Sphagnum can 'filter' N deposition, but effects on the plant and pore water depend on the N form

The ability of Sphagnum moss to efficiently intercept atmospheric nitrogen (N) has been assumed to be vulnerable to increased N deposition. However, the proposed critical load (20 kg N ha^-1 yr^-1) to exceed the capacity of the Sphagnum N filter has not been confirmed. A long-term (11 years) and realistic N manipulation on Whim bog was used to study the N filter function of Sphagnum (Sphagnum capillifolium) in response to increased wet N deposition. On this ombrotrophic peatland where ambient deposition was 8 kg N ha^-1 yr^-1, an additional 8, 24, and 56 kg N ha^-1 yr^-1 of either ammonium (NH₄⁺) or nitrate (NO₃^-) has been applied for 11 years. Nutrient status of Sphagnum and pore water quality from the Sphagnum layer were assessed. The N filter function of Sphagnum was still active up to 32 kg N ha^-1 yr^-1 even after 11 years. N saturation of Sphagnum and subsequent increases in dissolved inorganic N (DIN) concentration in pore water occurred only for 56 kg N ha^-1 yr^-1 of NH₄⁺ addition. These results indicate that the Sphagnum N filter is more resilient to wet N deposition than previously inferred. However, functionality will be more compromised when NH₄⁺ dominates wet deposition for high inputs (56 kg N ha^-1 yr^-1). The N filter function in response to NO₃^- uptake increased the concentration of dissolved organic N (DON) and associated organic anions in pore water. NH₄⁺ uptake increased the concentration of base cations and hydrogen ions in pore water though ion exchange. The resilience of the Sphagnum N filter can explain the reported small magnitude of species change in the Whim bog ecosystem exposed to wet N deposition. However, changes in the leaching substances, arising from the assimilation of NO₃^- and NH₄⁺, may lead to species change.