Lake Texcoco Research Articles

Mapping soil salinity using a combined spectral response index for bare soil and vegetation: A case study in the former lake Texcoco, Mexico

Mapping soil salinity is difficult due to its large spatial and temporal variability. Remote sensing is widely used to lower survey costs, but existing studies usually analyze bare soils and make little reference to the halophytic plants and their role as salinity indicators. This paper aims to correlate soil characteristics (electric conductivity in saturation extract (EC e) and sodium absorption ratio (SAR) with the spectral response of plant species and bare soils, integrating an algorithm to allow multi-scale mapping using remote sensors. Ground radiance was measured on different plant species and bare soils. A Combined Spectral Response Index (COSRI) was calculated for bare soils and vegetation by adjusting the normalized difference vegetation index (NDVI). EC e and SAR were determined in surface soil samples. Correlation coefficients between COSRI and soil salinity were obtained and a model was adjusted to predict soil salinity. Landsat-ETM and airborne digital images were used to calculate raster maps of COSRI, and EC e and SAR were estimated using adjusted models. Correlation between COSRI and EC e and SAR was of −0.885 and −0.857, respectively. Variance accounted for by exponential models for EC e and SAR was of 82.6% and 75.1%, respectively. It may be concluded that the method is an easy, low-cost procedure to map salt-affected areas.

Bioremediation of polycyclic aromatic hydrocarbon-contaminated saline–alkaline soils of the former Lake Texcoco

Polycyclic aromatic hydrocarbons (PAHs) such as phenanthrene, anthracene and Benzo[ a]pyrene (BaP) are toxic for the environment. Removing these components from soil is difficult as they are resistant to degradation and more so in soils with high pH and large salt concentrations as in soil of the former lake Texcoco, but stimulating soil micro-organisms growth by adding nutrients might accelerate soil restoration. Soil of Texcoco and an agricultural Acolman soil, which served as a control, were spiked with phenanthrene, anthracene and BaP, added with or without biosolid or inorganic fertilizer (N, P), and dynamics of PAHs, N and P were monitored in a 112-day incubation. Concentrations of phenanthrene did not change significantly in sterilized Acolman soil, but decreased 2-times in unsterilized soil and >25-times in soil amended with biosolid and NP. The concentration of phenanthrene in unsterilized soil of Texcoco was 1.3-times lower compared to the sterilized soil, 1.7-times in soil amended with NP and 2.9-times in soil amended with biosolid. In unsterilized Acolman soil, degradation of BaP was faster in soil amended with biosolid than in unamended soil and soil amended with NP. In unsterilized soil of Texcoco, degradation of BaP was similar in soil amended with biosolid and NP but faster than in the unamended soil. It was found that application of biosolid and NP increased degradation of phenanthrene, anthracene and BaP, but to a different degree in alkaline–saline soil of Texcoco compared to an agricultural Acolman soil.

Micromorphology of opaline features in soils on the sediments of the ex-Lago de Texcoco, México

The former Texcoco Lake (ex-Lago de Texcoco) is situated in the closed Basin in Mexico. The lake was artificially drained in the 20th century. Six soil profiles on the eastern border of the former lake were analysed for mineralogical and microfabric characteristics in order to differentiate between sedimentary and pedogenic processes. Large quantities of silica liberated as a result of weathering of the surrounding volcanic mountains partly precipitated in the lake. As a result, layers of an exceptional sediment, locally called “jaboncillo”, were deposited. They are characterised by a high Si-content and a very high (up to 500%) water retention. In addition several amorphous silica features were formed in these lacustrine conditions. In the centre of the lake opaline nodules, mainly composed of opal-A, formed in the jaboncillo. Near the former coast line concentric silico-calcitic nodules (ooliths) were formed as a result of fluctuations of the input of fresh water and biological activity. Several superposed layers of in situ silicified roots point to frequent changes in the shore line position. After emergence of the sediments, about 100 years ago, a translocation of silica, mainly caused by an alkalisation of the sediment, gave rise to the formation of different pedogenic types of opaline coatings and infillings, resulting locally in a duripan. The micromorphological and mineralogical properties of these silica features are discussed and sedimentary and pedogenic processes considered.

Notes on the reproduction and survival of Moina hutchinsoni Brehm, 1937 (Moinidae: Anomopoda) grown in media of varying salinity

Moina hutchinsoni, a North American species distributed in saline and alkaline waters, was isolated from temporary puddles in the former Texcoco Lake basin near Mexico City. Its growth under laboratory conditions was assayed at different salinity values. Two culture media were tested: reconstituted hard-water plus sea salts and diluted natural brine. The alga Ankistrodesmus falcatus was supplied as food at a concentration of 0.4 x 106 cells ml-1. Salinity levels ranged from 2 to 20. For all experiments, twenty replicates, consisting of one neonate female individually placed in a glass container with 80 ml culture volume, were used. All vessels were reviewed daily and the progeny counted and removed. Culture media and food were renewed each day. Data on the survival and reproduction of M. hutchinsoni were recorded during its entire life cycle. The high average longevity (22.6) days, number of clutches (17.8), and average total progeny (138.9 neonates). were recorded at a salinity of 10 of natural brine water. In the synthetic medium reconstituted hard water plus sea salts, the highest average longevity 9.2 days, one of the highest number of clutches (9.6), and the highest average total progeny (29.5) neonates. were recorded at a salinity of 4. Reproduction and survival was affected by salinity and the culture medium, with the best results recorded in diluted natural brine. The information presented here is the first biological data surveyed for this euryhaline species in experimental conditions.

Amorphous silica materials in soils and sediments of the Ex-Lago de Texcoco, Mexico: An explanation for its subsidence

Mexico City is built on the sediments of the former Texcoco lake (Ex-Lago de Texcoco), which are characterised by the presence of a siliceous deposit, up to 70 m in thickness, called “jaboncillo” (like soap). The jaboncillo is composed dominantly of amorphous silica and iron oxides and hydroxides, with minor amounts of phyllosilicates and detrital volcanic minerals. The mineralogy, the micromorphology and physical and chemical properties are discussed. The optical isotropic groundmass consists largely of amorphous silica and iron hydroxides. The bulk density, calculated for the dry material is about 0.26 g cm −3. Its water retention capacity is extremely high, up to 522% at field capacity, but this characteristic is partly lost after drying. Also the high solubility of Si, the high CEC and COLE are irreversibly reduced after drying, due to a denser packing of the originally randomly distributed hydrated silica spheres. Therefore, it is evident that drainage will irreversibly change physical characteristics, and increase risk of instability and subsidence of the sediments when used for construction.

Phylogenetic analysis of bacterial populations in waters of the former Texcoco Lake, Mexico

Molecular techniques were used to compare the compositions of the bacterial communities of the 2 following lagoons from the former soda Texcoco Lake, Mexico: the restored Facultativa lagoon and the Nabor Carrillo lagoon. Ribosomal intergenic spacer analysis (RISA) revealed that bacterial communities of the 2 lagoons were different and presented a relatively low diversity. Clone libraries of 16S rDNA genes were constructed, and significant phylotypes were distinguished by restriction fragment length polymorphism (RFLP). A representative clone from each phylotype was partially sequenced. Molecular identification and phylogenetic analyses based on ribosomal sequences revealed that the Facultativa lagoon harbored mainly gamma- and beta-Proteobacteria, low G+C Gram-positive bacteria, and several members of the Halobacteriaceae family of archaea. The Nabor Carrillo lagoon mainly included typical halophilic and alkaliphilic low G+C Gram-positive bacteria, gamma-Proteobacteria, and beta-Proteobacteria similar to those found in other soda lakes. Several probably noncultured new bacterial species were detected. Three strains were isolated from the Nabor Carrillo lagoon, their partial 16S rDNA sequences were obtained. On this basis, they were identified as Halomonas magadiensis (H1), Halomonas eurihalina (H2), and Staphylococcus sciuri (H3). This is the first study that uses molecular techniques to investigate potential genetic diversity in the Texcoco lakes. In this preliminary evaluation, we infer the presence of alkalophilic, halophilic, or haloalkaliphilic bacteria potentially useful for biotechnology.

The impacts of inorganic nitrogen application on mineralization of 14C-labelled maize and glucose, and on priming effect in saline alkaline soil

Organic matter dynamics and nutrient availability in saline alkaline soil of the former lake Texcoco will determine the success of a planned reforestation program. Uniformly labelled 14C-maize (MAI-treatment) and glucose (GLU-treatment) with or without 200 mg NH 4 + − N kg −1 soil (MAI-N treatment and GLU-N treatment, respectively) were added to soils with electrolytic conductivity (EC) 56 dS m −1 (soil A) and 12 dS m −1 (soil B) to investigate the importance of N availability on decomposition of organic material. Production of CO 2 and CO 2 14 and inorganic N dynamics were monitored. The amount of 14C-glucose mineralized increased 1.8-times in the soil A, but had no effect in the soil B when 200 mg NH 4 + − N kg −1 soil was added, while the amount of 14C-maize mineralized increased 1.7 and 1.3-times when 200 NH 4 + − N kg −1 soil was added in the soils A and B, respectively. Application of NH 4 + increased priming effect 3.7-times in the MAI-treatment of the soil A and 3.4-times in the GLU-treatment, while in the soil B the increase of priming effect was 4.1-times in the MAI-treatment and 3.7-times in the GLU-treatment. Of the 200 mg NH 4 + − N kg −1 added to both soils less than 10 mg NH 3–N kg −1 was volatilized within one day, while 22 and 44 mg NH 4 + − N kg −1 soil was fixed on the soil matrix in the soil A and the soil B, respectively. Therefore more than 100 mg NH 4 + −N kg −1 was immobilized into the microbial biomass within the first day. Concentration of nitrite ( NO 2 − ) increased sharply in all the treatments of soil A at the onset of the incubation followed by a decrease. A similar pattern was observed in the GLU-N and MAI-N treatments of the soil B, but not in the other treatments. A decrease in concentration of NO 3 − was observed in both soils followed by an increase in the MAI-N and GLU-N treatments of the soil B. It was found that application of NH 4 + had a stimulating effect on the decomposition of maize and glucose, and on the priming effect, while assimilatory reduction of NO 3 − resulted in an increase of NO 2 − in the soil A, and nitrification in the soil B.

Removal and transformation of recalcitrant organic matter from stabilized saline landfill leachates by coagulation–ozonation coupling processes

The Bordo Poniente sanitary landfill in Mexico City currently receives 11,500 ton/day of solid wastes. The landfill has been in operation since 1985, in what was formerly Texcoco Lake, now a dried-up lakebed. The physico-chemical characteristics of the leachate generated by this particular landfill are altered by the incorporation of freatic saline water present in the area. This paper reports the results from a study evaluating coagulation and ozonation as alternative processes for removing and transforming recalcitrant organic matter from stabilized saline landfill leachate. Coagulation with ferric sulfate was found to remove up to 67% of COD and 96% of leachate color. The remaining 33% COD was removed with ozone. Recalcitrant organic matter removal by ozonation is limited by the reaction kinetic due mainly to ozone's low reactivity with the organic compounds present in the leachates (amines, amides, alcohols, aliphatic compounds, and carboxylic acids). However, ozone contributes greatly to changing the recalcitrant characteristics of organic matter. Leachate biodegradability was found to be significantly enhanced through ozonation: BOD 5 values reach 265%, and the BOD 5/COD ratio increases from 0.003 to 0.015. Infrared analysis of ozonated leachates shows that the main by-products of recalcitrant organic matter ozonation are an increase in the hydroxyl and carboxylic groups, and the presence of aldehydes groups.

A Redescription of Moina Hutchinsoni, a Rare Cladoceran (Branchiopoda: Anomopoda) Found in Remnants of a Mexican Saline Lake, with Notes on Its Life History

Abstract Moina hutchinsoni Brehm, 1937, a nearctic anomopod cladoceran, was observed in shallow ephemeral ponds of the former Texcoco Lake, near Mexico City, Mexico. The type material, from Lake Winnemucca, Nevada, was considered lost; additional topotypic material could not be obtained from that locality because the site is completely dry. The species is redescribed herein based on specimens from Texcoco Lake and Border Lake, Nebraska. Specimens from these localities were compared to study the morphological variability of this species; differences were found in the cuticular hair patterns on head, body, antennules, and second antennae, in the number of feathered setae on the postabdomen, and in the structure of pectens in claws. The life cycle of this rare species was analyzed in the laboratory. In culture, diet consisted of the microalgae Ankistrodesmus falcatus (freshwater) and Chlorella sp. (marine); both algae were consumed by the cladoceran. Culture temperature ranged between 15° and 20°C, and salin...

An Air Pollution Modeling Study Using Three Surface Coverings Near the New International Airport of Mexico City

The dry lakebed of what once was the lake of Texcoco is the location selected for the New International Airport of Mexico City. This project will generate an important urban development near the airport with regional implications on air quality. Using a prognostic air quality model, the consequences of photochemical air pollution in the metropolitan area of Mexico City resulting from three possible coverings for the areas of the lakebed that are not occupied by the runway and terminal building are investigated. These coverings are desert, grassland, and water and occupy an area of 63 km2. This study is based on a representative high pollution episode. In addition to reducing the emission of primary natural particles, the water covering generates a land-water breeze capable of maintaining enough ventilation to reduce pollutant concentrations over a localized region of the metropolitan area and may enhance the wind speed on the coasts of the proposed lake.

Rapid immobilization of applied nitrogen in saline–alkaline soils

The dynamics of inorganic N are important in soil, and this applies particularly to the saline–alkaline soils of the former lake Texcoco in Mexico with high pH and salinity where a forestation program was started in the 1970s. In soils of lake Texcoco, in Mexico, more than 50% of applied N could not be accounted for one day after application of 200 mg kg−1 soil along with glucose amendment. It was not clear whether this was due to abiotic or biotic processes, the form of inorganic N applied or the result of applying an easily decomposable substrate. We investigated this by adding glucose and 200 mg kg−1 soil as (NH4)2SO4-N or KNO3-N to sterilized and unsterilized soil. The changes in inorganic and ninhydrin N, microbial biomass C and production of CO2 were then monitored. Between the time of applying N and extraction with 0.5 M K2SO4, i.e., after ca 2 h, approximately 110 mg NH4 +-N kg−1 dry soil could not be accounted for in the unsterilized and sterilized soil and that remained so for the entire incubation in the sterilized soil. After 1 day this increased to 140 mg NH4 +-N kg−1 dry soil in the unsterilized control and 170 mg NH4 +-N kg−1 dry soil in C amended soil. Volatilization of NH3 accounted for 56 mg NH4 +-N kg−1 so the rest appeared to be adsorbed on the soil matrix. The NH3 volatilization and NH4 + fixed in the soil matrix remained constant over time and no oxidation to NO2 − or NO3 − had occurred, so unaccounted N in unsterilized soil was probably incorporated into the microbial biomass in excess of what was required for metabolic activity. The unaccounted N was ca 70 mg NO3 −–N in nitrate amended soil after 3 days and 138 NO3 −–N when glucose was additionally added. Losses through abiotic processes were absent as inferred from changes in sterilized soil and the aerobic incubation inhibited possible losses through denitrification. It was inferred that NO3 − that could not be accounted for was taken up by micro-organisms in excess of what was required for metabolic activity.

Mineralization of 14 C-labelled maize in alkaline saline soils

The turnover of organic material determines the availability of plant nutrients in unfertilized soils, and this applies particularly to the alkaline saline soil of the former Lake Texcoco in Mexico. Uniformly labelled [14C] maize and its neutral detergent fibre (NDF) fraction, mainly containing cellulose and hemi-cellulose, were added to these soils to investigate dynamics of C and N and the importance of the NDF fraction. Soil with electrolytic conductivity (EC) of 1.2, 3.2, 24.6 and 32.7 dS m−1 was incubated aerobically, while CO2 and 14CO2 production, and inorganic N dynamics (NH4 +, NO2 −, NO3 −) were monitored. The amount of 14C-labelled maize mineralized after 97 days was >500 mg C kg−1 dry soil (D.S.) of the 1000 mg C kg−1 D.S. added in soils with EC≤ 24.6 dS m−1, but only 257 mg C kg−1 D.S. in soil with EC 32.7 dS m−1. The decomposition of the NDF fraction showed a lag, greatest in the soil with the largest EC and the amount of 14C-labelled NDF fraction mineralized after 97 days was > 300 mg C kg−1 D.S. in soils with EC ≤ 3.2 dS m−1, but in the soil with EC 32.7 dS m−1 it was only 118 mg C kg−1D.S. Application of 14C-labelled maize and the NDF fraction induced a priming effect, most accentuated at the onset of the incubation. The ratio between the amount of CO2 produced due to the priming effect and the 14CO2 produced was 16-times larger when 250 mg maize-C kg−1 D.S. was added and only 3-times when 2000 mg maize-C kg−1 D.S. was added. Oxidation of NO2 − occurred in soil with EC 32.7 dS m−1 as witnessed by decreases in concentration of NO2 − and increases in concentration of NO3 −. It was found that EC affected the decomposition of maize, the NDF fraction and the priming effect. Decomposition of cellulose and oxidation of NO2 − occurred in soil with EC 32.7 dS m−1 although cellulolytic micro-organisms and autotrophic NO2 − oxidizers could previously not be isolated from this soil.

Incorporation of glucose- 14 C and NH 4 + in microbial biomass of alkaline saline soil

The turnover of organic material determines the availability of plant nutrients in unfertilized soils, and this applies particularly to the alkaline saline soil of the former Lake Texcoco in Mexico. Approximately 10% of 1,000 mg of 14C-labelled glucose added to this soil remained in the microbial biomass C and half of 200 mg NH4 +-N added could not be accounted for within a day of application. We investigated whether the added 14C and NH4 + could be liberated by adding a further 1,000 mg 14C-labelled or unlabelled glucose C kg–1 dry soil and 200 mg NH4 +-N kg–1 dry soil to soils with electrolytic conductivity (EC) 1.2, 24.6 and 32.7 dS m–1 previously amended with 1,000 mg unlabelled glucose or 14C-labelled glucose. Application of unlabelled glucose at day 70 increased production of 14CO2 in soil with EC 32.7 dS m–1, but not in the other soils. Approximately 100 mg NH4 +-N kg–1 could not be accounted for within a day after application of 200 mg NH4 +-N kg–1 and that remained so for the entire 70 days. However, after the second application, NH4 + was released and oxidized to NO3 – in soil with EC ≤24.6 dS m–1, but only to NO2 – in soil with EC 32.7 dS m–1. It was found that the second application of glucose only affected dynamics of 14C in soil with EC 32.7 dS m–1. The soil of the former Lake Texcoco was depleted in N, and an EC of 32.7 dS m–1 inhibited oxidation of NO2 –.

A modeling study of air pollution modulation through land-use change in the Valley of Mexico

Using a prognostic air quality model it is shown that by the partial recovery of Lake Texcoco near the Metropolitan Area of Mexico City, a local reduction of about 11% to 15% can be achieved in concentration levels of ozone (O 3), carbon monoxide (CO), sulphur dioxide (SO 2) and chemical compounds with the potential of forming aerosols. This phenomenon takes place because of the land–lake breeze effect and local temperature modulation induced by the location and size of the recovered lake areas.

Geochemical evolution of groundwater beneath Mexico City

The geochemical evolution of groundwaters along a 24km flow beneath Central Mexico City path from the Sierra de las Cruces towards Lake Texcoco has been investigated using stable isotopes, radiocarbon and major and trace elements to determine the natural baseline conditions, the extent of any contamination and the effectiveness of the overlying aquitard seal. Modern groundwaters of low salinity (<200mgl−1) are found up to 11km from the outcrop area and groundwater ages of up to 6000yrbp occur in the middle part of the section. Groundwater stable isotope ratios δ18O and δ2H lie close to the global meteoric water line, indicating that the groundwater originates from local rainfall. The groundwater chemistry may be interpreted as the result of inputs from the source area with progressive water–rock interaction down the horizontal flow gradient. A redox boundary is found at 9km along the line of section, coincident with the start of the confined section. Relatively low nitrate concentrations (below 9mgl−1 NO3–N) are found in the aerobic waters; low concentrations of NO3 in the aerobic waters and low Cl reflect inputs prior to the modern development. Some elements (Cr, U, As, Se, Sb) increase their concentration with distance (time) as far as the redox boundary, but low concentrations occur in the reducing aquifer section. The chemistry of several major ions (Mg, Na/Cl, K) as well as trace elements such as Li, Rb, Ba reflect the weathering of the basaltic mineral assemblage (feldspars and mafic minerals) and their increases are generally proportional to residence time; phosphate, F and I concentrations indicate a probable source from apatite in the basaltic or rhyolitic rocks. A borehole in the east of the city (some 17km downgradient) intercepted thermal water (Si geothermometry indicates 163°C at depth). This water gives a distinctive composition indicating possible addition of metamorphic CO2 which has then reacted with the igneous rocks. Increases in B and Cl are derived from volatiles trapped in the glass or vesicular basalt. A thermal anomaly found in the middle section of the heavily pumped aquifer is interpreted as the up-coming of warmer water from medium to greater depth mainly from basalts, rhyolites and possibly limestones.The geochemistry indicates that groundwater beneath Mexico City is of good quality and there is no obvious evidence of leakage of inorganic compounds from surface sources of contamination through the aquitard. The younger groundwater drawn from the western outcrop area is generally of good inorganic quality. Increased drawdowns in the confined aquifer have induced flow of warmer water with higher Cl from depth. The resources in the aquifer represent an important reserve of good quality water which need to be properly managed as a high quality resource as part of integrated plans for the City's future supplies.

Simulating the dynamics of glucose and NH4+ in alkaline saline soils of the former Lake Texcoco with the Detran model

SummaryThe turnover of organic matter determines the availability of plant nutrients in unfertilized soils, and this applies particularly to the alkaline saline soil of the former Lake Texcoco in Mexico. We investigated the effects of alkalinity and salinity on dynamics of organic material and inorganic N added to the soil. Glucose labelled with 14C was added to soil of the former Lake Texcoco drained for different lengths of time, and dynamics of 14C, C and N were investigated with the Detran model. Soil was sampled from an undrained plot and from three drained for 1, 5 and 8 years, amended with 1000 mg 14C‐labelled glucose kg−1 and 200 mg NH4+‐N kg−1, and incubated aerobically. Production of 14CO2 and CO2, dynamics of NH4+, NO2– and NO3–, and microbial biomass 14C, C and N were monitored and simulated with the Detran model. A third stable microbial biomass fraction had to be introduced in the model to simulate the dynamics of glucose, because &gt; 90 mg 14C kg−1 soil persisted in the soil microbial biomass after 97 days. The observed priming effect was mostly due to an increased decay of soil organic matter, but an increased turnover of the microbial biomass C contributed somewhat to the phenomenon. The dynamics of NH4+ and NO3– in the NH4+‐amended soil could not be simulated unless an immobilization of NH4+ into the microbial biomass occurred in the first day of the incubation without an immediate incorporation of it into microbial organic material. The dynamics of C and a priming effect could be simulated satisfactorily, but the model had to be adjusted to simulate the dynamics of N when NH4+ was added to alkaline saline soils.

Dynamics of 14C-labelled glucose in alkaline saline soil

The application of glucose C or sewage sludge C to undrained alkaline saline soil of the former lake of Texcoco and soils drained for 1, 5 and 8 years significantly increased CO2 production only for the 8-year site. The added organic material could have been immobilised in the microbial biomass or sequestered. To elucidate possible mechanisms involved, we investigated the fate of 14C-labelled glucose when added to soil, under laboratory conditions. Soil was sampled from an undrained plot and from plots drained for 1 , 5 and 8 years, amended with 1000mg 14C-labelled glucose C kg−1 dry soil (DS) and 200mg NH4+–Nkg−1 DS, and incubated aerobically for 97 days at 22±1°C. 14CO2 and CO2 production, inorganic-N dynamics (NH4+, NO2−, NO3−), microbial biomass 14C, C and ninhydrin N, glucose and NH3 volatilization were monitored.The 14CO2 production was fastest in the soil drained for 8 years with electrolytic conductivity of 2.6dSm−1 and lowest in the soil drained for 1 year with conductivity of 79.9dSm−1. There was a lag between uptake of glucose and production of 14CO2, which increased with increased conductivity. A priming effect of 250, 674 and 221mg CO2–C kg−1 DS was observed in the undrained soil and soils drained for 1 and 5 years, but it was absent in the soil drained for 8 years. The priming effect was explained by a replacement of the microbial biomass by newly formed biomass in the undrained soil and soil drained for 5 years but partly by increased decomposition of soil organic matter in the soil drained for 1 year. Nitrification, as indicated by increases in NO3− concentrations, occurred even in the soil drained for 1 year, but oxidisers of NO2− were inhibited as indicated by temporal increases in concentrations of NO2−. Calculated efficiencies for 14C were affected by soil characteristics.We found that sequestration of added organic C as found in previous experiments was not confirmed but soil characteristics changed by drainage affected decomposition of added organic material. Care should be taken when salts are applied to soil to study possible effects of conductivity on C and N dynamics as micro-organisms adapt to these specific conditions.

Chemical and biological characteristics of alkaline saline soils from the former Lake Texcoco as affected by artificial drainage

Soils from the former Lake Texcoco are alkaline saline and were artificially drained and irrigated with sewage effluents since the late 1980s. Undrained soil and soil drained for 1, 5 and 8 years were sampled, characterized and incubated aerobically for 90 days at 22±1 °C while production of CO2, available P and concentrations of NH4+, NO2– and NO3– were monitored. Artificial drainage decreased pHH2O, water holding capacity, organic C, total N, and Na+, K+, Mg2+, B, Cl– and SO42– concentrations, increased inorganic C and Ca2+ concentrations more than 5-fold while total P was not affected. Microbial biomass C decreased with increased length of drainage but bacteria, actinomycetes, denitrifiers and cellulose-utilizing bacteria tended to show opposite trends. CO2 production was less in soils drained ≥5 years compared to undrained soil but more than in soils drained for 1 year. Emission of NH3 was negligible and concentrations of NH4+ remained constant over time in each soil. Nitrification, as witnessed by increases in NO3– concentrations, occurred in soil drained for 8 years. NO2– concentrations decreased in soils drained ≤1 year in the first 7 days of the incubation and remained constant thereafter. It was found that artificial drainage of soils from the former Lake Texcoco profoundly affected soil characteristics. Decreases in pH and Na+, K+, Cl– and SO42– concentrations made conditions more favourable for plant growth, although low concentrations of inorganic N and available P might be limiting factors.

Cladophorella netzahualpillii sp. nov. (Cladophorales, Ulvophyceae), a species reproducing by spores

A new species of Cladophorella, C. netzahualpillii Galicia & Novelo, is described from the walls of some tanks of brackish water near Texcoco Lake, Mexico. The characteristic features of this cladophoroid filamentous alga are its reproduction by spores, the absence of a system of prostrate filaments with only a few rhizoids and the simultaneous germination of series of akinetes to form a pseudoparenchymatous basal system. The thallus forms tufts 4–5 mm high, with mainly spherical intercalary and terminal akinetes, which have a lamellated wall and a thick sheath. The sporangia are mainly terminal and cylindrical or triangular in shape with spherical or oblong spores.

Mexico City and the biogeochemistry of global urbanization

Mexico City is far advanced in its urban evolution, and cities in currently developing nations may soon follow a similar course. This paper investigates the strengths and weaknesses of infrastructures for the emerging megacities. The major driving force for infrastructure change in Mexico City is concern over air quality. Air chemistry data from recent field campaigns have been used to calculate fluxes in the atmosphere of the Valley of Mexico, for compounds that are important to biogeochemistry including methane (CH 4 ), carbon monoxide (CO), nonmethane hydrocarbons (NMHCs), ammonia (NH 3 ), sulfur dioxide (SO 2 ), nitrogen oxides (NO x and NO y ), soot, and dust. Leakage of liquified petroleum gas approached 10% during sampling periods, and automotive pollutant sources in Mexico City were found to match those in developed cities, despite a lower vehicle-to-person ratio of 0.1. Ammonia is released primarily from residential areas, at levels sufficient to titrate pollutant acids into particles across the entire basin. Enhancements of reduced nitrogen and hydrocarbons in the vapor phase skew the distribution of NO y species towards lower average deposition velocities. Partly as a result, downwind nutrient deposition occurs on a similar scale as nitrogen fixation across Central America, and augments marine nitrate upwelling. Dust suspension from unpaved roads and from the bed of Lake Texcoco was found to be comparable to that occurring on the periphery of the Sahara, Arabian, and Gobi deserts. In addition, sodium chloride (NaCl) in the dust may support heterogeneous chlorine oxide (ClO x ) chemistry. The insights from our Mexico City analysis have been tentatively applied to the upcoming urbanization of Asia.