The aim of this study was to quantify the effects of fertiliser N on C stocks in trees (stems, stumps, branches, needles, and coarse roots) and soils (organic layer +0-10 cm mineral soil) by analysing data from 15 long-term (14-30 years) experiments in Picea abies and Pinus sylvestris stands in Sweden and Finland. Low application rates (30-50 kg N ha À1 year À1 ) were always more efficient per unit of N than high application rates (50-200 kg N ha À1 year À1 ). Addition of a cumulative amount of N of 600-1800 kg N ha À1 resulted in a mean increase in tree and soil C stock of 25 and 11 kg (C sequestered) kg À1 (N added) (''N-use efficiency''), respectively. The corresponding estimates for NPK addition were 38 and 11 kg (C) kg À1 (N). N-use efficiency for C sequestration in trees strongly depended on soil N status and increased from close to zero at C/N 25 in the humus layer up to 40 kg (C) kg À1 (N) at C/N 35 and decreased again to about 20 kg (C) kg À1 (N) at C/N 50 when N only was added.In contrast, addition of NPK resulted in high (40-50 kg (C) kg À1 (N)) N-use efficiency also at Nrich (C/N 25) sites. The great difference in N-use efficiency between addition of NPK and N at Nrich sites reflects a limitation of P and K for tree growth at these sites. N-use efficiency for soil organic carbon (SOC) sequestration was, on average, 3-4 times lower than for tree C sequestration. However, SOC sequestration was about twice as high at P. abies as at P. sylvestris sites and averaged 13 and 7 kg (C) kg À1 (N), respectively. The strong relation between N-use efficiency and humus C/N ratio was used to evaluate the impact of N deposition on C sequestration. The data imply that the 10 kg N ha À1 year À1 higher deposition in southern Sweden than in northern Sweden for a whole century should have resulted in 2.0 ± 1.0 (95% confidence interval) kg m À2 more tree C and 1.3 ± 0.5 kg m À2 more SOC at P. abies sites in the south than in the north for a 100-year period. These estimates are consistent with differences between south and north in tree C and SOC found by other studies, and 70-80% of the difference in SOC can be explained by different N deposition.

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