Distribution and morphological variation of tree ferns (Cyatheaceae) along an elevation gradient

Knowing how species and communities respond to environmental change is fundamental in the context of climate change. The search for patterns of abundance and phenotypic variation along altitudinal gradients can provide evidence on adaptive limits. We evaluated the species abundance and the variation in morphometric and stomatal characters in five tree ferns species (Cyathea fulva, C. divergens, C. myosuroides, Alsophila firma and Gymnosphaera salvinii) distributed along an elevation gradient in a well-preserved Mexican cloud forest. Variation at the community and species level was assessed using exploratory and multivariate data analysis methods. We wanted to explore if the species abundance is environmentally determined, to determine the degree of variation along the elevation gradient, to test for differences between zones and associations with elevation, humidity and soil nutrients, and to assess contribution of the intra- and interspecific variation to the community response to elevation and soil nutrients. The studied fern community showed strong species turnover along the elevation gradient, with some influence of soil nutrient concentration, supporting environmental determinism. All measured characters displayed variation along the gradient. Stomatal characters (size and density) had significantly less variation than morphometric characters (trunk diameter, stipe length and blade length), but stomatal density also shows interesting intraspecific patterns. In general, patterns within the fern community suggest a strong influence of species identity, especially of species inhabiting the lower edge of the cloud forest, which showed the clearest morphometric and stomatal patterns, associated to contrasting environments rather than to changes in elevation. The coincidence between morphometric and stomatal patterns in this area suggest hydraulic adjustments in response to contrasting environments. Our results provide evidence that tree ferns species respond to environmental changes through adjustments of morphometric plasticity and stomatal density, which is relevant to predict possible responses to variation in environmental conditions resulting from climate change.

La mineralización e inmovilización microbiana determinan la dinámica del azufre en el suelo

El azufre (S) es un nutriente esencial para los seres vivos, sin embargo, existen pocos trabajos sobre su dinámica en el suelo y sobre la importancia de la intervención de los microorganismos en la transformación de las moléculas de S. Esta revisión tiene como objetivo analizar la trascendencia de dos procesos que regulan la dinámica de este elemento: la mineralización en la que intervienen diversas enzimas y la inmovilización que atañe a la adquisición de compuestos con azufre, ambos determinantes en la biodisponibilidad de este elemento, relativa a su abundancia y a sus propiedades, que en gran medida realiza la comunidad microbiana del suelo (CMS). Lo anterior, con la finalidad de mostrar además de los pormenores de la capacidad bioquímica de estos mecanismos, el valor de su función en la naturaleza y su relevancia para la conservación del suelo, a través de un manejo adecuado del mismo.

Amino Acids in the Root Exudates of Agave lechuguilla Torr. Favor the Recruitment and Enzymatic Activity of Nutrient-Improvement Rhizobacteria

Agave lechuguilla is a widely distributed plant in arid ecosystems. It has been suggested that its microbiome is partially responsible for its great adaptability to the oligotrophic environments of the Chihuahuan Desert. To lead the recruitment of beneficial rhizobacteria, the root exudates are essential; however, the amino acids contained within these compounds had been largely overlooked. Thus, we investigated how the variations of amino acids in the rhizosphere at different growth stages of A. lechuguilla affect the rhizobacterial community composition, its functions, and activity of the beneficial bacteria. In this regard, it was found that arginine and tyrosine were related to the composition of the rhizobacterial community associated to A. lechuguilla, where the most abundant genera were from the phylum Proteobacteria and Bacteroidetes. Moreover, Firmicutes was largely represented by Bacillus in the phosphorus-mineralizing bacteria community, which may indicate its great distribution and versatility in the harsh environments of the Chihuahuan Desert. In contrast, we found a high proportion of Unknown taxa of nitrogen-fixing bacteria, reflecting the enormous diversity in the rhizosphere of these types of plants that remains to be explored. This work also reports the influence of micronutrients and the amino acids methionine and arginine over the increased activity of the nitrogen-fixing and phosphorus-mineralizing bacteria in the rhizosphere of lechuguillas. In addition, the results highlight the multiple beneficial functions present in the microbiome that could help the host to tolerate arid conditions and improve nutrient availability.

Squash root microbiome transplants and metagenomic inspection for in situ arid adaptations

Arid zones contain a diverse set of microbes capable of survival under dry conditions, some of which can form relationships with plants under drought stress conditions to improve plant health. We studied squash (Cucurbita pepo L.) root microbiome under historically arid and humid sites, both in situ and performing a common garden experiment. Plants were grown in soils from sites with different drought levels, using in situ collected soils as the microbial source. We described and analyzed bacterial diversity by 16S rRNA gene sequencing (N = 48) from the soil, rhizosphere, and endosphere. Proteobacteria were the most abundant phylum present in humid and arid samples, while Actinobacteriota abundance was higher in arid ones. The β-diversity analyses showed split microbiomes between arid and humid microbiomes, and aridity and soil pH levels could explain it. These differences between humid and arid microbiomes were maintained in the common garden experiment, showing that it is possible to transplant in situ diversity to the greenhouse. We detected a total of 1009 bacterial genera; 199 exclusively associated with roots under arid conditions. By 16S and shotgun metagenomics, we identified dry-associated taxa such as Cellvibrio, Ensifer adhaerens, and Streptomyces flavovariabilis. With shotgun metagenomic sequencing of rhizospheres (N = 6), we identified 2969 protein families in the squash core metagenome and found an increased number of exclusively protein families from arid (924) than humid samples (158). We found arid conditions enriched genes involved in protein degradation and folding, oxidative stress, compatible solute synthesis, and ion pumps associated with osmotic regulation. Plant phenotyping allowed us to correlate bacterial communities with plant growth. Our study revealed that it is possible to evaluate microbiome diversity ex-situ and identify critical species and genes involved in plant-microbe interactions in historically arid locations.

The role of wood anatomical traits in the coexistence of oak species along an environmental gradient

Oaks (Quercus) are a dominant woody plant genus in the northern hemisphere, which occupy a wide range of habitats and are ecologically diverse. We analysed the wood anatomical traits, the variables derived and the relative hydraulic conductivity of 21 oak species to identify their performance according to abiotic factors, leaf phenological patterns and phylogenetic restrictions by analysing the interspecific variation along an environmental gradient. First, we determine the causes of anatomical trait variation in the oaks, analysing the functional trade-offs related to distribution along the environmental gradient. We measure the phenotypic plasticity of the anatomical traits to determine the role of environment and geographic distance in the range of phenotypic plasticity. Second, we examined if oaks co-occurred along the environmental gradient. Then we analysed if wood anatomical traits reflect differences among their phylogenetic section, leaf habit and a phylogenetic section/leaf habit category. Last, we tested the phylogenetic signal. Our results showed that vessel diameter, vessel frequency, wood density and relative hydraulic conductivity are the main axes of trait variation in the species analysed among leaf habit categories. The aridity index and seasonal precipitation drive the variation in the analysed traits. Higher environmental distance resulted in a higher relative distance plasticity index among traits. Co-occurrence of oak species with different leaf habits and phylogenetic trajectories may promote complementary resource acquisition. The phylogenetic signal in the oak species studied was low, which implies labile wood traits.

The nutrient-improvement bacteria selected by Agave lechuguilla T. and their role in the rhizosphere community

Agave lechuguilla has one of the widest distributions among other agave species in the Chihuahuan Desert. Their capacity to grow in poorly developed soils and harsh conditions has been related to their association with plant growth-promoting rhizobacteria. In this work, we explored how soil properties and plant growth stage influence the composition of the rhizobacterial communities, their interactions, and the enzymatic activity and abundance of nitrogen-fixing bacteria and organic phosphorus-mineralizing bacteria in two subregions of the Chihuahuan Desert. We found that mature plants of lechuguilla stimulated the activity and abundance of nutrient-improvement rhizobacteria, and these soil samples had a higher content of total organic carbon, ammonium (NH4) and nitrite + nitrate (NO2+NO3). Nutrient availability seems to be an essential driver of the bacterial community's structure since the genera with more connections (hubs) were those with known mechanisms related to the availability of nutrients, such as env. OPS17 (Bacteroidetes), Gemmatimonadaceae uncultured, S0134terrestrial group, BD211terrestrial group (Gemmatimonadetes), Chthoniobacteracea and Candidatus Udaeobacter (Verrucomicrobia). This work shows that the late growth stages of lechuguilla recruit beneficial bacteria that favor its establishment and tolerance to harsh conditions of the arid lands.

Small-scale variation in a pristine montane cloud forest: evidence on high soil fungal diversity and biogeochemical heterogeneity

Montane cloud forests are fragile biodiversity hotspots. To attain their conservation, disentangling diversity patterns at all levels of ecosystem organization is mandatory. Biotic communities are regularly structured by environmental factors even at small spatial scales. However, studies at this scale have received less attention with respect to larger macroscale explorations, hampering the robust view of ecosystem functioning. In this sense, fungal small-scale processes remain poorly understood in montane cloud forests, despite their relevance. Herein, we analyzed soil fungal diversity and ecological patterns at the small-scale (within a 10 m triangular transect) in a pristine montane cloud forest of Mexico, using ITS rRNA gene amplicon Illumina sequencing and biogeochemical profiling. We detected a taxonomically and functionally diverse fungal community, dominated by few taxa and a large majority of rare species (81%). Undefined saprotrophs represented the most abundant trophic guild. Moreover, soil biogeochemical data showed an environmentally heterogeneous setting with patchy clustering, where enzymatic activities suggest distinctive small-scale soil patterns. Our results revealed that in this system, deterministic processes largely drive the assemblage of fungal communities at the small-scale, through multifactorial environmental filtering.

Soil organic matter dynamics and microbial metabolism along an altitudinal gradient in Highland tropical forests

The highland forests of tropical regions are highly vulnerable to climate change because changes in soil organic quality due to the increased soil water deficit conditions through rising temperatures. Several authors have reported that labile molecules dominate soil organic matter at higher elevations, and it is therefore more vulnerable to the rising temperatures associated with climate change. The objective of the present study was to analyze the effect of interaction between the chemical composition of organic matter derived from the dominant plant species and the metabolism of microbial community along an elevational gradient in a highland forest in Central Mexico. The study compared three vegetation-soil systems that represent three different elevational levels: Alnus-system (3100 m.a.s.l.), Abies-system (3500 m.a.s.l.) and Pinus-system (3700 m.a.s.l.). The SOM produced in the lowest site is more recalcitrant (i.e., higher Alkyl:O-Alkyl ratio) as a result of the lower water availability than in the highest site. The results of Threshold Elemental Ratio_C:N (TER_C:N) and TER_C:P for the organic layer were lower than their C:N and C:P ratios in the organic layer, supporting that the microbial community of the organic layer in the site of lowest elevation must be limited by the carbon source, rather than by N and P. However, these results were not found in the mineral soil, suggesting that the drivers of organic matter decomposition differ between the organic layer and the mineral soil. As a conclusion, our results suggest that the chemical recalcitrance of organic matter (at the lowest site) and temperature (at the highest site) reduce the microbial metabolic activity in the forest floor. Integrated study of plant-derived organic material and the microbial metabolism of the forest floor is therefore required to achieve a full understanding of the vulnerability of tropical mountain ecosystems to climate change.

Testing the Two-Step Model of Plant Root Microbiome Acquisition Under Multiple Plant Species and Soil Sources

The two-step model for plant root microbiomes considers soil as the primary microbial source. Active selection of the plant’s bacterial inhabitants results in a biodiversity decrease toward roots. We collected sixteen samples of in situ ruderal plant roots and their soils and used these soils as the main microbial input for single genotype tomatoes grown in a greenhouse. Our main goal was to test the soil influence in the structuring of rhizosphere microbiomes, minimizing environmental variability, while testing multiple plant species. We massively sequenced the 16S rRNA and shotgun metagenomes of the soils, in situ plants, and tomato roots. We identified a total of 271,940 bacterial operational taxonomic units (OTUs) within the soils, rhizosphere and endospheric microbiomes. We annotated by homology a total of 411,432 (13.07%) of the metagenome predicted proteins. Tomato roots did follow the two-step model with lower α-diversity than soil, while ruderal plants did not. Surprisingly, ruderal plants are probably working as a microenvironmental oasis providing moisture and plant-derived nutrients, supporting larger α-diversity. Ruderal plants and their soils are closer according to their microbiome community composition than tomato and its soil, based on OTUs and protein comparisons. We expected that tomato β-diversity clustered together with their soil, if it is the main rhizosphere microbiome structuring factor. However, tomato microbiome β-diversity was associated with plant genotype in most samples (81.2%), also supported by a larger set of enriched proteins in tomato rhizosphere than soil or ruderals. The most abundant bacteria found in soils was the Actinobacteria Solirubrobacter soli, ruderals were dominated by the Proteobacteria Sphingomonas sp. URGHD0057, and tomato mainly by the Bacteroidetes Ohtaekwangia koreensis, Flavobacterium terrae, Niastella vici, and Chryseolinea serpens. We calculated a metagenomic tomato root core of 51 bacterial genera and 2,762 proteins, which could be the basis for microbiome-oriented plant breeding programs. We attributed a larger diversity in ruderal plants roots exudates as an effect of the moisture and nutrient acting as a microbial harbor. The tomato and ruderal metagenomic differences are probably due to plant domestication trade-offs, impacting plant-bacteria interactions.

Detailed epidemiological analysis as a strategy for evaluating the actual behavior of tuberculosis in an apparently low-incidence region

Tuberculosis control in developing regions with apparent low incidence, like the low-income Mexican state of Michoacán, with mean annual incidence rates below 10/100,000 inhabitants, requires knowledge of the actual behavior of the disease. This can be determined using an epidemiological profile at sub-regional level, allowing disclosure of the clinical and social factors that may be hampering efforts to control tuberculosis. In this work, a detailed epidemiological profile was outlined using data of all new monthly cases registered in the National System of Epidemiological Surveillance Database for Michoacán municipalities from 2000 to 2012. Cases were grouped by gender and age, and sociodemographic data were obtained both from the National Institute of Statistics and Geography and from the United Nations Development Programme. Correlations were calculated by Chi-square, Mann-Whitney U, and Kruskal-Wallis H tests. We observed no statistically significant differences between notification rates for the years 2000, 2005 and 2010 (χ² = 0.222, p = 0.895). The percentage of cases is similar between all age groups older than 15, while some regions had low notification rates but high proportions of pediatric cases. Higher proportions of cases of extrapulmonary tuberculosis were observed in municipalities in northern Michoacán. No correlation was found between municipal Human Development Index values and municipal notification rates. Michoacán is undergoing an epidemiological transition with three regions having different epidemiological profiles and particular needs for effective prevention and containment of tuberculosis. Our work shows the importance of the spatial scale of epidemiological profiles for determining specific regional needs of surveillance and containment.

Divergence with gene flow is driven by local adaptation to temperature and soil phosphorus concentration in teosinte subspecies (Zea mays parviglumis and Zea mays mexicana)

Patterns of genomic divergence between hybridizing taxa can be heterogeneous along the genome. Both differential introgression and local adaptation may contribute to this pattern. Here, we analysed two teosinte subspecies, Zea mays ssp. parviglumis and ssp. mexicana, to test whether their divergence has occurred in the face of gene flow and to infer which environmental variables have been important drivers of their ecological differentiation. We generated 9,780 DArTseqTM SNPs for 47 populations, and used an additional data set containing 33,454 MaizeSNP50 SNPs for 49 populations. With these data, we inferred features of demographic history and performed genome wide scans to determine the number of outlier SNPs associated with climate and soil variables. The two data sets indicate that divergence has occurred or been maintained despite continuous gene flow and/or secondary contact. Most of the significant SNP associations were to temperature and to phosphorus concentration in the soil. A large proportion of these candidate SNPs were located in regions of high differentiation that had been identified previously as putative inversions. We therefore propose that genomic differentiation in teosintes has occurred by a process of adaptive divergence, with putative inversions contributing to reduced gene flow between locally adapted populations.

Direct and legacy effects of plant-traits control litter decomposition in a deciduous oak forest in Mexico

Background

Litter decomposition is a key process in the functioning of forest ecosystems, because it strongly controls nutrient recycling and soil fertility maintenance. The interaction between the litter chemical composition and the metabolism of the soil microbial community has been described as the main factor of the decomposition process based on three hypotheses: substrate-matrix interaction (SMI), functional breadth (FB) and home-field advantage (HFA). The objective of the present study was to evaluate the effect of leaf litter quality (as a direct plant effect, SMI hypothesis), the metabolic capacity of the microbial community (as a legacy effect, FB hypothesis), and the coupling between the litter quality and microbial activity (HFA hypothesis) on the litter decomposition of two contiguous deciduous oak species at a local scale.

Methods

To accomplish this objective, we performed a litterbag experiment in the field for 270 days to evaluate mass loss, leaf litter quality and microbial activity in a complete factorial design for litter quality and species site.

Results

The litter of Quercus deserticola had higher rate of decomposition independently of the site, while the site of Quercus castanea promoted a higher rate of decomposition independently of the litter quality, explained by the specialization of the soil microbial community in the use of recalcitrant organic compounds. The Home-Field Advantage Index was reduced with the decomposition date (22% and 4% for 30 and 270 days, respectively).

Discussion

We observed that the importance of the coupling of litter quality and microbial activity depends on decomposition stage. At the early decomposition stage, the home-advantage hypothesis explained the mass loss of litter; however, in the advanced decomposition stage, the litter quality and the metabolic capacity of the microbial community can be the key drivers.

Inferring changes in soil organic matter in post-wildfire soil burn severity levels in a temperate climate

Simple, rapid and reliable methods of assessing soil burn severity (SBS) are required in order to prioritize post-fire emergency stabilization actions. SBS proxies based on visual identification and changes in soil organic matter (SOM) content and quality can be related to other soil properties in order to determine the extent to which soil is perturbed following fire. This task is addressed in the present study by an approach involving the use of differential scanning calorimetry-thermogravimetric analysis (DSC-TGA) to determine changes in SOM generated in soils subjected to different levels of SBS. Intact topsoil monoliths comprising the organic horizons and the surface mineral soil (alumic-humic umbrisols) were collected from a representative P. pinaster stand in NW Spain. The monoliths were experimentally burned in a combustion wind tunnel to simulate different fire conditions (fuel bed comprising forest pine litter and wood; air flow, 0.6 m s^-1). Changes in OM properties in the soil organic layer and mineral soils samples (0-2 cm) at the different temperatures and SBS levels were identified. For both duff and mineral soil, the data revealed a temperature-induced increase in aromatic compounds and a concomitant decrease of carbohydrates and alkyl products. However, for a given temperature, the degree of carbonization/aromatization was lower in the mineral soil than in the duff, possibly due to the different composition of the OM and to the different combustion conditions. The low degree of aromatization of the organic matter suggests that this soil component could undergo subsequent biological degradation. SOM content and thermal recalcitrance (measured as T50) discriminated the SBS levels. Use of visual identification of SBS levels in combination with DSC-TGA enables rapid evaluation of the spatial variability of the effects of fire on SOM properties. This information is useful to predict soil degradation process and implement emergency soil stabilization techniques.

The response of soil microbial communities to variation in annual precipitation depends on soil nutritional status in an oligotrophic desert

Background

Soil microbial communities (SMC) play a central role in the structure and function of desert ecosystems. However, the high variability of annual precipitation could results in the alteration of SMC and related biological processes depending on soil water potential. The nature of the physiological adjustments made by SMC in order to obtain energy and nutrients remains unclear under different soil resource availabilities in desert ecosystems. In order to examine this dynamic, the present study examined the effects of variation in annual precipitation on physiological adjustments by the SMC across two vegetation-soil systems of different soil organic matter input in an oligotrophic desert ecosystem.

Methods

We collected soil samples in the Cuatro Ciénegas Basin (Mexico) under two vegetation covers: rosetophylous scrub (RS) and grassland (G), that differ in terms of quantity and quality of organic matter. Collections were conducted during the years 2011, 2012, 2013 and 2014, over which a noticeable variation in the annual precipitation occurred. The ecoenzymatic activity involved in the decomposition of organic matter, and the concentration of dissolved, available and microbial biomass nutrients, were determined and compared between sites and years.

Results

In 2011, we observed differences in bacterial taxonomic composition between the two vegetation covers. The lowest values of dissolved, available and microbial nutrients in both cover types were found in 2012. The G soil showed higher values of dissolved and available nutrients in the wet years. Significant positive correlations were detected between precipitation and the ratios Cmic:Nmic and Cmic:Pmic in the RS soil and Cmic:Pmic and Nmic:Pmic in the G soil. The slopes of the regression with Cmic and Nmic were higher in the G soil and lower in the RS soil. Moreover, the SMC under each vegetation cover were co-limited by different nutrients and responded to the sum of water stress and nutrient limitation.

Discussion

Soil community within both sites (RS and G) may be vulnerable to drought. However, the community of the site with lower resources (RS) is well adapted to acquire P resources by ecoenzyme upregulation during years with adequate precipitation, suggesting that this community is resilient after drought occurs. Under the Global Climate Change scenarios for desert ecosystems that predict reduced annual precipitation and an increased intensity and frequency of torrential rains and drought events, the soil microbial communities of both sites could be vulnerable to drought through C and P co-limitation and reallocation of resources to physiological acclimatization strategies in order to survive.

Spatial heterogeneity of physicochemical properties explains differences in microbial composition in arid soils from Cuatro Cienegas, Mexico

Arid ecosystems are characterized by high spatial heterogeneity, and the variation among vegetation patches is a clear example. Soil biotic and abiotic factors associated with these patches have also been well documented as highly heterogeneous in space. Given the low vegetation cover and little precipitation in arid ecosystems, soil microorganisms are the main drivers of nutrient cycling. Nonetheless, little is known about the spatial distribution of microorganisms and the relationship that their diversity holds with nutrients and other physicochemical gradients in arid soils. In this study, we evaluated the spatial variability of soil microbial diversity and chemical parameters (nutrients and ion content) at local scale (meters) occurring in a gypsum-based desert soil, to gain knowledge on what soil abiotic factors control the distribution of microbes in arid ecosystems. We analyzed 32 soil samples within a 64 m² plot and: (a) characterized microbial diversity using T-RFLPs of the bacterial 16S rRNA gene, (b) determined soil chemical parameters, and (c) identified relationships between microbial diversity and chemical properties. Overall, we found a strong correlation between microbial composition heterogeneity and spatial variation of cations (Ca², K⁺) and anions (HCO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}${}_{3}^{-}$\end{document}3−, Cl⁻, SO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}${}_{4}^{2-}$\end{document}42−) content in this small plot. Our results could be attributable to spatial differences of soil saline content, favoring the patchy emergence of salt and soil microbial communities.

Agricultural land-use change in a Mexican oligotrophic desert depletes ecosystem stability

Background

Global demand for food has led to increased land-use change, particularly in dry land ecosystems, which has caused several environmental problems due to the soil degradation. In the Cuatro Cienegas Basin (CCB), alfalfa production irrigated by flooding impacts strongly on the soil.

Methods

In order to analyze the effect of such agricultural land-use change on soil nutrient dynamics and soil bacterial community composition, this work examined an agricultural gradient within the CCB which was comprised of a native desert grassland, a plot currently cultivated with alfalfa and a former agricultural field that had been abandoned for over 30 years. For each site, we analyzed C, N and P dynamic fractions, the activity of the enzyme phosphatase and the bacterial composition obtained using 16S rRNA clone libraries.

Results

The results showed that the cultivated site presented a greater availability of water and dissolved organic carbon, these conditions promoted mineralization processes mediated by heterotrophic microorganisms, while the abandoned land was limited by water and dissolved organic nitrogen. The low amount of dissolved organic matter promoted nitrification, which is mediated by autotrophic microorganisms. The microbial N immobilization process and specific phosphatase activity were both favored in the native grassland. As expected, differences in bacterial taxonomical composition were observed among sites. The abandoned site exhibited similar compositions than native grassland, while the cultivated site differed.

Discussion

The results suggest that the transformation of native grassland into agricultural land induces drastic changes in soil nutrient dynamics as well as in the bacterial community. However, with the absence of agricultural practices, some of the soil characteristics analyzed slowly recovers their natural state.

Ecosystem engineering and manipulation of host plant tissues by the insect borer Oncideres albomarginata chamela

Ecosystem engineering by insect herbivores occurs as the result of structural modification of plants manipulated by insects. However, only few studies have evaluated the effect of these modifications on the plant responses induced by stem-borers that act as ecosystem engineers. In this study, we evaluated the responses induced by the herbivory of the twig-girdler beetle Oncideres albomarginata chamela (Cerambycidae: Lamiinae) on its host plant Spondias purpurea (Anacardiaceae), and its relationship with the ecosystem engineering process carried out by this stem-borer. Our results demonstrated that O. albomarginata chamela branch removal induced the development of lateral branches increasing the resources needed for the development of future insect generations, of its own offspring and of many other insect species. Detached branches represent habitats with high content of nitrogen and phosphorous, which eventually can be incorporated into the ecosystem, increasing nutrient cycling efficiency. Consequently, branch removal and the subsequent plant tissue regeneration induced by O. albomarginata chamela represent key mechanisms underlying the ecosystem engineering process carried out by this stem-borer, which enhances arthropod diversity in the ecosystem.

Microbial secondary succession in soil microcosms of a desert oasis in the Cuatro Cienegas Basin, Mexico

Ecological succession is one of the most important concepts in ecology. However for microbial community succession, there is a lack of a solid theoretical framework regarding succession in microorganisms. This is in part due to microbial community complexity and plasticity but also because little is known about temporal patterns of microbial community shifts in different kinds of ecosystems, including arid soils. The Cuatro Cienegas Basin (CCB) in Coahuila, Mexico, is an arid zone with high diversity and endemisms that has recently been threatened by aquifer overexploitation. The gypsum-based soil system of the CCB is one of the most oligotrophic places in the world. We undertook a comparative 16S rRNA 454 pyrosequencing study to evaluate microbial community succession and recovery over a year after disturbance at two sites. Results were related to concurrent measurements of humidity, organic matter and total C and N content. While each site differed in both biogeochemistry and biodiversity, both present similar pattern of change at the beginning of the succession that diverged in later stages. After one year, experimentally disturbed soil was not similar to established and undisturbed adjacent soil communities indicating recovery and succession in disturbed soils is a long process.

Comparative metagenomics of two microbial mats at Cuatro Ciénegas Basin I: ancient lessons on how to cope with an environment under severe nutrient stress

The Cuatro Ciénegas Basin (CCB) is an oasis in the desert of Mexico characterized by low phosphorus availability and by its great diversity of microbial mats. We compared the metagenomes of two aquatic microbial mats from the CCB with different nutrient limitations. We observed that the red mat was P-limited and dominated by Pseudomonas, while the green mat was N-limited and had higher species richness, with Proteobacteria and Cyanobacteria as the most abundant phyla. From their gene content, we deduced that both mats were very metabolically diverse despite their use of different strategies to cope with their respective environments. The red mat was found to be mostly heterotrophic, while the green mat was more autotrophic. The red mat had a higher number of transporters in general, including transporters of cellobiose and osmoprotectants. We suggest that generalists with plastic genomes dominate the red mat, while specialists with minimal genomes dominate the green mat. Nutrient limitation was a common scenario on the early planet; despite this, biogeochemical cycles were performed, and as a result the planet changed. The metagenomes of microbial mats from the CCB show the different strategies a community can use to cope with oligotrophy and persist.

Bacterial communities and the nitrogen cycle in the gypsum soils of Cuatro Ciénegas Basin, coahuila: a Mars analogue

The OMEGA/Mars Express hyperspectral imager identified gypsum at several sites on Mars in 2005. These minerals constitute a direct record of past aqueous activity and are important with regard to the search of extraterrestrial life. Gale Crater was chosen as Mars Science Laboratory Curiosity's landing site because it is rich in gypsum, as are some desert soils of the Cuatro Ciénegas Basin (CCB) (Chihuahuan Desert, Mexico). The gypsum of the CCB, which is overlain by minimal carbonate deposits, was the product of magmatic activity that occurred under the Tethys Sea. To examine this Mars analogue, we retrieved gypsum-rich soil samples from two contrasting sites with different humidity in the CCB. To characterize the site, we obtained nutrient data and analyzed the genes related to the N cycle (nifH, nirS, and nirK) and the bacterial community composition by using 16S rRNA clone libraries. As expected, the soil content for almost all measured forms of carbon, nitrogen, and phosphorus were higher at the more humid site than at the drier site. What was unexpected is the presence of a rich and divergent community at both sites, with higher taxonomic diversity at the humid site and almost no taxonomic overlap. Our results suggest that the gypsum-rich soils of the CCB host a unique microbial ecosystem that includes novel microbial assemblies.

Rainfall and labile carbon availability control litter nitrogen dynamics in a tropical dry forest

N cycling in tropical dry forests is driven by rainfall seasonality but the mechanisms involved are not well understood. We studied the seasonal variation in N dynamics and microbial biomass in the surface litter of a tropical dry forest ecosystem in Mexico over a 2-year period. Litter was collected at 4 different times of the year to determine changes in total, soluble, and microbial C and N concentrations. Additionally, litter from each sampling date was incubated under laboratory conditions to determine potential C mineralization rate, net N mineralization, net C and N microbial immobilization, and net nitrification. Litter C concentrations were highest in the early-dry season and lowest in the rainy season, while the seasonal changes in N concentrations varied between years. Litter P was higher in the rainy than in the early-dry season. Water-soluble organic C (WSOC) and water-soluble N concentrations were highest during the early- and late-dry seasons and represented up to 4.1 and 5.9% of the total C and N, respectively. NH (4) (+) and NO (3) (-) showed different seasonal and annual variations. They represented an average 23% of soluble N. Microbial C was generally higher in the dry than in the wet seasons, while microbial N was lowest in the late-dry and highest in the early-rainy seasons. Incubations showed that lowest potential C mineralization rates and C and N microbial immobilization occurred in rainy season litter, and were positively correlated to WSOC. Net nitrification was highest in rainy season litter. Our results showed that the seasonal pattern in N dynamics was influenced by rainfall seasonality and labile C availability, and not by microbial biomass. We propose a conceptual model to hypothesize how N dynamics in the litter layer of the Chamela tropical dry forest respond to the seasonal variation in rainfall.

Forest-to-pasture conversion influences on soil organic carbon dynamics in a tropical deciduous forest

On a global basis, nearly 42% of tropical land area is classified as tropical deciduous forest (TDF) (Murphy and Lugo 1986). Currently, this ecosystem has very high deforestation rates; and its conversion to cattle pasture may result in losses of soil organic matter, decreases in soil fertility, and increases in CO₂ flux to the atmosphere. The soil organic matter turnover rate in a TDF after pasture conversion was estimated in Mexico by determining natural abundances of¹³C. Changes in these values would be induced by vegetation changes from the C₃ (forest) to the C₄ (pasture) photosynthetic pathway. The rate of loss of remnant forest-soil organic matter (fSOM) was 2.9 t ha^-1 year^-1 in 7-year-old pasture and decreased to 0.66 t ha^-1 year^-1 by year 11. For up to 3 years, net fSOM level increased in pastures; this increment can be attributed to decomposition of remnant forest roots. The sand-associated SOM fraction was the most and the silt-associated fraction the least depleted. TDF conversion to pasture results in extremely high rates of loss of remnant fSOM that are higher than any reported for any tropical forest.