Christina Siebe

Patterns of β-diversity in a Mexican tropical dry forest

Abstract Patterns of β-diversity in a highly diverse tropical dry forest tree community are described; the contribution of environmental heterogeneity and distance to β-diversity was assessed. Significant differences in elevation, insolation, slope and soil water holding capacity (p < 0.01), variables related to water availability, were found among 8 30 m × 100 m transects laid along contrasting slopes of a system of three parallel microbasins. A gradient in elevation and insolation was found within north-facing transects, among 10 m × 10 m sites; south-facing transects showed an elevation gradient while crest transects showed a gradient in water holding capacity. In total 119 species were registered, with 27 to 64 species per transect, and 4 to 16 species per site. A large β-diversity was found among and within transects; two indices of β-diversity consistently showed a higher β-diversity within transects than among them. Among transects, 64% of the variance in species composition could be attributed to the environmental variables; an additional 22% to the spatial distribution of sites. Within transects, 42% of the deviance in β-diversity values was explained by insolation, and 19% by distance. β-diversity increased with distance and with difference in insolation among sites; north-facing transects, those with most contrasting insolation conditions, had the steepest increase in β-diversity with distance. Such increase was clearly associated with changes in species composition, not with changes in species richness. Nomenclature: Lott (1993; in press). Abbreviations: AWHC = Available water holding capacity; TDF = Tropical dry forest.

Accumulation of Pharmaceuticals, Enterococcus, and Resistance Genes in Soils Irrigated with Wastewater for Zero to 100 Years in Central Mexico

Irrigation with wastewater releases pharmaceuticals, pathogenic bacteria, and resistance genes, but little is known about the accumulation of these contaminants in the environment when wastewater is applied for decades. We sampled a chronosequence of soils that were variously irrigated with wastewater from zero up to 100 years in the Mezquital Valley, Mexico, and investigated the accumulation of ciprofloxacin, enrofloxacin, sulfamethoxazole, trimethoprim, clarithromycin, carbamazepine, bezafibrate, naproxen, diclofenac, as well as the occurrence of Enterococcus spp., and sul and qnr resistance genes. Total concentrations of ciprofloxacin, sulfamethoxazole, and carbamazepine increased with irrigation duration reaching 95% of their upper limit of 1.4 µg/kg (ciprofloxacin), 4.3 µg/kg (sulfamethoxazole), and 5.4 µg/kg (carbamazepine) in soils irrigated for 19–28 years. Accumulation was soil-type-specific, with largest accumulation rates in Leptosols and no time-trend in Vertisols. Acidic pharmaceuticals (diclofenac, naproxen, bezafibrate) were not retained and thus did not accumulate in soils. We did not detect qnrA genes, but qnrS and qnrB genes were found in two of the irrigated soils. Relative concentrations of sul1 genes in irrigated soils were two orders of magnitude larger (3.15×10−3±0.22×10−3 copies/16S rDNA) than in non-irrigated soils (4.35×10−5±1.00×10−5 copies/16S rDNA), while those of sul2 exceeded the ones in non-irrigated soils still by a factor of 22 (6.61×10–4±0.59×10−4 versus 2.99×10−5±0.26×10−5 copies/16S rDNA). Absolute numbers of sul genes continued to increase with prolonging irrigation together with Enterococcus spp. 23S rDNA and total 16S rDNA contents. Increasing total concentrations of antibiotics in soil are not accompanied by increasing relative abundances of resistance genes. Nevertheless, wastewater irrigation enlarges the absolute concentration of resistance genes in soils due to a long-term increase in total microbial biomass.

Environmental impact of wastewater irrigation in central Mexico: an overview

The current knowledge on the environmental impact and public health risks associated with wastewater irrigation in central Mexico is summarized and discussed. Wastewater reuse in that semiarid region represents a valuable resource in agricultural production, due to the irrigation supply and considerable nutrients input to the farming soil. Nevertheless, there is evidence of increased prevalence of parasitic infections among agricultural workers and their families exposed to raw wastewater irrigation. Negative environmental effects may result from long‐term wastewater application, due to heavy metal accumulation in soils, increasing amounts of immediately mobile and easily mobilizable metal fractions, as well as crops uptake. First investigations carried out on soil fertility suggest the occurrence of important nitrogen losses, which could represent potential risks to public health induced by elevated nitrate concentrations in groundwater. Emphasis is given on future research priorities and measures that h...

Does Long-Term Irrigation with Untreated Wastewater Accelerate the Dissipation of Pharmaceuticals in Soil?

Long-term irrigation with untreated wastewater may increase soil microbial adaptation to pollution load and lead to enhanced natural attenuation. We hypothesized that long-term wastewater irrigation accelerates the dissipation of pharmaceuticals. To test our hypothesis we performed an incubation experiment with soils from the Mezquital Valley, Mexico that were irrigated for 0, 14, or 100 years. The results showed that the dissipation half-lives (DT50) of diclofenac (<0.1-1.4 days), bezafibrate (<0.1-4.8 days), sulfamethoxazole (2-33 days), naproxen (6-19 days), carbamazepine (355-1,624 days), and ciprofloxacin were not affected by wastewater irrigation. Trimethoprim dissipation was even slower in soils irrigated for 100 years (DT50: 45-72 days) than in nonirrigated soils (DT50: 12-16 days), was negatively correlated with soil organic matter content and soil-water distribution coefficients, and was inhibited in sterilized soils. Applying a kinetic fate model indicated that long-term irrigation enhanced sequestration of cationic or uncharged trimethoprim and uncharged carbamazepine, but did not affect sequestration of fast-dissipating zwitterions or negatively charged pharmaceuticals. We conclude that microbial adaptation processes play a minor role for pharmaceutical dissipation in wastewater-irrigated soils, while organic matter accumulation in these soils can retard trimethoprim and carbamazepine dissipation.

Wastewater Irrigation Increases the Abundance of Potentially Harmful Gammaproteobacteria in Soils in Mezquital Valley, Mexico

ABSTRACT Wastewater contains large amounts of pharmaceuticals, pathogens, and antimicrobial resistance determinants. Only a little is known about the dissemination of resistance determinants and changes in soil microbial communities affected by wastewater irrigation. Community DNAs from Mezquital Valley soils under irrigation with untreated wastewater for 0 to 100 years were analyzed by quantitative real-time PCR for the presence of sul genes, encoding resistance to sulfonamides. Amplicon sequencing of bacterial 16S rRNA genes from community DNAs from soils irrigated for 0, 8, 10, 85, and 100 years was performed and revealed a 14% increase of the relative abundance of Proteobacteria in rainy season soils and a 26.7% increase in dry season soils for soils irrigated for 100 years with wastewater. In particular, Gammaproteobacteria, including potential pathogens, such as Pseudomonas, Stenotrophomonas, and Acinetobacter spp., were found in wastewater-irrigated fields. 16S rRNA gene sequencing of 96 isolates from soils irrigated with wastewater for 100 years (48 from dry and 48 from rainy season soils) revealed that 46% were affiliated with the Gammaproteobacteria (mainly potentially pathogenic Stenotrophomonas strains) and 50% with the Bacilli, whereas all 96 isolates from rain-fed soils (48 from dry and 48 from rainy season soils) were affiliated with the Bacilli. Up to six types of antibiotic resistance were found in isolates from wastewater-irrigated soils; sulfamethoxazole resistance was the most abundant (33.3% of the isolates), followed by oxacillin resistance (21.9% of the isolates). In summary, we detected an increase of potentially harmful bacteria and a larger incidence of resistance determinants in wastewater-irrigated soils, which might result in health risks for farm workers and consumers of wastewater-irrigated crops.

Effects of 100 years wastewater irrigation on resistance genes, class 1 integrons and IncP-1 plasmids in Mexican soil

Long-term irrigation with untreated wastewater can lead to an accumulation of antibiotic substances and antibiotic resistance genes in soil. However, little is known so far about effects of wastewater, applied for decades, on the abundance of IncP-1 plasmids and class 1 integrons which may contribute to the accumulation and spread of resistance genes in the environment, and their correlation with heavy metal concentrations. Therefore, a chronosequence of soils that were irrigated with wastewater from 0 to 100 years was sampled in the Mezquital Valley in Mexico in the dry season. The total community DNA was extracted and the absolute and relative abundance (relative to 16S rRNA genes) of antibiotic resistance genes (tet(W), tet(Q), aadA), class 1 integrons (intI1), quaternary ammonium compound resistance genes (qacE+qacEΔ1) and IncP-1 plasmids (korB) were quantified by real-time PCR. Except for intI1 and qacE+qacEΔ1 the abundances of selected genes were below the detection limit in non-irrigated soil. Confirming the results of a previous study, the absolute abundance of 16S rRNA genes in the samples increased significantly over time (linear regression model, p < 0.05) suggesting an increase in bacterial biomass due to repeated irrigation with wastewater. Correspondingly, all tested antibiotic resistance genes as well as intI1 and korB significantly increased in abundance over the period of 100 years of irrigation. In parallel, concentrations of the heavy metals Zn, Cu, Pb, Ni, and Cr significantly increased. However, no significant positive correlations were observed between the relative abundance of selected genes and years of irrigation, indicating no enrichment in the soil bacterial community due to repeated wastewater irrigation or due to a potential co-selection by increasing concentrations of heavy metals.

Optimising seedling management: Pouteria sapota, Diospyros digyna, and Cedrela odorata in a Mexican rainforest

Seedlings of three commercial native tree species were planted under heterogeneous light and nutrient conditions in primary rainforest, secondary forest, and open pasture in Los Tuxtlas (Veracruz, Mexico). Management consisted of weeding around seedlings, and watering during the dry period. The objective was to find those natural growth conditions that maximize height growth in the first 2 years after transplantation from a nursery. Using stepwise multiple linear regression, the combined effect of varying canopy openness, leaf nutrients, initial seedling height, and seed mass on the plant height at the end of the study were analysed. The effect of the light environment differed clearly between species: the fruit species Pouteria sapota (Jacquin) H.E. Moore & Stearn (Mamey) and Diospyros digyna Jacquin (Black Sapote) revealed an optimal canopy openness of 60 and 55%, respectively, while the timber species Cedrela odorata Linnaeus (Spanish Cedar) grew best under maximum openness. Consequently, P. sapota and D. digyna are recommendable for an enrichment or shelterwood system in the forest, while C. odorata is recommendable as a reforestation species in the open. For P. sapota and D. digyna, plant height variation at the end of the study was also explained by the leaf zinc concentration, initial plant height, and in P. sapota by the seed mass and leaf calcium/magnesium ratio (for C. odorata, seed mass and nutrients had not been measured). The regression model indicated that considerable height increases are possible by optimizing these growth parameters within the encountered ranges. For P. sapota, average height growth after 24 months in the field could be increased 2.5-fold, from 111 cm with average values to 280 cm with optimal values. For D. digyna, average height growth after 17 months could be increased 1.8-fold, from 78 to 138 cm. For C. odorata, the possible increase after 16 months was 2.7-fold, from 55 to 147 cm. # 2000 Elsevier Science B.V. All rights reserved.

Long-term Wastewater Irrigation Reduces Sulfamethoxazole Sorption, but Not Ciprofloxacin Binding, in Mexican Soils

As a consequence of population growth and urbanization, arable fields are increasingly irrigated with wastewater, but the related environmental and health risks (e.g., pollution with antibiotics) are poorly understood. We performed batch sorption experiments with sulfamethoxazole (SMX) and ciprofloxacin (CIP) and soils that had been irrigated with untreated wastewater for 0, 14, 35, and 100 yr. Sorption of CIP was always strong and largely irreversible irrespective of the duration of wastewater irrigation and the content and quality of soil organic matter (SOM) (Freundlich sorption coefficient, : 346-979 mg L kg; 1/: 0.62-0.76) but decreased with increasing soil pH due to a decreasing fraction of the cationic species. Sorption of SMX and sorption hysteresis were stronger in the nonirrigated soil (: 4.14 mg L kg ± 0.02; 1/: 0.69 ± 0.02) than in the irrigated soils (: 0.65-1.38 mg L kg; 1/: 0.68-0.75). Irrigation (e.g., competition with SMX accumulated in soil or with other organic compounds contained in wastewater) and SOM quality (i.e., increase of carboxylic moieties with increasing time of irrigation) had a stronger effect on SMX sorption and its hysteresis than soil organic carbon content. Whereas sorption of SMX can be reduced by long-term irrigation with wastewater, sorption of CIP is intense also after prolonged irrigation.

Environmental Risks and Cost-Effective Risk Management in Wastewater Use Systems

Wastewater use in agriculture has many potential benefits, yet it also poses environmental risks. In particular, the use of untreated or partially treated wastewater over the long run may result in negative impacts on irrigated crops, soils, and groundwater through the addition of excessive levels of metals and metalloids, nutrients, salts and specific ionic species, and micro- pollutants. The environmental risk reduction strategies for wastewater can be categorized into: (1) treatment of wastewater to a desired effluent quality; (2) on-farm wastewater treatment options; and (3) farm-based measures to reduce risks in areas irrigated by untreated or partially treated wastewater. However, the number of strategies that have been economically assessed and have proven to be cost-effective is rather limited, although all mention a positive impact. Despite limited examples, the economics of risk management reveal that cost-effective options for improving water quality by removing undesirable constituents are available at the treatment plant level and beyond.

Water flow paths are hotspots for the dissemination of antibiotic resistance in soil

Antibiotic resistance genes in soil pose a potential risk for human health. They can enter the soil by irrigation with untreated or insufficiently treated waste water. We hypothesized that water flow paths trigger the formation of antibiotic resistance, since they transport antibiotics, multi-resistant bacteria and free resistance genes through the soil. To test this, we irrigated soil cores once or twice with waste water only, or with waste water added with sulfamethoxazole (SMX) and ciprofloxacin (CIP). The treatments also contained a dye to stain the water flow paths and allowed to sample these separately from unstained bulk soil. The fate of SMX and CIP was assessed by sorption experiments, leachate analyses and the quantification of total and extractable SMX and CIP in soil. The abundance of resistance genes to SMX (sul1 and sul2) and to CIP (qnrB and qnrS) was quantified by qPCR. The sorption of CIP was larger than the dye and SMX. Ciprofloxacin accumulated exclusively in the water flow paths but the resistance genes qnrB and qnrS were not detectable. The SMX concentration in the water flow paths doubled the concentration of the bulk soil, as did the abundance of sul genes, particularly sul1 gene. These results suggest that flow paths do function as hotspots for the accumulation of antibiotics and trigger the formation of resistance genes in soil. Their dissemination also depends on the mobility of the antibiotic, which was much larger for SMX than for CIP.