Ciclo interno do azoto em arvores

Nitrogen (N) is a macro nutrient that often limits plant productivity, strongly affecting carbon assimilation and plant growth. N availability for plants conditions leaf growth and crown development (foliar area) as well as photosynthetic efficiency and plays a key role in biomass partitioning either between ephemeral and permanent components or aereal and subterranean components. As they grow, trees incorporate N from several external sources. Soil organic matter mineralization, N fertilization and atmospheric deposition are all important sources of N in forest ecosystems. In some cases biological N-fixing is also important. Besides external N sources, trees also use N stored in their own tissues to grow, through the process of internal cycling. In temperate and boreal zone species, this includes seasonal storage of N, followed by remobilization during growth periods. Internal cycling allows the seasonal growth of trees to be independent of nutrient uptake and can contribute with a significative portion of the annual growth requirements of N. Internal cycling must also be considered as a determinant factor to explain long-term responses to fertilizer application by the trees. In the present work the processes of internal cycling of N were quantified in tree species presenting different growth rates and phenology: blue-gum (Eucalyptus globulus Labill.), a fast growing, evergreen species; cork-oak (Quercus suber L.), also evergreen, but slower growing and ash (Fraxinus angustifolia Vah1.) a deciduous, riparian species. Young trees were grown in sand culture and fertilized with nutrient solutions containing contrasting concentrations of NH_4NO_3 allowing the plants to grow at different rates and have different amounts of nitrogen, and consequently pre-conditioning the storage and recycling capacity of the trees. Isotope labelling techniques were used to quantify the internal cycling of N. "1"5N labelling allowed for the direct quantification of remobilization or uptake of N in the trees. Internal cycling of N were studied in relation to nitrogen supply in Eucalyptus globulus and Quercus suber, and in relation to elevated CO_2 in Fraxinus angustifoliaAvailable from Fundacao para a Ciencia e a Tecnologia, Servico de Informacao e Documentacao, Av. D. Carlos I, 126, 1249-074 Lisboa, Portugal / FCT - Fundação para o Ciência e a TecnologiaSIGLEPTPortuga