Contrasting patterns of soil N-cycling in model ecosystems of Fennoscandian boreal forests

• The low plant productivity of boreal forests ingeneral has been attributed to low soil N supply and lowtemperatures. Exceptionally high productivity occurs intoe-slope positions, and has been ascribed to influx of Nfrom surrounding areas and higher rates of soil Nturnover in situ. Despite large apparent natural variationsin forest productivity, rates of gross soil N mineralizationand gross nitrification have never beencompared in Fennoscandian boreal forests of contrastingproductivity. We report contrasting patterns of soilN turnover in three model ecosystems, representing therange in soil C-to-N ratios (19–41) in Fennoscandianboreal forests and differences in forest productivity by afactor close to 3. Gross N mineralization was seventimes higher when soil, microbial, and plant C-to-Nratios were the lowest compared to the highest. Thisprocess, nitrification and potential denitrification correlatedwith inorganic, total and microbial biomass N, butnot microbial C. There was a constant ratio between soiland microbial C-to-N ratio of 3.7±0.2, across wide ratiosof soil C-to-N and fungi-to-bacteria. Soil N-cyclingshould be controlled by the supplies of C and N to themicrobes. In accordance with plant allocation theory, wediscuss the possibility that the high fungal biomass athigh soil C-to-N ratio reflects a particularly high supplyof plant photosynthates, substrates of high-quality C, tomycorrhizal fungi. Methods to study soil N turnoverand N retention should be developed to take into accountthe impact of mycorrhizal fungi on soil N-cycling.

Nyckelord

Bacteria/growth & development

Biomass

Ecosystem

Fungi/growth & development

Minerals/metabolism

Nitrates/metabolism

Nitrites/metabolism

Nitrogen/analysis/*metabolism

Photosynthesis

Population Dynamics

Soil

Soil Microbiology

Species Specificity

Trees/*physiology

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