Débora V. Aragão

Correlation but no causation between leaf nitrogen and maximum assimilation: The role of drought and reproduction in gas exchange in an understory tropical plant Miconia ciliata (Melastomataceae)

Alternative hypotheses were tested to explain a previously reported anomaly in the response of leaf photosynthetic capacity at light saturation (A(max)) in Miconia ciliata to dry-season irrigation. The anomaly is characterized by an abrupt increase in leaf A(max) for nonirrigated plants at the onset of the rainy season to values that significantly exceeded corresponding measurements for plants that were irrigated during the previous dry season. Hypothesis 1 posits that a pulse in leaf nitrogen increases CO(2) assimilation in nonirrigated plants at the onset of the wet season and is dampened for irrigated plants; this hypothesis was rejected because, although a wet-season nitrogen pulse did occur, it was identical for both irrigated and nonirrigated plants and was preceded by the increase in assimilation by nonirrigated plants. Hypothesis 2 posits that a reproduction-related, compensatory photosynthetic response occurs in nonirrigated plants following the onset of the wet season and is dampened in irrigated plants; consistent with hypothesis 2, high maximum assimilation rates for control plants in the wet season were significantly correlated with fruiting and flowering, whereas irrigation caused flowering and fruiting in the dry season, spreading M. ciliata reproductive activity in irrigated plants across the entire year.

Avaliação de indicadores de qualidade do solo sob alternativas de recuperação do solo no Nordeste Paraense

Os indicadores convencionais de fertilidade de solo, mais comumente utilizados, enfatizam o balanco acido-base e o conteudo de elementos quimicos, mas nao consideram adequadamente as propriedades biologicas do solo, responsaveis por grande parte do seu funcionamento. Os indicadores biologicos, ainda pouco difundidos, levam em consideracao a atividade microbiana do solo que e mais sensivel a dinâmica do solo. Em Marapanim-PA, e utilizando um desenho experimental inteiramente casualizado, com quatro repeticoes, avaliou-se o comportamento dos indicadores do solo carbono e nitrogenio da biomassa microbiana, fosfatase acida, materia orgânica, carbono orgânico e fosforo disponivel, em um solo com cultivo sequencial de mandioca e milho, preparo com corte-e-trituracao da capoeira original. Os fatores corte-e-trituracao, fosfato natural e adubacao verde foram arranjados nos seguintes tratamentos: capoeira; testemunha; fosfato natural (FN); feijao-de-porco [Canavalia ensiformis (L.) DC] (FP); FN + FP; FN + guandu [Cajanus cajan (L.) Millspaugh]. (G); FN + titonia [Tithonia diversifolia (Hemsley) A. Gray] (T); FP + G; FP + T. Em quatro epocas de amostragens, foram tiradas amostras compostas do solo (0 - 10 cm) de cada parcela para analise dos indicadores. Os indicadores estudados foram influenciados pela epoca de amostragem do solo. Os indicadores fosfatase acida, nitrogenio da biomassa microbiana e carbono da biomassa microbiana foram capazes de detectar os efeitos de maior numero de tratamentos de recuperacao do solo testados, enquanto a materia orgânica do solo foi o menos capaz. Ao longo do tempo, a resposta dos indicadores variou em funcao dos fatores estudados.