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

We investigated the transfer of N into the soil via N 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 (NH)SO and the other half equivalent amounts of (NH)SO at N natural abundance level. N was transferred from one side of the root system (N side) to the other side (N 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 (NH)SO were used, corresponding with annual N deposition in the Netherlands (30 kg N ha) and a twelfth of that (2.5 kg N ha). 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 N, while Norway spruce took up 21.5 and 32.1 %, respectively. Both species transported proportions of N 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.