Atta mexicana waste dumps are hot spots of soil nutrients in an urban Neotropical cloud forest environment

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Relationship between soil nutrient availability and plant species richness in a tropical semi‐arid environment

Question: What is the relationship between soil fertility and plant species richness in the ‘fertile islands’ occurring beneath two species of legume (Cercidium praecox and Prosopis laevigata)? Location: Tehuacán‐Cuicatlán region, central Mexico. Methods: Plant richness was measured in three micro‐environments (below canopies of C. praecox, below canopies of P. laevigata and in areas without canopies). The concentration of soil nutrients (C, N and P), C and N in the microbiota, and processes of ecosystem functioning (net C mineralization rate and N mineralization) were measured. The relationship between soil variables and plant richness were assessed with ANCOVAs. Results: Soil nutrients and species richness increases markedly under fertility islands. There were higher concentrations of C and N in the soil, faster rates of C mineralization, and higher species richness under P. laevigata canopies. The relationship between soil fertility and species richness was always positive except for total N, ammonium and net C mineralization rate under C. praecox, and for available P under P. laevigata. Conclusions: The sign of the relationship between soil fertility and species richness varies according to the nutrient and the micro‐environment. Positive relationships could result from between species complementarity and facilitation. Negative relationships could be explained by a specific limitation threshold for some soil resources (P and N for plants and C for the soil microbiota) which eliminate the possibilities of between species complementarity and facilitation above that threshold. As in all observational studies, these relationships should be considered only correlational.     

Membrane Elastic Properties and Cell Function

Recent studies indicate that the cell membrane, interacting with its attached cytoskeleton, is an important regulator of cell function, exerting and responding to forces. We investigate this relationship by looking for connections between cell membrane elastic properties, especially surface tension and bending modulus, and cell function. Those properties are measured by pulling tethers from the cell membrane with optical tweezers. Their values are determined for all major cell types of the central nervous system, as well as for macrophage. Astrocytes and glioblastoma cells, which are considerably more dynamic than neurons, have substantially larger surface tensions. Resting microglia, which continually scan their environment through motility and protrusions, have the highest elastic constants, with values similar to those for resting macrophage. For both microglia and macrophage, we find a sharp softening of bending modulus between their resting and activated forms, which is very advantageous for their acquisition of phagocytic functions upon activation. We also determine the elastic constants of pure cell membrane, with no attached cytoskeleton. For all cell types, the presence of F-actin within tethers, contrary to conventional wisdom, is confirmed. Our findings suggest the existence of a close connection between membrane elastic constants and cell function.     

Lead and Arsenic Levels in Women with Different Body Mass Composition

The objective of this study was to evaluate whether lead (Pb) and arsenic (As) levels in biological fluids were associated to the body composition in a group of reproductive-age women. Voluntary childbearing-age women (n = 107) were divided into three groups according to their body mass index (BMI: weight/height² (kg/m²): low weight (BMI<18.5 kg/m²), normal $ \left( {{\text{BMI}} > 19\kern1.5pt<\kern1.5pt24.9\,{{\text{kg}} \mathord{\left/{\vphantom {{\text{kg}} {{{\text{m}}^{\text{2}}}}}} \right.} {{{\text{m}}^{\text{2}}}}}} \right) $ \left( {{\text{BMI}} > 19\kern1.5pt<\kern1.5pt24.9\,{{\text{kg}} \mathord{\left/{\vphantom {{\text{kg}} {{{\text{m}}^{\text{2}}}}}} \right.} {{{\text{m}}^{\text{2}}}}}} \right) , and overweight (BMI>25 kg/m²). Body composition and fat mass percentage were determined by the isotopic dilution method utilizing deuterated water. Blood lead concentrations were determined by graphite furnace atomic absorption spectrometry and urinary arsenic (AsU) concentrations by inductively coupled plasma mass spectrometry. The type and frequency of food consumption and lifestyle-related factors were also registered. Most women had $ {\text{PbB}}\,{\text{levels}} > 2\kern1.5pt<\kern1.5pt10\,{\mu{{\text{ g}}} \mathord{\left/{\vphantom {\mu{{\text{ g}}} {\text{dL}}}} \right.} {\text{dL}}} $ {\text{PbB}}\,{\text{levels}} > 2\kern1.5pt<\kern1.5pt10\,{\mu{{\text{ g}}} \mathord{\left/{\vphantom {\mu{{\text{ g}}} {\text{dL}}}} \right.} {\text{dL}}} , and only 2.6% had AsU concentrations above 50 μg/L. The levels of these toxic elements were not found to be associated with the fat mass percentage.     

Relationship between soil P fractions and microbial biomass in an oligotrophic grassland-desert scrub system

Phosphorus (P) is an essential element of the biosphere, both as a constituent of living organisms and as a regulator of biological processes. The Cuatro Ciénegas Basin in the central Chihuahuan Desert of Mexico is characterized by extreme P oligotrophy. The aim of this study was to quantify P distribution in soil P fractions, P sorption capacity, and P in microbial biomass in a desert scrub and grassland soil system in the Churince area of the Cuatro Ciénegas Basin over summer and winter seasons. Our objective, as part of an exploration of ecosystem functioning, was to ascertain the relationship between soil P fractions and P in microbial biomass. Our results demonstrate a scarcity of P, mainly in grassland, and also a higher P sorption capacity in grassland soil than in desert scrub. Desert scrub soil retained more P (228 ± 5 μg g^?1 dry soil) than grassland soil (87 ± 10 μg g^?1 dry soil), mainly in inorganic forms, but grassland soil retained more P in accessible organic forms. We suggest that biotic controls regulated by access to water shape the dynamics of soil P availability in the Churince grassland-desert scrub system.