ANALYSIS OF UPTAKE AND ALLOCATION OF NITROGEN AND SULFUR-COMPOUNDS BYTREES IN THE FIELD

Due to anthropogenic activities trees that often occupy nitrogen-poor environments are exposed to high loads of atmospheric nitrogen. Nitrog en influx into the leaves by dry and wet deposition may then contribut e a considerable fraction to total nitrogen required for growth and de velopment of trees. This uncontrolled influx does not only affect nitr ogen, but also sulphur metabolism, because of regulatory interactions of nitrogen and sulphur assimilation by inter-pathway control. To asse ss the metabolic consequences of nitrogen influx via the leaves, analy sis of its influence on the uptake of nitrogen and sulphur compounds b y the roots and the distribution of these compounds within the trees a re required. Techniques that may be used for this purpose under field conditions include (a) depletion of artificial soil water for the anal ysis of root uptake, (b) xylem sap extraction by pressure and displace ment, (c) xylem sap flow analysis by heat balance methods, and (d) phl oem sap exudation by extraction of bark segments. To quantify the cont ribution of gaseous nitrogen influx into the leaves to total nitrogen nutrition, additional trace gas exchange studies by dynamic chamber te chniques are required. In the present report, these techniques are dis cussed with special emphasis on the application under field conditions .