Prevalence and drivers of abrupt vegetation shifts in global drylands.

The constant provision of plant productivity is integral to supporting the liability of ecosystems and human wellbeing in global drylands. Drylands are paradigmatic examples of systems prone to experiencing abrupt changes in their functioning. Indeed, space-for-time substitution approaches suggest that abrupt changes in plant productivity are widespread, but this evidence is less clear using observational time series or experimental data at a large scale. Studying the prevalence and, most importantly, the unknown drivers of abrupt (rather than gradual) dynamical patterns in drylands may help to unveil hotspots of current and future dynamical instabilities in drylands. Using a 20-y global satellite-derived temporal assessment of dryland Normalized Difference Vegetation Index (NDVI), we show that 50% of all dryland ecosystems exhibiting gains or losses of NDVI are characterized by abrupt positive/negative temporal dynamics. We further show that abrupt changes are more common among negative than positive NDVI trends and can be found in global regions suffering recent droughts, particularly around critical aridity thresholds. Positive abrupt dynamics are found most in ecosystems with low seasonal variability or high aridity. Our work unveils the high importance of climate variability on triggering abrupt shifts in vegetation and it provides missing evidence of increasing abruptness in systems intensively managed by humans, with low soil organic carbon contents, or around specific aridity thresholds. These results highlight that abrupt changes in dryland dynamics are very common, especially for productivity losses, pinpoint global hotspots of dryland vulnerability, and identify drivers that could be targeted for effective dryland management.

Climate mediates the biodiversity-ecosystem stability relationship globally.

The insurance hypothesis, stating that biodiversity can increase ecosystem stability, has received wide research and political attention. Recent experiments suggest that climate change can impact how plant diversity influences ecosystem stability, but most evidence of the biodiversity-stability relationship obtained to date comes from local studies performed under a limited set of climatic conditions. Here, we investigate how climate mediates the relationships between plant (taxonomical and functional) diversity and ecosystem stability across the globe. To do so, we coupled 14 years of temporal remote sensing measurements of plant biomass with field surveys of diversity in 123 dryland ecosystems from all continents except Antarctica. Across a wide range of climatic and soil conditions, plant species pools, and locations, we were able to explain 73% of variation in ecosystem stability, measured as the ratio of the temporal mean biomass to the SD. The positive role of plant diversity on ecosystem stability was as important as that of climatic and soil factors. However, we also found a strong climate dependency of the biodiversity-ecosystem stability relationship across our global aridity gradient. Our findings suggest that the diversity of leaf traits may drive ecosystem stability at low aridity levels, whereas species richness may have a greater stabilizing role under the most arid conditions evaluated. Our study highlights that to minimize variations in the temporal delivery of ecosystem services related to plant biomass, functional and taxonomic plant diversity should be particularly promoted under low and high aridity conditions, respectively.

Decoupling of soil nutrient cycles as a function of aridity in global drylands.

The biogeochemical cycles of carbon (C), nitrogen (N) and phosphorus (P) are interlinked by primary production, respiration and decomposition in terrestrial ecosystems. It has been suggested that the C, N and P cycles could become uncoupled under rapid climate change because of the different degrees of control exerted on the supply of these elements by biological and geochemical processes. Climatic controls on biogeochemical cycles are particularly relevant in arid, semi-arid and dry sub-humid ecosystems (drylands) because their biological activity is mainly driven by water availability. The increase in aridity predicted for the twenty-first century in many drylands worldwide may therefore threaten the balance between these cycles, differentially affecting the availability of essential nutrients. Here we evaluate how aridity affects the balance between C, N and P in soils collected from 224 dryland sites from all continents except Antarctica. We find a negative effect of aridity on the concentration of soil organic C and total N, but a positive effect on the concentration of inorganic P. Aridity is negatively related to plant cover, which may favour the dominance of physical processes such as rock weathering, a major source of P to ecosystems, over biological processes that provide more C and N, such as litter decomposition. Our findings suggest that any predicted increase in aridity with climate change will probably reduce the concentrations of N and C in global drylands, but increase that of P. These changes would uncouple the C, N and P cycles in drylands and could negatively affect the provision of key services provided by these ecosystems.

Increasing aridity reduces soil microbial diversity and abundance in global drylands.

Soil bacteria and fungi play key roles in the functioning of terrestrial ecosystems, yet our understanding of their responses to climate change lags significantly behind that of other organisms. This gap in our understanding is particularly true for drylands, which occupy ∼41% of Earth´s surface, because no global, systematic assessments of the joint diversity of soil bacteria and fungi have been conducted in these environments to date. Here we present results from a study conducted across 80 dryland sites from all continents, except Antarctica, to assess how changes in aridity affect the composition, abundance, and diversity of soil bacteria and fungi. The diversity and abundance of soil bacteria and fungi was reduced as aridity increased. These results were largely driven by the negative impacts of aridity on soil organic carbon content, which positively affected the abundance and diversity of both bacteria and fungi. Aridity promoted shifts in the composition of soil bacteria, with increases in the relative abundance of Chloroflexi and α-Proteobacteria and decreases in Acidobacteria and Verrucomicrobia. Contrary to what has been reported by previous continental and global-scale studies, soil pH was not a major driver of bacterial diversity, and fungal communities were dominated by Ascomycota. Our results fill a critical gap in our understanding of soil microbial communities in terrestrial ecosystems. They suggest that changes in aridity, such as those predicted by climate-change models, may reduce microbial abundance and diversity, a response that will likely impact the provision of key ecosystem services by global drylands.

Sewer catchment effects on wastewater and biosolids odour management.

The composition of wastewater in sewer catchments is known to affect the performance of wastewater treatment plants (WWTPs). However, there is limited knowledge as to how catchment characteristics, such as types of catchment industries, impact odour emissions from downstream sludge processing and biosolids management. Odorous emissions from biosolids processing at WWTPs can represent a significant community impact when the local population is exposed to odours. The main odorants emitted from biosolids are volatile sulfur compounds (VSCs), however, volatile organic compounds (VOCs) in emissions may also be perceptable after the removal of VSCs in odour abatement systems. Types of compounds present in emissions throughout biosolids processing at five WWTPs of varying sizes and levels of treatment (primary only and primary and secondary) were analysed. The ratio of total VSCs to VOCs in emissions, and the sensorial importance of each class varied between the sites. As a number of the VOCs in emissions were of industrial origin, this variation is likely dependent on industrial flows into the upstream sewer catchment. The impact of different emission compositions on both activated carbon and biologically based odour abatement systems were discussed.

Ecology and performance of aerobic granular sludge treating high-saline municipal wastewater.

The successful development of aerobic granular sludge (AGS) for secondary wastewater treatment has been linked to a dedicated anaerobic feeding phase, which enables key microbes such as poly-phosphate-accumulating organisms (PAOs) and glycogen-accumulating organisms to gain a competitive advantage over floc-forming organisms. The application of AGS to treat high-saline sewage and its subsequent impacts on microbial ecology, however, are less well understood. In this study, the impacts of high-saline sewage on AGS development, performance and ecology were investigated using molecular microbiology methods. Two feeding strategies were compared at pilot scale: a full (100%) anaerobic feed; and a partial (33%) anaerobic feed. The results were compared to a neighbouring full-scale conventional activated sludge (CAS) system (100% aerobic). We observed that AGS developed under decreased anaerobic contact showed a comparable formation, stability and nitrogen removal performance to the 100% anaerobically fed system. Analysis of the microbial ecology showed that the altered anaerobic contact had minimal effect on the abundances of the functional nitrifying and denitrifying bacteria and Archaea; however, there were notable ecological differences when comparing different sized granules. In contrast to previous work, a large enrichment in PAOs in AGS was not observed in high-saline wastewater, which coincided with poor observed phosphate removal performance. Instead, AGS exhibited a substantial enrichment in sulfide-oxidising bacteria, which was complemented by elemental analysis that identified the presence of elemental sulfur precipitation. The potential role for these organisms in AGS treating high-saline wastewater is discussed.

Structure and functioning of dryland ecosystems in a changing world.

Understanding how drylands respond to ongoing environmental change is extremely important for global sustainability. Here we review how biotic attributes, climate, grazing pressure, land cover change and nitrogen deposition affect the functioning of drylands at multiple spatial scales. Our synthesis highlights the importance of biotic attributes (e.g. species richness) in maintaining fundamental ecosystem processes such as primary productivity, illustrate how N deposition and grazing pressure are impacting ecosystem functioning in drylands worldwide, and highlight the importance of the traits of woody species as drivers of their expansion in former grasslands. We also emphasize the role of attributes such as species richness and abundance in controlling the responses of ecosystem functioning to climate change. This knowledge is essential to guide conservation and restoration efforts in drylands, as biotic attributes can be actively managed at the local scale to increase ecosystem resilience to global change.

Plant species richness and shrub cover attenuate drought effects on ecosystem functioning across Patagonian rangelands.

Drought is an increasingly common phenomenon in drylands as a consequence of climate change. We used 311 sites across a broad range of environmental conditions in Patagonian rangelands to evaluate how drought severity and temperature (abiotic factors) and vegetation structure (biotic factors) modulate the impact of a drought event on the annual integral of normalized difference vegetation index (NDVI-I), our surrogate of ecosystem functioning. We found that NDVI-I decreases were larger with both increasing drought severity and temperature. Plant species richness (SR) and shrub cover (SC) attenuated the effects of drought on NDVI-I. Grass cover did not affect the impacts of drought on NDVI-I. Our results suggest that warming and species loss, two important imprints of global environmental change, could increase the vulnerability of Patagonian ecosystems to drought. Therefore, maintaining SR through appropriate grazing management can attenuate the adverse effects of climate change on ecosystem functioning.

Soil biodiversity supports the delivery of multiple ecosystem functions in urban greenspaces.

While the contribution of biodiversity to supporting multiple ecosystem functions is well established in natural ecosystems, the relationship of the above- and below-ground diversity with ecosystem multifunctionality remains virtually unknown in urban greenspaces. Here we conducted a standardized survey of urban greenspaces from 56 municipalities across six continents, aiming to investigate the relationships of plant and soil biodiversity (diversity of bacteria, fungi, protists and invertebrates, and metagenomics-based functional diversity) with 18 surrogates of ecosystem functions from nine ecosystem services. We found that soil biodiversity across biomes was significantly and positively correlated with multiple dimensions of ecosystem functions, and contributed to key ecosystem services such as microbially driven carbon pools, organic matter decomposition, plant productivity, nutrient cycling, water regulation, plant-soil mutualism, plant pathogen control and antibiotic resistance regulation. Plant diversity only indirectly influenced multifunctionality in urban greenspaces via changes in soil conditions that were associated with soil biodiversity. These findings were maintained after controlling for climate, spatial context, soil properties, vegetation and management practices. This study provides solid evidence that conserving soil biodiversity in urban greenspaces is key to supporting multiple dimensions of ecosystem functioning, which is critical for the sustainability of urban ecosystems and human wellbeing.

Grazing and ecosystem service delivery in global drylands.

Grazing represents the most extensive use of land worldwide. Yet its impacts on ecosystem services remain uncertain because pervasive interactions between grazing pressure, climate, soil properties, and biodiversity may occur but have never been addressed simultaneously. Using a standardized survey at 98 sites across six continents, we show that interactions between grazing pressure, climate, soil, and biodiversity are critical to explain the delivery of fundamental ecosystem services across drylands worldwide. Increasing grazing pressure reduced ecosystem service delivery in warmer and species-poor drylands, whereas positive effects of grazing were observed in colder and species-rich areas. Considering interactions between grazing and local abiotic and biotic factors is key for understanding the fate of dryland ecosystems under climate change and increasing human pressure.

Analysis of nitrous oxide emissions from aerobic granular sludge treating high saline municipal wastewater.

Conventional activated sludge (CAS)-based wastewater treatment processes have the potential to emit high concentrations of nitrous oxide (N2O) during nitrification and denitrification, which can significantly impact the environmental performance and carbon footprint of wastewater treatment operations. While N2O emissions from CAS have been extensively studied, there is little knowledge of N2O emissions from aerobic granular sludge (AGS) which is now an increasingly popular secondary treatment alternative. The N2O emissions performance of AGS needs to be investigated to ensure that the positive benefits of AGS, such as increased capacity and stable nutrient removal, are not offset by higher emissions. This study quantified N2O emissions from a pilot-scale AGS reactor operated under a range of organic loading rates. A second CAS pilot plant was operated in parallel and under identical loading rates to allow for side-by-side comparison of N2O emissions from floc-based activated sludge. Under low loadings of <0.6 kg COD/m3/d the N2O emission factor from AGS and CAS were similar, at around 1.46 ± 0.1% g N2Oemitted/g ammonium loaded. A step increase in the organic loading rate increased N2O emissions from AGS more so than CAS which appeared to be attributed to the reactor feeding strategy that was required for AGS formation. The use of a separate anaerobic feeding phase which was followed by the aeration phase, resulted in extended periods of low dissolved oxygen (DO) concentrations combined with an initial high biomass ammonium loading rate, which favours N2O production and was exacerbated at higher organic loads. Conversely, the combined feeding plus aeration operation (aerobic feed) employed by the CAS system enabled a more even biomass ammonium loading rate and DO supply. This work has shown that while AGS has many operational benefits, the impacts that aeration profile, loading rate and feeding strategy have on N2O emissions must be considered.

Global ecosystem thresholds driven by aridity.

Aridity, which is increasing worldwide because of climate change, affects the structure and functioning of dryland ecosystems. Whether aridification leads to gradual (versus abrupt) and systemic (versus specific) ecosystem changes is largely unknown. We investigated how 20 structural and functional ecosystem attributes respond to aridity in global drylands. Aridification led to systemic and abrupt changes in multiple ecosystem attributes. These changes occurred sequentially in three phases characterized by abrupt decays in plant productivity, soil fertility, and plant cover and richness at aridity values of 0.54, 0.7, and 0.8, respectively. More than 20% of the terrestrial surface will cross one or several of these thresholds by 2100, which calls for immediate actions to minimize the negative impacts of aridification on essential ecosystem services for the more than 2 billion people living in drylands.

The MARAS dataset, vegetation and soil characteristics of dryland rangelands across Patagonia.

We present the MARAS (Environmental Monitoring of Arid and Semiarid Regions) dataset, which stores vegetation and soil data of 426 rangeland monitoring plots installed throughout Patagonia, a 624.500 km2 area of southern Argentina and Chile. Data for each monitoring plot includes basic climatic and landscape features, photographs, 500 point intercepts for vegetation cover, plant species list and biodiversity indexes, 50-m line-intercept transect for vegetation spatial pattern analysis, land function indexes drawn from 11 measures of soil surface characteristics and laboratory soil analysis (pH, conductivity, organic matter, N and texture). Monitoring plots were installed between 2007 and 2019, and are being reassessed at 5-year intervals (247 have been surveyed twice). The MARAS dataset provides a baseline from which to evaluate the impacts of climate change and changes in land use intensity in Patagonian ecosystems, which collectively constitute one of the world´s largest rangeland areas. This dataset will be of interest to scientists exploring key ecological questions such as biodiversity-ecosystem functioning relationships, plant-soil interactions and climatic controls on ecosystem structure and functioning.

Comparing the performance of aerobic granular sludge versus conventional activated sludge for microbial log removal and effluent quality: Implications for water reuse.

The application of aerobic granular sludge (AGS) technology has increased in popularity, largely due to the smaller physical footprint, enhanced biological nutrient removal performance and ability to perform with a more stable operation when compared to conventional activated sludge (CAS) systems. To date, the ability of AGS to remove microbial pathogens such as; Escherichia coli, Giardia, and Cryptosporidium has not been reported. This study compared the log10 removal performance of commonly used pathogen surrogates (sulfite-reducing clostridia spores, f-RNA bacteriophage, E. coli and total coliforms) by AGS and CAS during the start-up phase, through to maturation. Results showed that AGS performed as well as CAS for the log10 removal performance of all microbial surrogates, except for spores which were removed more effectively by AGS most likely due to greater adherence of spores to the AGS biomass compared to CAS mixed liquor. Results suggest that AGS is capable of meeting or exceeding CAS-equivalent health-based targets for pathogen removal in the context of water recycling as well as not adversely affecting the secondary effluent water quality (suspended solids, turbidity and particle size) in terms of ultraviolet light transmissivity (254 nm). These findings confirmed for the first time that the adoption of AGS operation would not adversely impact downstream tertiary disinfection processes from altered water quality, nor would it require further pathogen treatment interventions in addition to what is already required for CAS systems.

Dynamics of volcanic ash remobilisation by wind through the Patagonian steppe after the eruption of Cordón Caulle, 2011.

Wind erosion of freshly-deposited volcanic ash causes persistent storms, strongly affecting ecosystems and human activity. Wind erosion of the volcanic ash was measured up to 17 months after the ash deposition, at 7 sites located within the ash-deposition area. The mass flux was measured up to 1.5 m above ground level. Mass transport rates were over 125 times the soil wind-erosion rates observed before the ash deposition, reaching up to 6.3 kg m-1 day-1. Total mass transport of ash during the 17 months ranged between 113.6 and 969.9 kg m-1 depending on topographic location and wind exposure. The vertical distribution of the mass flux at sites with higher vegetation cover was generally inverted as compared to sites with lower vegetation cover. This situation lasted 7 months and then a shift towards a more uniform vertical distribution was observed, in coincidence with the beginning of the decline of the mass transport rates. Decay rates differed between sites. Despite changes over time, an inverse linear correlation between the mass transports and the mass-flux gradients was found. Both the mass-flux gradients and the average mass-transport rates were not linked with shear-stress partition parameters, but with the ratio: ash-fall thickness to total vegetation cover.

Microbial diversity drives multifunctionality in terrestrial ecosystems.

Despite the importance of microbial communities for ecosystem services and human welfare, the relationship between microbial diversity and multiple ecosystem functions and services (that is, multifunctionality) at the global scale has yet to be evaluated. Here we use two independent, large-scale databases with contrasting geographic coverage (from 78 global drylands and from 179 locations across Scotland, respectively), and report that soil microbial diversity positively relates to multifunctionality in terrestrial ecosystems. The direct positive effects of microbial diversity were maintained even when accounting simultaneously for multiple multifunctionality drivers (climate, soil abiotic factors and spatial predictors). Our findings provide empirical evidence that any loss in microbial diversity will likely reduce multifunctionality, negatively impacting the provision of services such as climate regulation, soil fertility and food and fibre production by terrestrial ecosystems.

Understanding the impacts of allocation approaches during process-based life cycle assessment of water treatment chemicals.

Chemicals are an important component of advanced water treatment operations not only in terms of economics but also from an environmental standpoint. Tools such as life cycle assessment (LCA) are useful for estimating the environmental impacts of water treatment operations. At the same time, LCA analysts must manage several fundamental and as yet unresolved methodological challenges, one of which is the question of how best to "allocate" environmental burdens in multifunctional processes. Using water treatment chemicals as a case study example, this article aims to quantify the variability in greenhouse gas emissions estimates stemming from methodological choices made in respect of allocation during LCA. The chemicals investigated and reported here are those most important to coagulation and disinfection processes, and the outcomes are illustrated on the basis of treating 1000 ML of noncoagulated and nondisinfected water. Recent process and economic data for the production of these chemicals is used and methodological alternatives for solving the multifunctionality problem, including system expansion and mass, exergy, and economic allocation, are applied to data from chlor-alkali plants. In addition, Monte Carlo simulation is included to provide a comprehensive picture of the robustness of economic allocation results to changes in the market price of these industrial commodities. For disinfection, results demonstrate that chlorine gas has a lower global warming potential (GWP) than sodium hypochlorite regardless of the technique used to solve allocation issues. For coagulation, when mass or economic allocation is used to solve the multifunctionality problem in the chlor-alkali facility, ferric chloride was found to have a higher GWP than aluminum sulfate and a slightly lower burden where system expansion or exergy allocation are applied instead. Monte Carlo results demonstrate that when economic allocation is used, GWP results were relatively robust and resilient to the changes in commodity prices encountered during the study period, with standard deviations less than 6% for all chlor-alkali-produced chemicals reported here. Overall outcomes from the study demonstrate the potential variability in LCA results according to the allocation approach taken and emphasize the need for a consensus approach to water sector LCAs.

Climate and soil attributes determine plant species turnover in global drylands.

AIM: Geographic, climatic, and soil factors are major drivers of plant beta diversity, but their importance for dryland plant communities is poorly known. This study aims to: i) characterize patterns of beta diversity in global drylands, ii) detect common environmental drivers of beta diversity, and iii) test for thresholds in environmental conditions driving potential shifts in plant species composition. LOCATION: 224 sites in diverse dryland plant communities from 22 geographical regions in six continents. METHODS: Beta diversity was quantified with four complementary measures: the percentage of singletons (species occurring at only one site), Whittake's beta diversity (ß(W)), a directional beta diversity metric based on the correlation in species occurrences among spatially contiguous sites (ß(R2)), and a multivariate abundance-based metric (ß(MV)). We used linear modelling to quantify the relationships between these metrics of beta diversity and geographic, climatic, and soil variables. RESULTS: Soil fertility and variability in temperature and rainfall, and to a lesser extent latitude, were the most important environmental predictors of beta diversity. Metrics related to species identity (percentage of singletons and ß(W)) were most sensitive to soil fertility, whereas those metrics related to environmental gradients and abundance ((ß(R2)) and ß(MV)) were more associated with climate variability. Interactions among soil variables, climatic factors, and plant cover were not important determinants of beta diversity. Sites receiving less than 178 mm of annual rainfall differed sharply in species composition from more mesic sites (> 200 mm). MAIN CONCLUSIONS: Soil fertility and variability in temperature and rainfall are the most important environmental predictors of variation in plant beta diversity in global drylands. Our results suggest that those sites annually receiving ~ 178 mm of rainfall will be especially sensitive to future climate changes. These findings may help to define appropriate conservation strategies for mitigating effects of climate change on dryland vegetation.

Plant species richness and ecosystem multifunctionality in global drylands.

Experiments suggest that biodiversity enhances the ability of ecosystems to maintain multiple functions, such as carbon storage, productivity, and the buildup of nutrient pools (multifunctionality). However, the relationship between biodiversity and multifunctionality has never been assessed globally in natural ecosystems. We report here on a global empirical study relating plant species richness and abiotic factors to multifunctionality in drylands, which collectively cover 41% of Earth's land surface and support over 38% of the human population. Multifunctionality was positively and significantly related to species richness. The best-fitting models accounted for over 55% of the variation in multifunctionality and always included species richness as a predictor variable. Our results suggest that the preservation of plant biodiversity is crucial to buffer negative effects of climate change and desertification in drylands.

Infection caused by Rahnella aquatilis.

The authors report a case of a 27-year-old African American woman with sickle cell disease who developed septic shock caused by a Rahnella aquatilis infection associated with a peripherally inserted central venous catheter. The infection was treated successfully with ciprofloxacin. R aquatilis is a Gram-negative rod, first isolated from freshwater in 1976, which has been linked to human disease in rare instances, most commonly in immunosuppressed individuals. In addition to this case report, the authors also review the literature.