title: Sphagnum can 'filter' N deposition, but effects on the plant and pore water depend on the N form.
abstract: 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 (20kgNha(-1)yr(-1)) to exceed the capacity of the Sphagnum N filter has not been confirmed. A long-term (11years) 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 8kgNha(-1)yr(-1), an additional 8, 24, and 56kgNha(-1)yr(-1) of either ammonium (NH4(+)) or nitrate (NO3(-)) has been applied for 11years. 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 32kgNha(-1)yr(-1) even after 11years. N saturation of Sphagnum and subsequent increases in dissolved inorganic N (DIN) concentration in pore water occurred only for 56kgNha(-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 (56kgNha(-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.
authors.affiliation: Kyushu University Forest, Kyushu University, 394 Tsubakuro, Sasaguri, Fukuoka 811-2415, Japan. Electronic address: mchiwa@forest.kyushu-u.ac.jp.
authors.forename: Masaaki
authors.initials: M
authors.lastname: Chiwa
authors.affiliation: Centre for Ecology & Hydrology (CEH) Edinburgh, Bush Estate, Penicuik, Midlothian EH260QB, UK.
authors.forename: Lucy J
authors.initials: LJ
authors.lastname: Sheppard
authors.affiliation: Centre for Ecology & Hydrology (CEH) Edinburgh, Bush Estate, Penicuik, Midlothian EH260QB, UK.
authors.forename: Ian D
authors.initials: ID
authors.lastname: Leith
authors.affiliation: Centre for Ecology & Hydrology (CEH) Edinburgh, Bush Estate, Penicuik, Midlothian EH260QB, UK.
authors.forename: Sarah R
authors.initials: SR
authors.lastname: Leeson
authors.affiliation: Centre for Ecology & Hydrology (CEH) Edinburgh, Bush Estate, Penicuik, Midlothian EH260QB, UK.
authors.forename: Y Sim
authors.initials: YS
authors.lastname: Tang
authors.affiliation: Centre for Ecology & Hydrology (CEH) Edinburgh, Bush Estate, Penicuik, Midlothian EH260QB, UK.
authors.forename: J Neil
authors.initials: JN
authors.lastname: Cape
mesh_terms: D064751:Ammonium Compounds; D017753:Ecosystem; D004784:Environmental Monitoring; D009566:Nitrates; D009584:Nitrogen / Q000737:chemistry / Q000378:metabolism*; D044003:Sphagnopsida / Q000502:physiology*
chemical_list: D064751:Ammonium Compounds; D009566:Nitrates; D009584:Nitrogen
keywords: Base cations; Dissolved organic nitrogen; Manipulation experiment; N uptake; Tissue N