Seasonal N changes in alnus orientalis and populus nigra and N2 fixation by exotic alder species in Syria

Two experiments were conducted.The first was to study nodulation and N2 fixation of several introduced alder species (Alnus glutinosa, A. incana, A. rubra and A. viridis) grown in soil from beneath Alnus orientalis. The second was to determine pattern of N changes in leaves and bark of Alnus orientalis and populus nigra natural stands during two successive years. Results showed that frankia in soil from underneath Alnus orientalis nodulated and fixed nitrogen on roots of local alder as well as on roots of introduced alder species from distant and ecologically diverse localities. However, differences were found among species in the number of nodules formed and amount of nitrogen fixed. Percentages of nitrogen derived from atmosphere (%Ndfa) ranged from 5% in A.viridis to 66% in A. orientalis. Microscopic study of Alnus orientalis nodules showed the presence of vesicles, and frankia belonging to Sp-type. Foliar N concentration was higher in alder than in poplar. Total N concentration in alder leaves remained relatively constant at about 3% during summer, whereas N concentration in poplar decreased sharply in leaves and increased in bark. No substantial increase in N concentration was found in alder bark, and the fallen leaves were rich in nitrogen. During autumn, leaf N concentration in poplar decreased by 43% and 51% for the first and the second year, respectively, whereas N concentrations in bark increased by 71% and 100%. Total N concentrations in alder leaves decreased only by 8-16% while, values in the adjacent bark remained stable. In contrast to poplar, it seems that Alnus orientalis does not exhibit net leaf retranslocation to bark tissues.(author)