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Microbial crust develops on the surface of the soil due to colonization of microorganisms as cyanobacteria and macro organisms such as bryophytes and lichens. Initial support is a physical crust that dominates in arid and semi-arid areas such as the de Quíbor, state Lara, Venezuela. The objective of this study is to show the interaction of the crust organ microbial mineral through different structures anchoring morphological and organic excretions microorganisms contribute to the onset of the formation of micro aggregates and soil retention. The morphological diversity of microbiological structures of the crust was studied by Scanning Electron Microscopy (SEM). The results show different sizes of micro aggregates, which show the effect of rhizoids and rhizines, and also cyanobacteria into entanglement of soil aggregates. Aggregation occurs by the action of the same organisms, as the entanglement of bryophytes and rhizines rhizoids along fungal hyphae, facilitates stabilizing micro aggregates entrapment. Secretions of polysaccharides and / or cyanobacteria gummy exudate and lichens generate an adhesive effect of the mineral particles remain bound, which provides the initiation and formation of micro aggregates, improving the structure and thus protects the soil from hidric erosion. Source

Polissar P.J.,Lamont Doherty Earth Observatory | Abbott M.B.,University of Pittsburgh | Wolfe A.P.,University of Alberta | Vuille M.,Albany State University | Bezada M.,Libertador Experimental Pedagogical University
Proceedings of the National Academy of Sciences of the United States of America | Year: 2013

Holocene variations of tropical moisture balance have been ascribed to orbitally forced changes in solar insolation. If this model is correct, millennial-scale climate evolution should be antiphased between the northern and southern hemispheres, producing humid intervals in one hemisphere matched to aridity in the other. Here we show that Holocene climate trends were largely synchronous and in the same direction in the northern and southern hemisphere outer-tropical Andes, providing little support for the dominant role of insolation forcing in these regions. Today, seasurface temperatures in the equatorial Pacific Ocean modulate rainfall variability in the outer tropical Andes of both hemispheres, and we suggest that this mechanism was pervasive throughout the Holocene. Our findings imply that oceanic forcing plays a larger role in regional South American climate than previously suspected, and that Paci fic sea-surface temperatures have the capacity to induce abrupt and sustained shifts in Andean climate. Source

Stansell N.D.,University of Pittsburgh | Abbott M.B.,University of Pittsburgh | Rull V.,CSIC Botanic Institute of Barcelona | Rodbell D.T.,Union College at Schenectady | And 2 more authors.
Earth and Planetary Science Letters | Year: 2010

A radiocarbon dated sediment record from Laguna de Los Anteojos, a cirque lake in the Mérida Andes of Venezuela, indicates that warmer and wetter atmospheric conditions occurred in the northern tropics at the onset of the Bølling (~. 14,600. cal. yr BP), and abruptly colder and drier conditions around the time of the Younger Dryas (YD). Geochemical and clastic sediment analyses from Los Anteojos show that glaciers advanced at ~. 12,850. cal. yr BP, reached their YD maximum extent at ~. 12,650. cal. yr BP, and then retreated until complete deglaciation of the watershed at ~. 11,750. cal. yr BP. The onset of warmer conditions that ended the coldest phase of the YD occurred several hundred years earlier at Los Anteojos than in the high latitudes of the Northern Hemisphere. During the peak YD glacial advance, glacier equilibrium-line altitudes in the region were ~. 360 to 480. m lower, and temperature was ~. 2.2 to 2.9 °C colder than modern. Independent palynological evidence from the Los Anteojos sediment core indicates that the northern Andes were more arid and at least 2.3 °C colder during the YD. The direction and timing of glacial fluctuations in Venezuela are consistent with observations of marine sediment records from the Cariaco Basin that suggest abrupt cooling occurred at ~. 12,850. cal. yr BP, followed by a shift to higher temperature after ~. 12,300. cal. yr BP. The timing and pattern of climatic changes in northern South America are also consistent with paleoclimate records from the southern Tropical Andes that suggest a southward shift in the position of the Intertropical Convergence Zone occurred at the start of the cooling event, followed by a return to wetter conditions in northern South America during the late stages of the YD. The early warming of the tropical atmosphere and invigoration of the hydrologic cycle likely contributed to the shift to increased temperature in the higher latitudes of the Northern Hemisphere at the end of the late Glacial stage. © 2010 Elsevier B.V. Source

Murillo Pulgarin J.A.,University of Castilla - La Mancha | Garcia Bermejo L.F.,University of Castilla - La Mancha | Duran A.C.,Libertador Experimental Pedagogical University
Phytochemical Analysis | Year: 2011

Introduction - The cobalt is an essential element for leguminous plants but may be harmful for other species; for that reason determination of Co(II) is very important for the management of polluted areas and for discover plants with capacity for the hyperaccumulation of heavy metals, which has produced a growing necessity of fast, sensitive and selective analytical techniques. Objective - To develop an analytical procedure for the determination of cobalt in plant tissue by coupling the ionic chromatography to the luminol-based chemiluminescence detection. Methodology - The sample was digested in a mixture of concentrated nitric acid and hydrogen peroxide, using an microwave oven to dissolve the Co(II). The solution containing Co(II) ions was injected to an ionic chromatograph using oxalic acid as the eluent. The detection was based on the catalytic effect of Co(II) on the luminol chemiluminescence using perborate or percarbonate as oxidants. Experimental variables, such as concentrations, pH, flow rates and acid digestion conditions were optimised. Results - Well-resolved chromatographic peaks were obtained. The height and area showed linear dependences with the Co(II) concentration, which were used to quantify the heavy metal, with recoveries up to 95%. The microwave irradiation (60 s) was sufficient for the complete mineralisation of 200 mg of sample, employing 2 mL of the acid mixture. The method was free from the interferences, requiring less than 12 minutes to complete the analysis. Conclusion - The method was simple and rapid for the determination of cobalt in plant tissue with detection limits comparable to those obtained with more sophisticated and expensive analytical equipments. Copyright © 2010 John Wiley & Sons, Ltd. Source

On the floor of Quíbor, grows discontinuously a crust formed by lichens, bryophytes and cyanobacteria. The purpose of this research is to identify variations in the behaviours of some soil properties by comparing soil samples with and without crust. This is a field research developed in three phases: The first, called field, in order to collect samples of soil, with support in the random digits table and from two longitudinal transects, they are used on every ten meters a quadrat square 25 × 25 cm; the second phase, laboratory, in which soil properties determined: a) physical as % moisture; b) chemicals such as salinity, pH, Electrical Conductivity (EC) and organic carbon (OC); and c) biochemical, including organic matter (OM) and carbon dioxide (CO2) given off; the third phase, statistical analysis, "t" test was used in order to identify significant changes between the two sample groups. We conclude that the presence of the crust on the soil surface increases metabolic activity, the OC and OM. As for pH, EC, salinity and% H there were no significant changes. Source

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