INVESTIGATIONS ON THE NITROGEN-METABOLISM OF FOREST TREES BY MATHEMATICAL-MODELING OF NATURAL ISOTOPE RATIOS

Based on an experimental investigation of nitrogen concentrations and deltaN-15-values in different parts of Picea abies (L.) trees and soil samples and of their dependence on the age of plant parts and on dama ging stress, mathematical models which represent main features of nitr ogen turnover in the forest tree - soil system and associated isotope variations are developed. These models consider parts of 3 cycles of n itrogen: (1) transport of soluble nitrogen within trees, (2) turnover of protein nitrogen in different plant parts, and (3) the exchange of N between tree and soil via litter and the uptake of mineralized soil N by plant roots. Isotope effects of protein synthesis and mineralizat ion of organic soil N are included. The models give a qualitative expl anation of the data and lead to a better understanding of isotopic var iations as indicators of forest damage.