title: Nitrogen balance along a northern boreal forest fire chronosequence.
abstract: Fire is a major natural disturbance factor in boreal forests, and the frequency of forest fires is predicted to increase due to climate change. Nitrogen (N) is a key determinant of carbon sequestration in boreal forests because the shortage of N limits tree growth. We studied changes in N pools and fluxes, and the overall N balance across a 155-year non stand-replacing fire chronosequence in sub-arctic Pinus sylvestris forests in Finland. Two years after the fire, total ecosystem N pool was 622 kg ha-1 of which 16% was in the vegetation, 8% in the dead biomass and 76% in the soil. 155 years after the fire, total N pool was 960 kg ha-1, with 27% in the vegetation, 3% in the dead biomass and 69% in the soil. This implies an annual accumulation rate of 2.28 kg ha-1 which was distributed equally between soil and biomass. The observed changes in N pools were consistent with the computed N balance +2.11 kg ha-1 yr-1 over the 155-year post-fire period. Nitrogen deposition was an important component of the N balance. The biological N fixation increased with succession and constituted 9% of the total N input during the 155 post-fire years. N2O fluxes were negligible (≤ 0.01 kg ha-1 yr-1) and did not differ among post-fire age classes. The number and intensity of microbial genes involved in N cycling were lower at the site 60 years after fire compared to the youngest and the oldest sites indicating potential differences in soil N cycling processes. The results suggest that in sub-arctic pine forests, the non-stand-replacing, intermediate-severity fires decrease considerably N pools in biomass but changes in soil and total ecosystem N pools are slight. Current fire-return interval does not seem to pose a great threat to ecosystem productivity and N status in these sub-arctic forests.
authors.affiliation: Department of Forest Sciences, University of Helsinki, Helsinki, Finland.
authors.forename: Marjo
authors.identifier: http://orcid.org/0000-0001-9963-4748
authors.initials: M
authors.lastname: Palviainen
authors.affiliation: Department of Environmental and Biological Sciences, University of Eastern Finland, Kuopio, Finland.
authors.forename: Jukka
authors.initials: J
authors.lastname: Pumpanen
authors.affiliation: Department of Forest Sciences, University of Helsinki, Helsinki, Finland.
authors.forename: Frank
authors.initials: F
authors.lastname: Berninger
authors.affiliation: Department of Forest Sciences, University of Helsinki, Helsinki, Finland.
authors.forename: Kaisa
authors.initials: K
authors.lastname: Ritala
authors.affiliation: Institute of Mountain Hazards and Environment, Chengdu, China.
authors.forename: Baoli
authors.initials: B
authors.lastname: Duan
authors.affiliation: Department of Food and Environmental Sciences, University of Helsinki, Helsinki, Finland.
authors.forename: Jussi
authors.initials: J
authors.lastname: Heinonsalo
authors.affiliation: Department of Food and Environmental Sciences, University of Helsinki, Helsinki, Finland.|Collaborative Innovation Center of Sustainable Forestry in Southern China, College of Forestry, Nanjing Forestry University, Nanjing, China.
authors.forename: Hui
authors.initials: H
authors.lastname: Sun
authors.affiliation: Department of Forest Sciences, University of Helsinki, Helsinki, Finland.
authors.forename: Egle
authors.initials: E
authors.lastname: Köster
authors.affiliation: Department of Forest Sciences, University of Helsinki, Helsinki, Finland.
authors.forename: Kajar
authors.initials: K
authors.lastname: Köster
mesh_terms: D001110:Arctic Regions; D018533:Biomass; D057965:Carbon Sequestration; D004190:Disasters; D017753:Ecosystem; D005387:Finland; D005390:Fires; D065928:Forests; D009584:Nitrogen / Q000378:metabolism*; D028223:Pinus / Q000737:chemistry / Q000378:metabolism; D012987:Soil / Q000737:chemistry; D014197:Trees / Q000737:chemistry / Q000378:metabolism
chemical_list: D012987:Soil; D009584:Nitrogen