Nitrogen deposition does not enhance Sphagnum decomposition

Area of Science:

• Ecology
• Biogeochemistry
• Environmental Science

Background:

• Long-term nitrogen (N) additions to peatlands alter bryophyte growth, species dominance, and decomposition rates.
• Previous studies primarily used ammonium nitrate (NH4NO3), overlooking the distinct impacts of nitrate (NO3(-)) or ammonium (NH4(+)) dominance in wet deposition.

Purpose of the Study:

• To investigate the effects of elevated wet deposition of nitrate (NO3(-)) versus ammonium (NH4(+)), alone or with phosphorus (P) and potassium (K), on Sphagnum decomposition and hummock pore water chemistry.
• To understand the influence of different nitrogen forms on Sphagnum quality and mass loss in an ombrotrophic bog.

Main Methods:

• Field experiment applying different forms and rates of nitrogen (8 or 56 kg N ha(-1) yr(-1)) over background levels (8 kg N ha(-1) yr(-1)) for 5-11 years.
• Analysis of Sphagnum quality for decomposers, mass loss, and hummock pore water chemistry.
• Assessment of carbon (C) and N cycling dynamics in the N-treated bog.

Main Results:

• Nitrogen (N) addition, particularly as ammonium (NH4(+)), increased N concentration in Sphagnum but did not enhance mass loss.
• Sphagnum mass loss was primarily influenced by moss species and climatic factors.
• High N applications altered hummock pore water chemistry, and C and N cycling appeared decoupled.

Conclusions:

• Moss species, climate variability, and direct negative N effects (e.g., NH4(+) toxicity) are key drivers of Sphagnum decomposition.
• Indirect effects, such as pH changes and altered plant species dominance under elevated NO3(-), also influence C and N dynamics.
• The form of nitrogen deposition significantly impacts peatland ecosystem processes, decoupling C and N cycling.