Doses e fontes de nitrogênio na produtividade do eucalipto e nas frações da matéria orgânica em solo da região do cerrado de Minas Gerais
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2011-11-08
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Revista Brasileira de Ciência do Solo
Resumo
Apesar da grande quantidade de N acumulada em plantações de eucalipto de alta produtividade, o aumento em volume do tronco em resposta à aplicação de N não tem sido expressivo nem consistente. O objetivo deste trabalho foi verificar o efeito de doses e fontes de N sobre o crescimento e o acúmulo de N em plantas de eucalipto, na serapilheira, além do impacto nas frações da matéria orgânica do solo (MOS). O experimento foi instalado em campo, no município de Itamarandiba-MG, em blocos ao acaso com três repetições, consistindo da aplicação em cobertura de doses (0, 60, 120 e 240 kg ha^-1) e fontes de N distintas (sulfato de amônio e nitrato de amônio) em clone de eucalipto (AEC1528®). O efeito dos tratamentos sobre o crescimento e acúmulo de N nas plantas foi avaliado aos 30 meses de idade, abatendo-se árvores com DAP médio e separando-as em lenho, casca, galhos e folhas, para determinação da produção de matéria seca e dos teores e conteúdos de nutrientes das plantas. Amostras de solo e de serapilheira foram coletadas para análises de nutrientes. Os teores de C e N total da matéria orgânica particulada (MOP) e da matéria orgânica associada à fração mineral (MOAM) foram determinados por espectrometria de massa de razão isotópica, após separação física da MOS. As análises estatísticas consistiram de análise de variância e de regressão. A aplicação de adubos nitrogenados promoveu aumento no crescimento volumétrico do tronco e na matéria seca da parte aérea. A dose de N como sulfato de amônio para obter 90 % da produção máxima foi de 74 kg ha^-1, a qual resultou em incremento de 42,3 % no volume de tronco em relação à testemunha sem adubação nitrogenada. Na dose de 120 kg ha^-1 de N, não houve diferença de resposta à aplicação de sulfato de amônio e nitrato de amônio. Não foram detectadas alterações nos estoques de C e N da MOS com a adubação nitrogenada. No entanto, houve aumento da absorção de Ca, Mg e S. A taxa de recuperação aparente de N no campo foi maior na dose de 120 kg ha^-1 de N, atingindo 34,4 %.
In spite of the high amount of N accumulated in high-yielding, short-rotation eucalyptus plantations, no significant or consistent increases in trunk volume have been observed in response to N fertilization. This experiment evaluated the effect of N fertilization on the growth and chemical composition of eucalyptus as well as its role in soil organic matter (SOM) mineralization. The field experiment was carried out in a randomized block design with three replications, in Itamarandiba- MG. Two N sources (ammonium sulphate and ammonium nitrate) of increasing N rates (0, 60, 120, 240 kg ha^-1) were tested on the eucalyptus clones AEC1528®. The effect of the treatments on tree growth and N accumulation were evaluated in 30 month-old trees with average DBH, which were harvested and separated in stem, bark, branches, and leaves to determine dry matter production and nutrient contents. Soil and forest litter samples were collected for chemical analyses. Total C and N content of particulate organic matter (POM) and organic matter associated to the mineral fraction (OMAM) were quantified by mass spectrometry (IRMS) after physical separation of SOM. The statistical analyses consisted of variance analysis and regression. Nitrogen fertilization increased trunk volume and aerial dry matter. The estimated N rate to obtain 90 % of the maximum trunk volume was 74 kg ha^-1 in the form of ammonium sulfate, and increased the yield by 42.3 % in comparison to the control plot without N fertilization. In response to N rates of 120 kg ha^-1, no difference was observed between ammonium sulfate and ammonium nitrate. No alterations in SOM C and N stocks were detected due to N application. However, there was an increase in Ca, Mg and S absorption. The apparent N recovery rate in the field was highest at the N rate of 120 kg ha^-1, reaching 34.4%.
In spite of the high amount of N accumulated in high-yielding, short-rotation eucalyptus plantations, no significant or consistent increases in trunk volume have been observed in response to N fertilization. This experiment evaluated the effect of N fertilization on the growth and chemical composition of eucalyptus as well as its role in soil organic matter (SOM) mineralization. The field experiment was carried out in a randomized block design with three replications, in Itamarandiba- MG. Two N sources (ammonium sulphate and ammonium nitrate) of increasing N rates (0, 60, 120, 240 kg ha^-1) were tested on the eucalyptus clones AEC1528®. The effect of the treatments on tree growth and N accumulation were evaluated in 30 month-old trees with average DBH, which were harvested and separated in stem, bark, branches, and leaves to determine dry matter production and nutrient contents. Soil and forest litter samples were collected for chemical analyses. Total C and N content of particulate organic matter (POM) and organic matter associated to the mineral fraction (OMAM) were quantified by mass spectrometry (IRMS) after physical separation of SOM. The statistical analyses consisted of variance analysis and regression. Nitrogen fertilization increased trunk volume and aerial dry matter. The estimated N rate to obtain 90 % of the maximum trunk volume was 74 kg ha^-1 in the form of ammonium sulfate, and increased the yield by 42.3 % in comparison to the control plot without N fertilization. In response to N rates of 120 kg ha^-1, no difference was observed between ammonium sulfate and ammonium nitrate. No alterations in SOM C and N stocks were detected due to N application. However, there was an increase in Ca, Mg and S absorption. The apparent N recovery rate in the field was highest at the N rate of 120 kg ha^-1, reaching 34.4%.
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Palavras-chave
Ammonium, Eucalyptus sp., Soil organic matter, Nitrate, Forest litter