No evidence of positive feedback between litter deposition and seedling growth rate in Neotropical savannas

Purpose: Plant-soil feedbacks are important drivers of ecosystem dynamics and have been hypothesized to affect woody encroachment in savannas. Woody encroachment is expected to increases savanna soil fertility through deposition of organic matter, favoring further establishment of woody individuals. In this context, we tested if litter input promotes forest seedling growth in dystrophic savanna soils, and if this was accompanied by an increase in microbial activity. Methods: In a glasshouse experiment, we planted woody seedlings of three forest species in savanna soils either mixed or not (control) with litter from closely related savanna or forest species (10 species). We evaluated the growth of the woody seedlings as well as the response of the soil microbiota activity and biomass to litter addition. Results: Litter addition had either no effect or negative effects on seedling growth, and different seedling species responded differently to litter addition. However, we did find microbial activity to increase in response to litter addition, especially through the input of litter rich in phosphorus (P) and carbon (C). Conclusions: Our results indicate that litter input does not favor woody seedlings growth in savanna soil. Instead, litter input showed a potential to hinder seedling growth, especially of fast-growing species. Furthermore, litter input consistently increased soil microbiota activity, mainly through the input of P and C, highlighting the importance of energy and P in the nutrient dynamics of Cerrado. Thus, our results did not support the hypothesis that litter deposition triggers a positive feedback with woody encroachment via increased seedling growth

Redox activities and ROS, NO and phenylpropanoids production by axenically cultured intact olive seedling roots after interaction with a mycorrhizal or a pathogenic fungus

Las raíces de las plántulas de olivo, en cultivo axénico, fueron colocadas alternativamente en contacto con Rhizophagus irregulares (micorrícicos) o con hongos Verticillim dahliae (patógenos). También se incluyeron tratamientos MeJA. Las raíces intactas (generación de anión superóxido, superóxido dismutasa y actividades de peroxidasa) se midieron en las actividades in vivo del apoplasto. Todos nuestros resultados mostraron que las actividades redox apoplásticas de raíces de las plántulas intactas en contacto con el hongo micorriza compatible fueron claramente atenuados en comparación con el hongo patógeno o tratado con MeJA, incluso en las primeras etapas usadas en el tratamiento. Los fenoles totales, flavonoides y glucósidos fenilpropanoides, también fueron cuantificados. Las raíces en contacto con el hongo micorriza no mejoraron la biosíntesis de compuestos fenólicos con respecto a los controles, mientras que los de contacto con el patógeno mejoraron de forma significativa la biosíntesis de todas las fracciones fenólicas medidas. Las especies reactivas del oxígeno y la acumulación de óxido nítrico en las raíces fueron examinadas por microscopía de fluorescencia. Todos ellas presentaron una acumulación mucho mayor en las raíces en contacto con el patógeno que con el hongo micorriza. En total, estos resultados indican que las raíces de las plántulas intactas de olivo, claramente diferenciadas entre micorrizas y hongos patógenos, atenuan las reacciones de defensa contra la primera para facilitar su creación, mientras que induce una reacción de defensa fuerte y sostenida contra el segundo. Ambas especies reactivas de oxígeno y nitrógeno parecían estar involucrados en estas respuestas desde los primeros momentos de contacto. Sin embargo, se necesitan más investigaciones para aclarar la diafonía propuesta entre ellos y sus respectivas funciones en estas respuestas ya que las imágenes de fluorescencia de las raíces revelaron que las especies reactivas del oxígeno se acumulan principalmente en el apoplasto (congruente con las actividades redox medidas en este compartimento), mientras el óxido nítrico se almacena principalmente en el citosol.Roots of intact olive seedlings, axenically cultured, were alternatively placed in contact with Rhizophagus irregularis (mycorrhizal) or Verticillim dahliae (pathogenic) fungi. MeJA treatments were also included. In vivo redox activities in the apoplast of the intact roots (anion superoxide generation, superoxide dismutase and peroxidase activities) were measured. All our results showed that apoplastic redox activities of intact seedling roots in contact with the compatible mycorrhizal fungus were clearly attenuated in comparison with the pathogenic fungus or treated with MeJA, even at the early stages of treatment used. Total phenolics, flavonoids and phenylpropanoid glycosides were also quantified. Roots in contact with the mycorrhizal fungus did not enhance the biosynthesis of phenolic compounds with respect to controls, while those in contact with the pathogenic one significantly enhanced the biosynthesis of all phenolic fractions measured. Reactive oxygen species and nitric oxid accumulation in roots were examined by fluorescence microscopy. All of them presented much higher accumulation in roots in contact with the pathogenic than with the mycorrhizal fungus. Altogether these results indicate that intact olive seedling roots clearly differentiated between mycorrhizal and pathogenic fungi, attenuating defense reactions against the first to facilitate its establishment, while inducing a strong and sustained defense reaction against the second. Both reactive oxygen and nitrogen species seemed to be involved in these responses from the first moments of contact. However, further investigations are required to clarify the proposed crosstalk between them and their respective roles in these responses since fluorescence images of roots revealed that reactive oxygen species were mainly accumulated in the apoplast (congruently with the measured redox activities in this compartment) while nitric oxid was mainly stored in the cytosol.-- Ministerio de Ciencia e Innovación. Proyecto CGL2009-12406 -- Junta de Extremadura. Proyecto PRI09A023peerReviewe

Bactérias amonificantes e nitrificantes e teores de amônio e nitrato afetados por plantas de cobertura e fertilizantes nitrogenados.

O objetivo do trabalho foi avaliar o efeito das plantas de cobertura e da fonte de N nas bactérias nitrificantes e amonificantes do solo, bem como nos teores de nitrato e amônio. O experimento foi conduzido em um latossolo vermelho distrófico sobre sistema plantio direto (SPD) por seis anos. O delineamento experimental foi em blocos casualizados, no esquema de parcelas subdivididas, com quatro repetições. As parcelas foram constituídas por seis espécies de plantas de cobertura do solo (Brachiaria brizantha, Brachiaria decumbens, Brachiaria humidicola, Brachiaria ruziziensis, Pennisetum americanum e Crotalaria spectabilis) e as subparcelas pelo controle e três fontes de N (1- controle, sem aplicação de N, 2- nitrato de cálcio, 3- sulfato de amônio e 4-sulfato de amônio + dicianodiamida (DCD)) aplicadas imediatamente após a emergência do arroz na dose de 40 kg ha-1 de N. Foram avaliadas a atividade das bactérias e os teores de nitrato e amônio no solo aos 15 DAE. As plantas de cobertura milheto (Pennisetum americanum), crotalaria (Crotalaria spectabilis); Brachiaria brizantha, B. decumbens; e B. humidicola proporcionaram os maiores teores de amônio no solo; O uso do inibidor de nitrificação (dicianodiamida-DCD) inibiu parte das bactérias nitrificantes e proporcionou os maiores teores de amônio no solo. A atividade das bactérias amonificantes e nitrificantes foi maior nas parcelas cultivadas com braquiárias. Plantas de cobertura aliada ao uso de DCD pode ser uma estratégia para aumentar os teores de amônio do solo cultivado sobre SPD

Gene probing reveals the widespread distribution, diversity and abundance of isoprene-degrading bacteria in the environment

Background: Approximately 500 Tg of isoprene are emitted to the atmosphere annually, an amount similar to that of methane, and despite its significant effects on the climate, very little is known about the biological degradation of isoprene in the environment. Isolation and characterisation of isoprene degraders at the molecular level has allowed the development of probes targeting isoA encoding the α-subunit of the isoprene monooxygenase. This enzyme belongs to the soluble diiron centre monooxygenase family and catalyses the first step in the isoprene degradation pathway. The use of probes targeting key metabolic genes is a successful approach in molecular ecology to study specific groups of bacteria in complex environments. Here, we developed and tested a novel isoA PCR primer set to study the distribution, abundance, and diversity of isoprene degraders in a wide range of environments. Results: The new isoA probes specifically amplified isoA genes from taxonomically diverse isoprene-degrading bacteria including members of the genera Rhodococcus, Variovorax, and Sphingopyxis. There was no cross-reactivity with genes encoding related oxygenases from non-isoprene degraders. Sequencing of isoA amplicons from DNA extracted from environmental samples enriched with isoprene revealed that most environments tested harboured a considerable variety of isoA sequences, with poplar leaf enrichments containing more phylogenetically diverse isoA genes. Quantification by qPCR using these isoA probes revealed that isoprene degraders are widespread in the phyllosphere, terrestrial, freshwater and marine environments. Specifically, soils in the vicinity of high isoprene-emitting trees contained the highest number of isoprene-degrading bacteria. Conclusion: This study provides the molecular ecology tools to broaden our knowledge of the distribution, abundance and diversity of isoprene degraders in the environment, which is a fundamental step necessary to assess the impact that microbes have in mitigating the effects of this important climate-active gas

Avaliação da diversidade de fungos MA em raízes de pupunha através de mini biblioteca de amplicons de região do rDNA 18S.

A avaliação da diversidade de fungos micorrízicos arbusculares por meio de técnicas moleculares tem sido preconizada como uma alternativa vantajosa à avaliação baseada em taxonomia clássica devido à possibilidade de reduzir a subjetividade envolvida na caracterização morfológica de esporos, base da taxonomia clássica, e de identificar espécies colonizando as raízes, impossível de ser realizada de outra forma. Para estimar a diversidade de fungos MA associada às raízes de pupunha cultivada em SAF, duas bibliotecas de região do rDNA 18S foram construídas e seqüenciadas a partir de duas amostras simples de raízes coletadas em duas plantas diferentes

Bacterial community and chemical composition of mixed fresh cactus forage and buffel grass hay during aerobic exposure.

The chemical composition of cactus forage becomes a favorable culture medium for accelerated microbial activity when exposed to air, as it contains high content of non-fiber carbohydratesand water. Thus, the aim of this study was to evaluate the bacterial community dynamics of different mixtures, using fresh forage of cactus and buffel grass hay as a function of the period of exposure to air. The experimental design used was a 5 × 5 factorial completely randomized (five levels of cactus forage × five times of exposure to air), with five replications. The peak of Escherichia coli population growth was after 16.06 h of exposure to air, observed in treatments of 90% and 100% cactus forage. There was an increase in microbial richness and uniformity of all treatments after six hours. The most abundant genera were Weissella, Lactobacillus, Bacteroides, Pseudomonas, Sphingobacterium, and Sphingomonas. The diet with 100% cactus forage showed a predominance of Weissella, Lactobacillus, and Leuconostoc. With 20% cactus forage, there was a greater apparent abundance of Pseudomonas, Sphingomonas, and Sphingobacterium. Aerobic exposure of mixtures of cactus forage with buffel grass hay increases the proliferation of microorganisms with pathogenic potential in the diet. Aerobic exposure of mixtures of cactus forage with buffel grass hay increases the proliferation of microorganisms with pathogenic potential in the diet. Therefore, an exposure period of fewer than six hours with 20% cactus forage is recommended to minimize levels of E. coli. Avoiding negative effects of the multiplication of pathogenic microorganisms on animal and human healt

Unexpected root growth and functioning in very deep rooted eucalyptus tree plantations: ontogeny or adaptation to abiotic stresses?

Functional and architectural plant root models simulate root architecture, growth dynamics, mortality, fine root distribution and water and nutrient uptake within the whole soil profile but with a unique set of parameters, usually measured within the topsoil only. They implicitly hypothesize that root growth and functioning is homogeneous throughout soil depth. Moreover, various observations have shown that trees have the capacity to explore very deep soil layers whatever soil and climate conditions, even where no drought is encountered. However, deep rooting strategies are often assumed to result from an adaptation of trees to withdraw water at depth when topsoil dries out. Here we report on changes in fine root behavior, morphology and anatomy with soil depth when comparing shallow roots to those growing down to 17-m in eucalypt plantations in Brazil, using various methods: root intersect measurements on pit walls, permanent pits equipped with (mini)rhizotrons, in-growth and sequential cores, analyses of rhizosphere properties and root anatomy, as well as modeling approaches. We show that very small densities of deep fine roots can have a key functional role for tree survival during extreme drought periods and make it possible to take up nutrients at depth, leached from the upper soil layers or naturally present in the subsoil. Unexpectedly, lower fine root mortality, higher elongation rate, fewer number but larger xylem vessels, higher increase of organic C and available K concentrations in the rhizosphere were found for deep compared to shallow fine roots. Very fast fine root exploration at depth > 4m was found in humid tropics, with no water stress conditions and high resource requirements of trees. Our results suggest a territorial strategy providing access to resources at great depth, suggesting an ontogenetic determinism in addition to the effect of the environment on deep fine root colonization