Whole-tree seasonal nitrogen uptake and partitioning in adult Fagus sylvatica L. and Picea abies L. [Karst.] trees exposed to elevated ground-level ozone

The effect of long-term exposure of twice-ambient O3 (2 × O3) on whole-tree nitrogen (N) uptake and partitioning of adult beech and spruce was studied in a mixed forest stand, SE-Germany. N uptake as 15N tracer and N pools were calculated using N concentrations and biomass of tree compartments. Whole-tree N uptake tended to be lower under 2 × O3 in both species compared to trees under ambient O3 (1 × O3). Internal partitioning in beech showed significantly higher allocation of new N to roots, with mycorrhizal root tips and fine roots together receiving about 17% of new N (2 × O3) versus 7% (1 × O3). Conversely, in spruce, N allocation to roots was decreased under 2 × O3. These contrasting effects on belowground N partitioning and pool sizes, being largely consistent with the pattern of N concentrations, suggest enhanced N demand and consumption of stored N with higher relevance for tree-internal N cycling in beech than in spruce.