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

Area of Science:

• Ecology
• Microbiology
• Botany

Background:

• Tropical cloud forests host diverse mosses with symbiotic nitrogen-fixing cyanobacteria.
• Biological nitrogen fixation (BNF) by these symbionts is a key source of nitrogen (N) in these ecosystems.
• Nutrient availability, particularly N and phosphorus (P), significantly influences BNF rates.

Purpose of the Study:

• To investigate the impact of increased nitrogen (N) and phosphorus (P) availability on BNF in three tropical cloud forest moss species.
• To understand how nutrient enrichment affects the contribution of moss-cyanobacteria associations to ecosystem nitrogen pools.

Main Methods:

• Laboratory experiment involving the addition of N and P (individually and combined) to three moss species: *Campylopus* sp., *Dicranum* sp., and *Thuidium peruvianum*.
• Measurement of biological nitrogen fixation (BNF) rates under different nutrient treatments.
• Analysis of species-specific responses to nutrient enrichment.

Main Results:

• Nitrogen addition rapidly and almost completely inhibited BNF across all tested mosses.
• Phosphorus addition yielded variable effects, with some species showing promotion and others inhibition of BNF.
• *Campylopus* sp. exhibited consistently low BNF rates.
• *Dicranum* sp. showed decreased BNF with all nutrient additions.
• *T. peruvianum* demonstrated enhanced BNF with phosphorus addition.

Conclusions:

• Moss species exhibit distinct responses to nutrient availability, influencing their contribution to ecosystem nitrogen cycling.
• Increased nitrogen input in tropical cloud forests is likely to significantly reduce BNF via the moss-cyanobacteria pathway.
• This reduction in BNF could limit nitrogen availability in these sensitive ecosystems.