title: Contrasting patterns of soil N-cycling in model ecosystems of Fennoscandian boreal forests.
abstract: The low plant productivity of boreal forests in general has been attributed to low soil N supply and low temperatures. Exceptionally high productivity occurs in toe-slope positions, and has been ascribed to influx of N from surrounding areas and higher rates of soil N turnover in situ. Despite large apparent natural variations in forest productivity, rates of gross soil N mineralization and gross nitrification have never been compared in Fennoscandian boreal forests of contrasting productivity. We report contrasting patterns of soil N turnover in three model ecosystems, representing the range in soil C-to-N ratios (19-41) in Fennoscandian boreal forests and differences in forest productivity by a factor close to 3. Gross N mineralization was seven times higher when soil, microbial, and plant C-to-N ratios were the lowest compared to the highest. This process, nitrification and potential denitrification correlated with inorganic, total and microbial biomass N, but not microbial C. There was a constant ratio between soil and microbial C-to-N ratio of 3.7+/-0.2, across wide ratios of soil C-to-N and fungi-to-bacteria. Soil N-cycling should be controlled by the supplies of C and N to the microbes. In accordance with plant allocation theory, we discuss the possibility that the high fungal biomass at high soil C-to-N ratio reflects a particularly high supply of plant photosynthates, substrates of high-quality C, to mycorrhizal fungi. Methods to study soil N turnover and N retention should be developed to take into account the impact of mycorrhizal fungi on soil N-cycling.
authors.affiliation: Department of Forest Ecology, Swedish University of Agricultural Sciences, SLU, 90183 Umeå, Sweden. mona.n.hogberg@sek.slu.se
authors.forename: Mona N
authors.initials: MN
authors.lastname: Högberg
authors.forename: David D
authors.initials: DD
authors.lastname: Myrold
authors.forename: Reiner
authors.initials: R
authors.lastname: Giesler
authors.forename: Peter
authors.initials: P
authors.lastname: Högberg
mesh_terms: D001419:Bacteria / Q000254:growth & development; D018533:Biomass; D017753:Ecosystem; D005658:Fungi / Q000254:growth & development; D008903:Minerals / Q000378:metabolism; D009566:Nitrates / Q000378:metabolism; D009573:Nitrites / Q000378:metabolism; D009584:Nitrogen / Q000032:analysis / Q000378:metabolism*; D010788:Photosynthesis; D011157:Population Dynamics; D012987:Soil; D012988:Soil Microbiology; D013045:Species Specificity; D014197:Trees / Q000502:physiology*
chemical_list: D008903:Minerals; D009566:Nitrates; D009573:Nitrites; D012987:Soil; D009584:Nitrogen