Nitrogen uptake, retranslocation and potential N₂-fixation in Scots pine and Norway spruce seedlings

Scots pine (Pinus sylvestris L.) has higher early growth rates than Norway spruce (Picea abies (L.) H. Karst.). To help efforts to understand possible reasons for this difference and identify appropriate regeneration methods, we labeled seedlings with ¹⁵N in the nursery to probe nitrogen (N) uptake and retranslocation in Scots pine and Norway spruce seedlings at both harsh and fertile sites in southern and northern Sweden. For this, ¹⁵N dilution was measured during the first two years after planting. We also examined the potential N₂-fixation capacity in fine roots after 5–7 growing seasons. Use of ¹⁵N-labeled seedlings enabled clear discernment of contributions of uptake of new N and retranslocation of old N to new foliage. Scots pine seedlings had higher proportions of N derived from uptake (N_dfu) than Norway spruce seedlings, and higher growth. Scots pine seedlings were less responsive to site preparation treatments compared to Norway spruce that has a greater need for appropriate silvicultural measures to grow well. After the second growing season, the contribution of N derived from retranslocation (N_dfr) to N in new foliage had diminished to 10–20% at all except the harshest site, where both species were more dependent on N_dfr. The potential N₂-fixation capacity in fine roots of the two species differed, but in both cases the contribution of N₂-fixation to N-acquisition was negligible.