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

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.

Keywords: cyanobacteria; ecosystem ecology; global change; mosses; nitrogen fixation; nutrient limitation; phosphorus; tropical cloud forest.

Figure 1

Epifluorescence microscopy image of one shoot of Thuidium peruvianum moss collected in a tropical montane cloud forest in south-eastern Peru (40× magnification—Olympus BX61), image with a blue filter (A) and image with a green filter (B), note the epiphytic cyanobacteria filaments (arrows) in association with the moss shoot. Images: L. A. Clasen.

Figure 2

Mean nitrogenase activity (acetylene reduction in nmol g dw⁻¹ h⁻¹, ±SE) across all time points in response to the nutrient additions (control, nitrogen (N), phosphorus (P) and nitrogen and phosphorus (NP)) across three moss species (Campylopus sp., Dicranum sp. and Thuidium peruvianum) collected in a tropical montane cloud forest in south-eastern Peru. Different letters (a/b) show statistically significant (Tukey HSD test) differences between treatments. n=18 (three moss species, six replicates per treatment per moss species).

Figure 3

Mean nitrogenase activity (acetylene reduction in nmol g dw⁻¹ h⁻¹; ±SE, n = 24, across all time points) of the whole experiment in response to nutrient additions (control, nitrogen (N), phosphorus (P) and nitrogen and phosphorus (NP)) in three moss species (Campylopus sp., Dicranum sp. and Thuidium peruvianum) collected in a tropical montane cloud forest in south-eastern Peru. Different letters (a/b/ab) show statistically significant (Tukey HSD test) differences between treatments.

Figure 4

Mean nitrogenase activity (acetylene reduction in nmol g dw^-1 h^-1 ±SE, n = 6) over time (1, 7, 23 and 30 days after nutrient additions) in response to nutrient additions (control, nitrogen, phosphorus and nitrogen plus phosphorus) in three moss species (Campylopus sp., Dicranum sp. and Thuidium peruvianum) collected in a tropical mountain cloud forest in Peru. R² and p values are given. Trendlines show significant results (p < 0.05). Note the different scales at the axis.

Figure 5

Mean response ratio (ln (natural logarithm) RR, n = 24, ±SE, across all time points) of nitrogenase activity across the treatments (nitrogen (N), nitrogen and phosphorus (NP) and phosphorus (P)) relative to the control treatment, in three moss species (Campylopus sp., Dicranum sp. and Thuidium peruvianum) collected in a tropical mountain cloud forest in Peru. Different letters (a/b/ab) show statistically significant (Tukey HSD test) differences between treatments.

Figure 6

Mean pH values for each moss species (Campylopus sp., Dicranum sp. and Thuidium peruvianum) after nutrient additions (control, nitrogen (N), nitrogen and phosphorus (NP) and phosphorus (P)). Different letters (a/b/ab/ac/bc/c) show statistically significant (Tukey test) differences between treatments.