Analysis of uptake and allocation of nitrogen and sulphur compounds by trees in the field

Due to anthropogenic activities trees that often occupy nitrogen-poor environments are exposed to high loads of atmospheric nitrogen. Nitrogen influx into the leaves by dry and wet deposition may then contribute a considerable fraction to total nitrogen required for growth and development of trees. This uncontrolled influx does not only affect nitrogen, but also sulphur metabolism, because of regulatory interactions of nitrogen and sulphur assimilation by inter-pathway control. To assess the metabolic consequences of nitrogen influx via the leaves, analysis of its influence on the uptake of nitrogen and sulphur compounds by the roots and the distribution of these compounds within the trees are required. Techniques that may be used for this purpose under field conditions include (a) depletion of artificial soil water for the analysis of root uptake, (b) xylem sap extraction by pressure and displacement, (c) xylem sap flow analysis by heat balance methods, and (d) phloem sap exudation by extraction of bark segments. To quantify the contribution of gaseous nitrogen influx into the leaves to total nitrogen nutrition, additional trace gas exchange studies by dynamic chamber techniques are required. In the present report, these techniques are discussed with special emphasis on the application under field conditions.