchez for assistance with canopy access and experimental set-up, B. Turner and D. Agudo for assistance with isotope sample analyses, N. Wurzburger and members of the Hedin lab for helpful discussions and feedback on drafts and two anonymous reviewers for their helpful comments.
© 2019 by the Ecological Society of America
- biological nitrogen fixation
- molybdenum limitation
- phosphorus limitation
- tropical forest canopy