The commercial plantations of eucalyptus in Brazil are usually located in soils of low natural fertility, thus the addition of fertilizers is necessary in order to obtain high productivities. In spite of the high amount of nitrogen (N) accumulated in plantations of high productivity, increases in volume and biomass in response to the application of N fertilizers has not been expressive. The absence or little response of eucalyptus to N fertilization could be attributed to organic matter mineralization. However, recent observations, in two field experiments, showed contrasting response of eucalyptus to N application. In one case, the effect of ammonium sulphate could be attributed to the sulphur of the fertilizer, fact supported by a strong eucalypt response to single superphosphate in comparison with triple superphosphate. However, in the same site, the effect remained with the application of N either as ammonium-N or nitrate-N sources. Could this response be an indirect effect for the presence of other nutritional factors on the mineralization of soil organic matter? This research consisted of two experiments, aiming to evaluate the effect of N on the growth and chemical composition of eucalyptus. The first experiment, under field conditions, in the Itamarandiba County - MG, was carried out in a randomized block design with three replications, and consisted of the application of N rates from two sources, ammonium sulphate (AS) and ammonium nitrate (AN). Treatment effects were evaluated on 30 month-old trees by measuring stem volume, aboveground biomass and N accumulation in the trees. Soil and litter samples were randomly collected for chemical analyses. Total C and N content of the particulate organic matter (POM) and the organic matter associated to the mineral fraction (OMAM) were quantified by mass spectrometry (IRMS). The second experiment was conducted in a greenhouse, using a soil with higher and other with lower organic matter content, four rates of N and four rates of S, in a randomized block design with three replications. The N source was 15NH4 15NO3 with 60 % 15N atoms. Two clonal seedlings of eucalypts were grown into 3.6 L pots. Completely developed eucalypt leaves were collected at four different times after imposing the treatments for N determination by dry combustion in a continuous flow isotope ratio mass spectrometer to obtain the percentage of 15N atoms in excess, to assess the contribution of the applied fertilizer to plant N content. At the end of the experimental period (80 days) seedling height and biomass were determined and tissue samples submitted to nutrient determination. Nitrogen fertilization promoted increase in growth, in the field and in the greenhouse. Seedling growth rate in response to fertilizer was stronger when cultivated in the soil poorer in organic matter. Alterations in the C e N stocks in the POM and OMAM due to the nitrogen application could not be detected. However, there was an increase in the uptake of other nutrients. Apparent N recovery, in the field, was highest at the rate of 120 kg ha-1 of N, reaching 34,4 %. In the greenhouse trial N recovery from the fertilizer, as indicated by 15N, reached 30 % during the first 21 days, reducing thereafter.
