Diversidade florística e fitossociológica do tapete herbáceo da caatinga na reserva legal do Salitre, Juazeiro-BA.

Para amostragem das herbáceas foi adotado o método das parcelas, com 50m2 de área amostral em cada microhabitate

Nutrient cycling at Dois Irmãos forest (Recife, PE, Brazil) through litterfall

Na mata de Dois lrmãos, em Recife, PE, foram determinadas, durante três anos, as massas de folhas, ramos, inflorescências e frutos que caíam ao solo, e as quantidades de P, K, Ca, Mg, Mn, Fe e Zn contidas nestas partes vegetais. O material foi recolhido cada 14 dias sobre telas coletoras de 1 m2. Trimestralmente, foram colhidas e analisadas amostras de folhas novas e velhas das árvores próximas às telas coletoras, e amostras da camada de folhedo sobre o solo. As massas caídas variaram pouco, de ano para ano, sendo, em média, de 6,4 t.ha-1 de folhas e 1,7 t.ha-1 de ramos. Inflorescências e frutos corresponderam a apenas 3% da massa total. A queda de folhas foi mais pronunciada de outubro a janeiro; a de ramos, de junho a julho; e a de inflorescências e frutos, de dezembro a abril. A massa de folhedo foi, em média, de 39 t.ha-1. Não houve variações significativas nos teores de nutrientes ao longo dos anos, exceto para os de K, que foram 2 a 3 vezes maiores na época seca que na de chuvas. Em geral, os teores de P, K e Mg foram mais altos nas folhas ainda nas árvores, decrescendo nas folhas caídas, e mais ainda no folhedo e ramos, o oposto acontecendo com os teores de Ca, Mn, Fe e Zn. Com o material caído, são depositados nos solos da mata anualmente, 49 kg.ha-1 de Ca, 23 de K, 14 de Mg, 3 de P, 1,3 de Fe e 0,4 de Mn e Zn. Retidos no folhedo, há três vezes mais K e 4 a 7 vezes mais dos outros elementos, que as quantidades depositadas anualmente.The amounts of leaves, twigs, flowers and fruits falling to the ground and their contents of P, K, Ca, Mg, Mn, Fe and Zn, were determined during three years. The material was collected every 14 days from eleven 1 m2 collectors. Every three months samples of young and old leaves from trees around the collectors and samples of the litter above the soil were also gathered and analysed. The amounts of litterfall varied little from year to year, averaging 6.4 t/ha of leaves and 1.7 t/ha of twigs, yearly. Flowers and fruits amounted to only 3% of total litterfall. Leaf fall was mostly from October to January, twig fall from June to July, and flowers and fruits from December to April. Litterlayer on the ground averaged 39 t/ha. There were no significant differences in nutrient contents of all plant parts along the year except for K content, which was 2 to 3 times higher in the dry season than in the wet season. In general, P, K and Mg contents were highest in leaves collected from the trees, decreasing in fallen leaves and decreasing still more in litterlayer and twigs, while the opposite occurred with Ca, Mn, Fe and Zn contents. Through litterfall 49 kg/ha of Ca, 23 of K, 14 of Mg, 3 of P, 1.3 of Fe and 0.4 of Mn and Zn return to the ground yearly. In the litterlayer on the ground there were 3 times more K and 4 to 7 times more of all other nutrients than in annual litterfall.

Biomass and nutrient dynamics associated with slash fires in neotropical dry forests

Unprecedented rates of deforestation and biomass burning in tropical dry forests are dramatically influencing biogeochemical cycles, resulting in resource depletion, declines in biodiversity, and atmospheric pollution. We quantified the effects of defores- tation and varying levels of slash-fire severity on nutrient losses and redistribution in a second-growth tropical dry forest ("Caatinga") near Serra Talhada, Pernambuco, Brazil. Total aboveground biomass prior to burning was 74 Mg/ha. Nitrogen and phosphorus concentrations were highest in litter, leaves attached to slash, and fine wood debris (<0.64 cm diameter). While these components comprised only 30% of the prefire aboveground biomass, they accounted for -60% of the aboveground pools of N and P. Three experi- mental fires were conducted during the 1989 burning season. In these treatments con- sumption was 78, 88, and 95% of the total aboveground biomass. As much as 96% of the prefire aboveground N and C pools and 56% of the prefire aboveground P pool was lost during combustion processes. Nitrogen losses exceeded 500 kg/ha and P losses exceeded 20 kg/ha in the fires of the greatest severity. With increasing fire severity, the concentrations of N and P in ash decreased while the concentration of Ca increased. This indicates greater amounts of these nutrients were volatilized (i.e., greater ecosystem losses occurred) with increasing fire severity. Following fire, up to 47% of the residual aboveground N and 84% of the residual aboveground P were in the form of ash, which was quickly lost from the site via wind erosion. Fires appeared to have a minor immediate effect on total N, C, or P in the soils. However, soils in forests with no history of cultivation had significantly higher concentrations of C and P than second-growth forests. Based upon the measured losses of nutrients from these single slash-burning events, it would likely require a century or more of fallow for reaccumulation to occur. However, current fallow periods in this region are 15 yr or less

Evidence of random magnetic anisotropy in ferrihydrite nanoparticles based on analysis of statistical distributions

We show that the magnetic anisotropy energy of antiferromagnetic ferrihydrite depends on the square root of the nanoparticles volume, using a method based on the analysis of statistical distributions. The size distribution was obtained by transmission electron microscopy, and the anisotropy energy distributions were obtained from ac magnetic susceptibility and magnetic relaxation. The square root dependence corresponds to random local anisotropy, whose average is given by its variance, and can be understood in terms of the recently proposed single phase homogeneous structure of ferrihydrite.Comment: 6 pages, 2 figure

Effects of the rainy season on growth of Mimosa tenuiflora in dry tropical forest, Brazil.

Resumo

Characteristics of nodule bacteria from Mimosa spp grown in soils of the Brazilian semiarid region.

The Brazilian Northeastern dry forest (Caatinga) is one of the diversification centers of Mimosa species. We determined the characteristics of native rhizobia isolates from nodules of Mimosa tenuiflora and Mimosa paraibana grown in pots with soils collected under Caatinga vegetation and compared the restriction ribosomal DNA profiles of the isolates with those of 16 reference strains. All plants formed abundant indeterminate nodules and all nodule isolates formed fast growing colonies. No colony altered the medium to an alkaline reaction and most of them produced low or medium amounts of extracellular polysaccharides. White and creamy colonies predominated among the isolates but orange and green colonies were present. Differences among the isolates from the Mimosa species tested are indicated by the greater phenotypic diversity of those obtained from M. tenuiflora. The analysis of the 16S rDNA gene suggests that the isolates from M. tenuiflora and M. paraibana are closely related and closer to B-rhizobia than to &#945;-rhizobia. However, the similarity with all the tested B-rhizobia reference strains was relatively low suggesting that the isolates may belong to different bacteria species

Spin torque resonant vortex core expulsion for an efficient radio-frequency detection scheme

Spin-polarised radio-frequency currents, whose frequency is equal to that of the gyrotropic mode, will cause an excitation of the core of a magnetic vortex confined in a magnetic tunnel junction. When the excitation radius of the vortex core is greater than that of the junction radius, vortex core expulsion is observed, leading to a large change in resistance, as the layer enters a predominantly uniform magnetisation state. Unlike the conventional spin-torque diode effect, this highly tunable resonant effect will generate a voltage which does not decrease as a function of rf power, and has the potential to form the basis of a new generation of tunable nanoscale radio-frequency detectors

Functional analysis of soil chemical attributes using Geographic Information Systems to assess nutrient deficiencies.

Information on soil fertility is essential to determine the need for correcting measures. However, the use of appropriate techniques to provide information on the specific potential of each soil in many agricultural areas is scarce. Soil fertility in an area with family farming was evaluated based on spatialization of soil chemical attributes to indicate their usage limitations. The study area was on the water basin of the Natuba River, in the Mata Centro Zone, Pernambuco state, Brazil. Samples were taken from soils under the most representative agricultural uses and under native vegetation following a slope gradient (top, middle and bottom). Samples were analyzed for their base saturation, cation exchange capacity, aluminum saturation and phosphorus concentration. These parameters were used to indicate their degree of limitation to agricultural use in order to create a georeferenced database and were mapped using a Geographic Information System (GIS). The cultivated Rhodic Acrisol and Eutric Gleisol had the highest fertility, while the Xanthic Acrisol, Chromic Acrisol and Xanthic Ferralsol had the highest usage limitations. Areas used to grow vegetables had higher nutrient availability than those cropped to sugarcane. Spatialization in the GIS system contributed to the overall fertility analysis, allowing a clear visualization of areas that need soil improvement.Título em português: Análise funcional dos atributos químicos do solo usando Sistemas de Informação Geográfica para avaliar as deficiências de nutrientes