Do Nitrogen and Phosphorus Additions Affect Nitrogen Fixation Associated with Tropical Mosses?

Tropical cloud forests are characterized by abundant and biodiverse mosses which grow epiphytically as well as on the ground. Nitrogen (N)-fixing cyanobacteria live in association with most mosses, and contribute greatly to the N pool via biological nitrogen fixation (BNF). However, the availability of nutrients, especially N and phosphorus (P), can influence BNF rates drastically. To evaluate the effects of increased N and P availability on BNF in mosses, we conducted a laboratory experiment where we added N and P, in isolation and combined, to three mosses (Campylopus sp., Dicranum sp. and Thuidium peruvianum) collected from a cloud forest in Peru. Our results show that N addition almost completely inhibited BNF within a day, whereas P addition caused variable results across moss species. Low N-2 fixation rates were observed in Campylopus sp. across the experiment. BNF in Dicranum sp. was decreased by all nutrients, while P additions seemed to promote BNF in T. peruvianum. Hence, each of the three mosses contributes distinctively to the ecosystem N pool depending on nutrient availability. Moreover, increased N input will likely significantly decrease BNF associated with mosses also in tropical cloud forests, thereby limiting N input to these ecosystems via the moss-cyanobacteria pathway.

Automated summary

Tropical cloud forests are home to diverse mosses that grow abundantly and contribute to the nitrogen pool through biological nitrogen fixation (BNF). However, the availability of nutrients, especially nitrogen and phosphorus, can significantly affect BNF rates in mosses. A laboratory experiment in Peru showed that nitrogen addition inhibited BNF within a day, while phosphorus addition had variable effects on different moss species. Each moss species has a distinct contribution to the ecosystem's nitrogen pool depending on nutrient availability. Increased nitrogen input is likely to decrease BNF associated with mosses in tropical cloud forests, thus limiting nitrogen input to these ecosystems via the moss-cyanobacteria pathway.