Biological nitrogen fixation (BNF) is critical for the nitrogen cycle of forests, but we know little about the factors that control the microbial nitrogen fixers that live freely on the vast area of leaf surfaces 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 during precipitation. The heterogeneity of BNF rates reported from canopies, both between sites and within sites, indicates strong regulation by limiting factors, with nutrient limitation by phosphorus (P) or molybdenum (Mo) likely. We experimentally evaluated the nutrient response of free-living cyanobacterial communities in a lowland tropical forest canopy in Panama. By exposing nutrient-infused artificial substrates to in-situ canopy conditions, we were able to observe the response of free-living canopy nitrogen fixers to nitrogen, phosphorus and molybdenum additions, while controlling for microenvironmental heterogeneity.
Results/Conclusions
Colonization of substrates by cyanobacteria was negatively impacted by N addition but greatly enhanced by P addition. Phosphorus caused significant increases in heterocyst abundance (where nitrogen fixation occurs) and nitrogen fixation rates. Molybdenum addition alone did not impact colonization or heterocyst production, but showed a very strong interaction with P to increase fixation rates. We also found weaker but significant positive effects of potassium (K) additions. The high nitrogen fixation rates and rapid growth (over <6 months) of cyanobacteria in response to P and Mo addition suggest that canopy microorganisms are capable of making important contributions to tropical forest nitrogen cycles, notably in response to allochthonous dust deposition.