Nonrainfall water origins and formation mechanisms

Dryland ecosystems cover 40% of the total land surface on Earth and are defined broadly as zones where precipitation is considerably less than the potential evapotranspiration. Nonrainfall waters (for example, fog and dew) are the least-studied and least-characterized components of the hydrological cycle, although they supply critical amounts of water for dryland ecosystems. The sources of nonrainfall waters are largely unknown for most systems. In addition, most field and modeling studies tend to consider all nonrainfall inputs as a single category because of technical constraints, which hinders prediction of dryland responses to future warming conditions. This study uses multiple stable isotopes (2H, 18O, and 17O) to show that fog and dew have multiple origins and that groundwater in drylands can be recycled via evapotranspiration and redistributed to the upper soil profile as nonrainfall water. Surprisingly, the non–ocean-derived (locally generated) fog accounts for more than half of the total fog events, suggesting a potential shift from advection-dominated fog to radiation-dominated fog in the fog zone of the Namib Desert. This shift will have implications on the flora and fauna distribution in this fog-dependent system. We also demonstrate that fog and dew can be differentiated on the basis of the dominant fractionation (equilibrium and kinetic) processes during their formation using the 17O-18O relationship. Our results are of great significance in an era of global climate change where the importance of nonrainfall water increases because rainfall is predicted to decline in many dryland ecosystems. Fog and dew in the Namib Desert have multiple origins and their formation can be differentiated using stable isotopes. Fog and dew in the Namib Desert have multiple origins and their formation can be differentiated using stable isotopes

The Impact of Rainfall on Soil Moisture Dynamics in a Foggy Desert.

Soil moisture is a key variable in dryland ecosystems since it determines the occurrence and duration of vegetation water stress and affects the development of weather patterns including rainfall. However, the lack of ground observations of soil moisture and rainfall dynamics in many drylands has long been a major obstacle in understanding ecohydrological processes in these ecosystems. It is also uncertain to what extent rainfall controls soil moisture dynamics in fog dominated dryland systems. To this end, in this study, twelve to nineteen months’ continuous daily records of rainfall and soil moisture (from January 2014 to August 2015) obtained from three sites (one sand dune site and two gravel plain sites) in the Namib Desert are reported. A process-based model simulating the stochastic soil moisture dynamics in water-limited systems was used to study the relationships between soil moisture and rainfall dynamics. Model sensitivity in response to different soil and vegetation parameters under diverse soil textures was also investigated. Our field observations showed that surface soil moisture dynamics generally follow rainfall patterns at the two gravel plain sites, whereas soil moisture dynamics in the sand dune site did not show a significant relationship with rainfall pattern. The modeling results suggested that most of the soil moisture dynamics can be simulated except the daily fluctuations, which may require a modification of the model structure to include non-rainfall components. Sensitivity analyses suggested that soil hygroscopic point (sh) and field capacity (sfc) were two main parameters controlling soil moisture output, though permanent wilting point (sw) was also very sensitive under the parameter setting of sand dune (Gobabeb) and gravel plain (Kleinberg). Overall, the modeling results were not sensitive to the parameters in non-bounded group (e.g., soil hydraulic conductivity (Ks) and soil porosity (n)). Field observations, stochastic modeling results as well as sensitivity analyses provide soil moisture baseline information for future monitoring and the prediction of soil moisture patterns in the Namib Desert

The impact of fog on soil moisture dynamics in the Namib Desert

Soil moisture is a crucial component supporting vegetation dynamics in drylands. Despite increasing attention on fog in dryland ecosystems, the statistical characterization of fog distribution and how fog affects soil moisture dynamics have not been seen in literature. To this end, daily fog records over two years (Dec 1, 2014–Nov 1, 2016) from three sites within the Namib Desert were used to characterize fog distribution. Two sites were located within the Gobabeb Research and Training Center vicinity, the gravel plains and the sand dunes. The third site was located at the gravel plains, Kleinberg. A subset of the fog data during rainless period was used to investigate the effect of fog on soil moisture. A stochastic modeling framework was used to simulate the effect of fog on soil moisture dynamics. Our results showed that fog distribution can be characterized by a Poisson process with two parameters (arrival rate λ and average depth α (mm)). Fog and soil moisture observations from eighty (Aug 19, 2015–Nov 6, 2015) rainless days indicated a moderate positive relationship between soil moisture and fog in the Gobabeb gravel plains, a weaker relationship in the Gobabeb sand dunes while no relationship was observed at the Kleinberg site. The modeling results suggested that mean and major peaks of soil moisture dynamics can be captured by the fog modeling. Our field observations demonstrated the effects of fog on soil moisture dynamics during rainless periods at some locations, which has important implications on soil biogeochemical processes. The statistical characterization and modeling of fog distribution are of great value to predict fog distribution and investigate the effects of potential changes in fog distribution on soil moisture dynamics

Water regime history drives responses of soil Namib Desert microbial communities to wetting events

Despite the dominance of microorganisms in arid soils, the structures and functional dynamics of microbial communities in hot deserts remain largely unresolved. The effects of wetting event frequency and intensity on Namib Desert microbial communities from two soils with different water-regime histories were tested over 36 days. A total of 168 soil microcosms received wetting events mimicking fog, light rain and heavy rainfall, with a parallel “dry condition” control. T-RFLP data showed that the different wetting events affected desert microbial community structures, but these effects were attenuated by the effects related to the long-term adaptation of both fungal and bacterial communities to soil origins (i.e. soil water regime histories). The intensity of the water pulses (i.e. the amount of water added) rather than the frequency of wetting events had greatest effect in shaping bacterial and fungal community structures. In contrast to microbial diversity, microbial activities (enzyme activities) showed very little response to the wetting events and were mainly driven by soil origin. This experiment clearly demonstrates the complexity of microbial community responses to wetting events in hyperarid hot desert soil ecosystems and underlines the dynamism of their indigenous microbial communities.The South African National Research Foundation (NRF 88303) and the University of Pretoria.http://www.nature.com/scientificreportsam201

Cyanobacteria drive community composition and functionality in rock-soil interface communities

Most ecological research on hypoliths, significant primary producers in hyperarid deserts, has focused on the diversity of individual groups of microbes (i.e. bacteria). However, microbial communities are inherently complex, and the interactions between cyanobacteria, heterotrophic bacteria, protista and metazoa, are likely to be very important for ecosystem functioning. Cyanobacterial and heterotrophic bacterial communities were analysed by pyrosequencing, while metazoan and protistan communities were assessed by T-RFLP analysis. Microbial functionality was estimated using carbon substrate utilization. Cyanobacterial community composition was significant in shaping community structure and function in hypoliths. Ecological network analysis showed that most significant co-occurrences were positive, representing potential synergistic interactions. There were several highly interconnected associations (modules) and specific cyanobacteria were important in driving the modular structure of hypolithic networks. Together, our results suggest that hypolithic cyanobacteria have strong effects on higher trophic levels and ecosystem functioning.National Research Foundation (South Africa) and the University of Pretoria.http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1755-09982016-02-28hb201

Cyanobacteria drive community composition and functionality in rock-soil interface communities

Most ecological research on hypoliths, significant primary producers in hyperarid deserts, has focused on the diversity of individual groups of microbes (i.e. bacteria). However, microbial communities are inherently complex, and the interactions between cyanobacteria, heterotrophic bacteria, protista and metazoa, are likely to be very important for ecosystem functioning. Cyanobacterial and heterotrophic bacterial communities were analysed by pyrosequencing, while metazoan and protistan communities were assessed by T-RFLP analysis. Microbial functionality was estimated using carbon substrate utilization. Cyanobacterial community composition was significant in shaping community structure and function in hypoliths. Ecological network analysis showed that most significant co-occurrences were positive, representing potential synergistic interactions. There were several highly interconnected associations (modules) and specific cyanobacteria were important in driving the modular structure of hypolithic networks. Together, our results suggest that hypolithic cyanobacteria have strong effects on higher trophic levels and ecosystem functioning.National Research Foundation (South Africa) and the University of Pretoria.http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1755-09982016-02-28hb201

Southern African geomorphology : looking back, moving forward

No abstract availablehttp://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1468-0459hb201

A sequential co-extraction method for DNA, RNA and protein recovery from soil for future system-based approaches

A co-extraction protocol that sequentially isolates core biopolymer fractions (DNA, RNA, protein) from edaphic microbial communities is presented. In order to confirm compatibility with downstream analyses, bacterial T-RFLP profiles were generated from the DNA- and RNA-derived fractions of an arid-based soil, with metaproteomics undertaken on the corresponding protein fraction.National Research Foundation Grant no. 81779 (South Africa).http://www.elsevier.com/ locate/jmicmethhb2016Genetic

The influence of surface soil physicochemistry on the edaphic bacterial communities in contrasting terrain types of the Central Namib Desert

Notwithstanding, the severe environmental conditions, deserts harbour a high diversity of adapted micro-organisms. In such oligotrophic environments, soil physicochemical characteristics play an important role in shaping indigenous microbial communities. This study investigates the edaphic bacterial communities of three contrasting desert terrain types (gravel plains, sand dunes and ephemeral rivers) with different surface geologies in the Central Namib Desert. For each site, we evaluated surface soil physicochemistries and used explorative T-RFLP methodology to get an indication of bacterial community diversities. While grain size was an important parameter in separating the three terrain types physicochemically and specific surface soil types could be distinguished, the desert edaphic bacterial communities displayed a high level of local spatial heterogeneity. Ten variables contributed significantly (P < 0.05) to the variance in the T-RFLP data sets: fine silt, medium and fine sand content, pH, S, Na, Zn, Al, V and Fe concentrations, and 40% of the total variance could be explained by these constraining variables. The results suggest that local physicochemical conditions play a significant role in shaping the bacterial structures in the Central Namib Desert and stress the importance of recording a wide variety of environmental descriptors to comprehensively assess the role of edaphic parameters in shaping microbial communities.University of Pretoria and the South African National Research Foundation (NRF).http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1472-46692016-09-30hb201