ABSTRACT: Soil enzymes play a fundamental role in nutrient cycling in forest systems. The stoichiometry of C, N, and P–acquiring enzymes has been used to indicate nutrient limitation in the soil. However, the enzymatic stoichiometry remains poorly understood in pure and mixed eucalypt plantations. Thus, this study aims to assess the activity of enzymes in the soil to address the hypothesis that the introduction of N2-fixing trees could influence the enzymatic stoichiometry on C, N, and P cycling. The activity of β-glucosidase (BG), urease (U), and acid phosphatase (AP) was assessed in soil (0-20 cm depth) of pure Eucalyptus grandis without (E) and with N fertilization (E+N), and a mixed system with E. grandis and Acacia mangium (E+A), and a pure A. mangium (A) plantation at 27 and 39 months after planting. The activities of BG/U, BG/AP, and U/AP were used to calculate the enzyme C/N, C/P, and N/P ratios, respectively. Rates of N–acquiring enzymes were higher in E and E+N, while soil microorganisms invested in P–acquiring enzymes in A and E+A. The vector length and angle demonstrated that C demand by microorganisms does not change in relation to N and P, regardless of the treatment. However, N demand decreased in relation to P in A and E+A (mainly at 27 months). Our results suggest that enzymes activity in pure eucalypt systems is limited for their soil-litter nutrient contents. At the same time while acacia and mixed plantation seem to invest in P–acquiring enzymes to improve biological N2 fixation promoted by diazotrophic bacteria associated to acacia.

RESUMO A mandioca é, geralmente, plantada em solos arenosos e de baixa fertilidade, tornando eminente a necessidade da mesma em realizar associações benéficas com micro-organismos que possam auxiliar seu crescimento. Nessa ótica, é evidente a possibilidade de seleção de isolados bacterianos eficientes em promover o crescimento da cultura, os quais podem fornecer subsídios para futuros inoculantes. O objetivo deste trabalho foi isolar, identificar, selecionar e avaliar a diversidade genética de bactérias endofíticas de raízes e caules de mandioca, cultivada em Garanhuns - PE, com características envolvidas na promoção de crescimento vegetal. O isolamento foi realizado em meio de cultura semissólido LGI-P. Os isolados selecionados foram avaliados quanto ao potencial de fixar N2, quanto à capacidade de produzir ácido indol acético, quanto à capacidade de solubilizar fosfato inorgânico e produzir exopolissacarídeos. Alguns isolados bacterianos tiveram o gene 16S rRNA sequenciados pelo método de Sanger. No total, foram obtidos 52 isolados de bactérias endofíticas de mandioca. Em relação ao potencial para fixar N2, 15% dos isolados foram positivos. Quanto à produção de AIA, 78% dos isolados produziram este fitohormônio em meio com o acréscimo de L-triptofano. Cerca de 31% dos isolados foram capazes de solubilizar fosfato inorgânico e 60% apresentaram produção de exopolissacarídeos. A identificação de 19 isolados possibilitou o agrupamento em seis gêneros bacterianos, sendo eles: Achromobacter, Bacillus, Burkholderia, Enterobacter, Pantoea e Pseudomonas. Plantas de mandioca cultivadas em Garanhuns - PE apresentam interação com diferentes grupos de bactérias endofíticas e há grupos bacterianos com diversas características envolvidas na promoção de crescimento vegetal.

ABSTRACT Cassava is mostly planted in sandy soils which are usually of low fertility, thereby making it necessary to perform beneficial associations with microorganisms that can promote their growth. In this perspective, the possibility of selecting bacterial isolates efficient in promoting the growth of the culture is evident, which can provide subsidies for future inoculants. The objective of this study was to isolate, identify, select and evaluate the genetic diversity of endophytic bacteria in roots and stems of cassava grown in Garanhuns - PE, with features involved in promoting plant growth. The isolation was performed on culture medium semisolid LGI-P. The selected isolates were evaluated for the potential to fix N2, as the ability to produce indole acetic acid, for their ability to solubilize inorganic phosphate and produce exopolysaccharides. Some bacterial isolates had their 16S rRNA gene sequenced by the Sanger method. A total of 52 endophytic bacteria isolates were obtained from cassava. Regarding the potential to fix N2, 15% of the isolates were positive. As for the production of IAA, 78% of the isolates produced this phytohormone in a medium with increased L-tryptophan. Approximately 31% of the isolates were able to solubilize inorganic phosphate and 60% had exopolysaccharide. The identification of 19 isolates allowed the grouping into six bacterial genera, namely: Achromobacter, Bacillus, Burkholderia, Enterobacter, Pantoea and Pseudomonas. Cassava plants grown in Garanhuns - PE present interaction with different groups of endophytic bacteria and there are bacterial groups with several characteristics involved in promoting plant growth.