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. We here 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.

Cover and heterocyst dataCyanobacterial cover and heterocyst frequency for each treatment.CranedataSimplified.csvRaw isotope dataRaw data from stable isotope analyses (C and N) used for fixation calculationsNfixdata.csvFixation RatesContains the same columns as Nfixdata.csv, with very small samples (<500ug) removed, plus columns added for calculated fixation rates.