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

ABSTRACT

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.