Split-root labelling to investigate ¹⁵N rhizodeposition by Pinus sylvestris and Picea abies

We investigated the transfer of ¹⁵N into the soil via ¹⁵N uptake and release by tree roots, which involves the principles of the split-root technique. One half of the root system received an injection of (¹⁵NH₄)₂SO₄ and the other half equivalent amounts of (NH₄)₂SO₄ at ¹⁵N natural abundance level. ¹⁵N was transferred from one side of the root system (¹⁵N side) to the other side (¹⁴N side) and released into the soil. The method was conducted with Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies [L.] Karst). Two concentration levels of (NH₄)₂SO₄ were used, corresponding with annual N deposition in the Netherlands (30 kg N ha^–1) and a twelfth of that (2.5 kg N ha⁻¹). Samples were taken 3 and 6 weeks after labelling and divided into needles + stem, roots, rhizosphere and bulk soil. Already 3 weeks after labelling, Scots pine took up 23.7 % of the low and 9.1 % of the high amounts of ¹⁵N, while Norway spruce took up 21.5 and 32.1 %, respectively. Both species transported proportions of ¹⁵N to the rhizosphere (0.1–0.2 %) and bulk soil (0.3–0.9 %). The method is a useful tool to investigate the fate of root-derived N in soils, for example, for the formation of stable forms of soil organic matter.