ABSTRACT. Signal grass (Urochloa decumbens) and guinea grass (Megathyrsus maximus) are African grasses that are well established in the Brazilian Savannah and we tested their adaptation to different light intensity. Plants were grown for 45 days under 0% shade (full sun) and 25, 40, and 80% induced shade to evaluate their photosynthetic performance. Light curves showed higher values of electron transport rate, photochemical quenching, and effective quantum yield in plants subjected to 0 and 25% shade for signal grass and in 25 and 40% shade for guinea grass. The potential quantum yield evaluations revealed that signal grass felt the effects of excessive light around 11:30 am on plants subjected to 0 and 25% shade. Conversely, guinea grass showed these photoinhibition effects at the same shade level but in a longer time range (9:30 am to 1:30 pm). As shade increased, there was a reduction tendency of the pigment content in signal grass and the opposite was observed for guinea grass. Stomatal conductance showed different values during the day and among different shade levels and there were no differences in relative water content between treatments and species. Results indicated better photosynthetic performance for signal grass under high intensity and better photosynthetic performance for guinea grass subjected to intermediate and higher levels of shade. Altogether, the results indicate that guinea grass seems to be a more appropriate choice for silvopasture systems.

Nitrogen, in general, is the largest limiting plant growth nutrient in the tropics and is required as a synthetic fertilizer to improve plants productivity. Therefore, studies aiming in understanding and using nitrogen fixation by leguminous trees have been done as a low-cost alternative for chemical fertilizer. Native legume trees such as Inga vera have been recommended in the rehabilitation of degraded areas due their ability to establish symbiosis with nitrogen fixation organisms replacing nitrogen fertilization. These species are able to increase soil organic matter, nitrogen and phosphorus availability. Thus, the present study aims to assess the inoculation effects of native rhizobia strains on nodulation, dry matter production, nitrogen and phosphorus leaf incorporation in I. vera seedlings. With this purpose, four strains were obtained from inga nodules and tested in a greenhouse. The inoculation of I, vera seedlings with native rhizobium strains promoted an increase in shoot dry mass as well as in leaf nitrogen content. According to symbiotic efficiency equation, this approach ranged from 50 to 80% indicating that the nitrogen fertilization for this species can be partially replaced by rhizobia inoculation.

O objetivo deste trabalho foi avaliar a diversidade funcional e genética de bactérias associadas à rizosfera de genótipos de milho contrastantes quanto à eficiência de uso de fósforo, por meio do teste de fontes de carbono no sistema EcoPlate e da eletroforese em gel de gradiente desnaturante (DGGE) dos fragmentos amplificados dos genes 16S ribossomais (rDNA) das bactérias. Foram coletadas amostras de solo da rizosfera de linhagens e híbridos contrastantes quanto à eficiência de uso de fósforo, cultivados em Latossolo Vermelho-Escuro fase cerrado, com baixo e alto teor de P. Bactérias da rizosfera de híbridos e linhagens eficientes, sob estresse de P, analisadas pelo sistema EcoPlate, tenderam a se agrupar conforme a análise de componentes principais, o que indica que utilizaram fontes de carbono semelhantes. Não houve diferença na diversidade bacteriana, analisada pela DGGE, entre bactérias associadas a genótipos eficientes e ineficientes no uso de P. Com base no sequenciamento do 16S rDNA, foi verificado que a rizosfera de genótipos de milho sob estresse de P parece selecionar grupos específicos de bactérias. A estrutura populacional genética e metabólica de bactérias da rizosfera foi mais influenciada pelo teor de fósforo no solo do que pela eficiência das plantas em usar o fósforo.

The objective of this work was to evaluate the functional and genetic diversity of bacteria associated to the rhizosphere of maize genotypes contrasting for phosphorus use efficiency by means of the EcoPlate carbon source test and denaturing gradient gel electrophoresis (DGGE) of amplified 16S ribosomal DNA (rDNA) fragments of bacteria. Rhizosphere soil samples of maize genotypes (hybrids and lineages) contrasting for phosphorus use efficiency cultivated in an Oxisol with high and low P content were collected. Bacteria from the rhizosphere of P-efficient maize genotypes under P stress conditions analyzed by the EcoPlate system tended to group together according to main components analysis, which indicates that these bacteria used similar carbon sources. Microbial diversity analyzed by DGGE did not differ between P-efficient and inefficient maize genotypes. Based on the 16S rDNA fragment sequencing, the rhizosphere of maize genotypes growing on low-P soils seemed to select specific bacteria species. The genetic and metabolic structure of the rhizosphere bacterial community was more strongly influenced by the level of P in the soil than by the genotypes with contrasting P-use efficiency.

O objetivo foi acompanhar a decomposição de folhas de D. nigra e de E. grandis em laboratório, enterradas por 12 meses, em solo de Mata Atlântica e de eucaliptal com 40 anos de plantio. Os valores de pH obtidos para os solos de mata e de eucaliptal foram 5,61 e 4,47 e de alumínio 0,32 e 1,89mEqx100, respectivamente. Um grande número de microrganismos (fungos totais, solubilizadores de fosfato e de celulolíticos) foram encontrados no solo de mata. Uma perda de massa significativa foi observada após 4 meses de decomposição nas folhas de ambas espécies no solo de mata (30%), enquanto em solo de eucaliptal esta perda foi observada somente após 8 meses (40%). O conteúdo inicial de N e P em folhas de D. nigra é maior do que nas de E. grandis, embora os conteúdos de lignina e celulose sejam semelhantes. A razão C/N é maior nas folhas do eucalipto, indicando que sua degradação deve ser lenta. Apesar disto, não foram observadas diferenças significativas nas taxas de decomposição entre os tratamentos. Ocorreu uma perda significativa de P, lignina e celulose nos tratamentos. Estes resultados mostraram diferenças entre a decomposição de ambas as espécies e a influência do solo de mata e de eucaliptal no processo de decomposição.

The objective was to evaluate the decomposition of D. nigra and E. grandis leaves under laboratory conditions when incubated in soils from Atlantic Forest and from a 40-year-old eucalypt plantation for a year. The obtained values of pH in forest and eucalypt plantation soils were 5.61 and 4.47 and for aluminum 0.32 and 1.89mEqx100, respectively. A great number of microorganisms (total fungi, phosphate solubilising and cellulolytic organisms) were found in forest soil. Litterbag study revealed a higher mass loss of the leaves in forest soil in the first four months (c. 30%). In the eucalypt plantation soils loss was observed only after eight months (c. 40%). The initial contents of N and P were greater in D. nigra than in E. grandis leaves, although lignin and cellulose contents were similar. The C/N ratio was higher in E. grandis indicating that its degradation could be slower. Nonetheless, there were no significant differences in the decay rates (k) among the treatments. Phosphorus, lignina and cellulose were lost at the end of the experiment in all treatments. The results show the differences between decomposition of both species and the influence of forest and eucalypt plantation soils in the decomposition process.