1. Impact of long-term N fertilisation on C stocks in trees and soils in northern Europe Riitta Hyvönen, Tryggve Persson, Stefan Andersson, Bengt Olsson, Göran Ågren & Sune Linder Biogeochemistry (Online)

2. Objectives To determine the efficiency of added N on C storage in trees and soils in 15 long-term experiments in Sweden and Finland To use data on N-use efficiency to draw conclusions about the role of 100 years of N deposition on C stocks

4. Methods Long-term (14-30 yrs) experiments with repeated (3-30 times) addition of N and/or NPK in doses of 30-200 kg N ha -1 occasion -1. Tree biomass: Stem diam, no. of trees ha -1 and Marklund (1988) expansion factors, C=50% of dry wt Soil sampling: Organic layer + 0-10 cm min. soil on, at least, one occasion

5. Tree biomass C (above and below ground) in control plots (0) and after annual additions of ammonium nitrate (N), potassium and phosphorus (PK) or NPK at different doses (N1=33, N2=67, N3=109 kg N ha -1 yr -1 ) at two sites.

6. Cum. increase in tree biomass C in excess of control plots as a function of the cumulative amount of N added. N1=33, N2=67, N3=109 kg N ha -1 yr -1. Conclusion: Low application doses of N more efficient to sequester C than high N doses. Example: 35, 17, 9 kg (C) kg -1 (N) for N1, N2 and N3, respectively, at Norrliden).

7. N-use efficiency for C sequestration in tree biomass as a function of C/N ratios in humus layers of control plots after addition of N (left) or NPK (right) fertiliser at P. abies (PA) and Pinus sylvestris (PS) sites. Conclusion: Soil N status (C/N) seems to regulate N-use efficiency in trees, but low C/N ratio often reflects low availability of P and K as seen by NPK addition.

8. N-use efficiency for C sequestration in soil (org. layer + 0-10 cm min. soil) as a function of C/N ratios in humus layers of control plots after addition of N or NPK fertilisers

9. Relationship between N-use efficiencies in trees and soils for P. abies and P. sylvestris sites with both tree and soil studies. Figures from both N and NPK fertilisation. Conclusion: (1) Higher SOC seq. in P abies than in P sylvestris soils. (2) High SOC seq. also when trees don´t sequester extra C indicates reduced decomposition.

10. Mean (  SE, n=4) C pools, C mineralisation rates (at 15°C) and calculated field Rh to a depth of 10 cm in the mineral soil at Stråsan in control (0) and N-fertilised (N1) treatments in 2004. N1 had received, on average, 33 kg N ha -1 y -1 during 39 yrs.

11. Estimates of cumulative N deposition in S and N Sweden during 1900-2000 and resulting tree and soil C pools as calculated from the N-use efficiencies

12. Mean SOC content (±95% c.i.) for forest floor and total soil at dry and mesic forest sites in southern (55-60 o N, n= 445), central (60 – 64 o N, n=448) and northern Sweden (64-68 o N, n=329). After Mats Olsson, pers. comm.

13. Conclusions N-use efficiencies for trees (P abies) varied (0-50 kg (C) kg -1 (N)) depending on C/N ratio and PK availability. N-use efficiencies for SOC was generally lower than for trees and averaged 13 kg (C) kg -1 (N)) for P abies and 7 for P sylvestris. 10 kg N ha -1 yr -1 higher N deposition in S than in N Sweden for a whole century should have resulted in 2.0±1.0 kg m -2 more tree C and 1.3±0.5 kg m -2 more SOC.