No abstract

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 N2 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.

<p>Moss-associated cyanobacteria have been show to contribute greatly to the nitrogen (N) pools of pristine ecosystems. Moisture and temperature are the most important abiotic factors directly influencing the rates of biological nitrogen fixation (BNF) in mosses. However, the availability of some macro and micronutrients can also be correlated to changes in those rates, and this is linked to the functional role of nutrients involved in the nitrogenase enzyme complex. To identify the effects of nutrient availability on BNF we conducted two experiments with mosses from contrasting ecosystems, subarctic and tropical, in which we applied N and phosphorous (P) along an elevation gradient in the subarctic (Sweden), as well as in a laboratory experiment using three different moss species from a tropical cloud forest (Peru). Nitrogenase activity in mosses was assessed via the acetylene reduction assay (ARA) at different time points after the additions. Our results show that N addition rapidly reduce BNF rates associated with mosses from both ecosystems. But, P additions lead to variable results, especially in tropical mosses, where we detected species-specific responses towards the nutrients additions. Further, the effects of nutrient additions depended on the positioning along the elevation gradient in the subarctic, reflecting differences in nutrient availability and thereby, limitation. Our findings show how the availability of essential macronutrients (i.e. N and P) can play a fundamental role in promoting or inhibiting BNF in mosses, and that time frame and species identity can be significant in modulating the response of BNF to nutrient availability.</p>