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Nitrogen is the nutrient mainly limiting forest growth on mineral soil sites in the boreal regions. The objective of this study was to find out the response of stem wood N to repeated fertilizations and to find out their long-lasting effects on soil organic matter composition, focusing on C and N cycling processes and concentrations of condensed tannins. The site was located in a relatively unfertile Scots pine ( Pinus sylvestris  L.) stand in eastern Finland. The treatments were three levels of N fertilization (0, 150, 300 kg/ha) applied four times at 5-year intervals with the last addition 29 years ago. The N additions had not changed the pH of the humus layer but resulted in higher availability of N. The C-to-N ratio of organic matter decreased with increasing N addition. The treatment of 300 kg/ha increased the net N mineralization rate and the ratio of net N mineralization/microbial biomass N and decreased the amount of C in the microbial biomass and its C-to-N ratio and the concentration of condensed tannins. Net nitrification and extractable nitrate were negligible in all soils. In soil diffusive fluxes, NH 4 -, NO 3 - and amino acid-N were all detected by in situ microdialysis sampling; the results showed large variation but supported higher N availability in N fertilized soil. The N fertilization increased tree-ring widths and the effect lasted for about 10 years after the last fertilization event. Nitrogen content and the N isotopic ratio  15 N/ 14 N ( δ  15 N) in tree-rings increased both after the first N addition in the treatment of 300 kg/ha. In conclusion, soil properties still indicated higher N availability in the N fertilized soil after three decades since the latest fertilization, but the response of tree diameter growth had faded out after a much shorter period.

Long-term response of soil and stem wood properties to repeated nitrogen fertilization in a N-limited Scots pine stand