Maintenance of tropical forest biodiversity and its ecosystem services depend on mutualistic relationships between plants and other organisms. These mutualisms may affect the ability of plant species from different functional groups to respond to CO2 elevation. In tropical forests, phosphorus (P) is one of the main limiting nutrients related to forest productivity, which adds further uncertainty to the role of the Amazon as a carbon sink as atmospheric CO2 concentration increases. Here, we used a nitrogen-fixing plant species, Inga edulis, to investigate if the plant coordination with three types of mutualists (rhizobium, mycorrhizas, and ant bodyguards) are affected by both phosphorus and CO2 factors. We specifically asked 1) if extrafloral nectaries (EFNs) activity is influenced by the presence of rhizobia and mycorrhizas in the roots, 2) if the attraction of ants to extrafloral nectaries is influenced by the presence of rhizobia and mycorrhizas on the roots, and 3) if the phosphorus addition and [eCO2] affect mutualistic interactions above and belowground.

Within the AmazonFACE Program, we used open-top chambers (OTC) with ambient and elevated CO2 [eCO2] (~ +200ppm) (n=4), and carried out a pot experiment where Inga edulis seeds were sown in soils with and without phosphorus addition (6 per OTC, n=12 per treatment). After a year and a half of cultivation, the activity of EFNs and ant attendance to the EFN were monitored for five months. After two years of experiment, the seedlings were harvested, and the roots searched for nodules and mycorrhizas.

We found that root-rhizobia interactions were the main factor directly influencing the number of activate EFNs and ants attending to EFNs and the relationship were stronger in the P added soils and under [eCO2]. Mycorrhizas did not affect the EFNs activity, but negatively influenced the number of ants attending to EFNs in P added soils. This pattern is one of the few results revealing responses of CO2 elevation on plant coordination with multiple mutualistic partners. Under [eCO2] condition, plant demand for N increases, reducing the carbon cost of nitrogen fixation, favoring plant investment in indirect defense via ant-EFN interactions only if the plant is not nutrient limited. However, plants may allocate carbon to nutrient acquisition via mycorrhizas in P added soils and not for ants-EFN interactions, possibly lowering the biological defense against herbivores. Together, suggest that nutrient limitation regulate carbon allocation under eCO2 and asymmetric mutualisms can drive complex response of plants within nitrogen-fixing functional group.