The consequences of using microlysimeters: Why microlysimeters grossly overestimate dew amounts in arid regions? A critical review.

Although commonly considered the gold standard for measurement of non-rainfall water (NRW), providing reasonable reliable data for vegetated soils, microlysimeters (MLs) tend to grossly overestimate NRW (primarily in form of dew) on barren soil. In arid and semiarid regions, the reported values may be overestimated by hundreds and even 1000 %. This bias is attributed to (i) the effect of the structure and dimension of the ML (ii) the tacit assumption that the weight difference between morning and the previous midday/evening results from dew or (iii) the belief that the MLs will provide reliable values if the difference in weight would be calculated only from the evening or night. For instance, from the time during which the air temperature reaches the dewpoint temperature or from the time during which condensation takes place on an adjacent leaf-wetness sensor. Calculating dew by the weight difference of MLs led to the notions that the fine-textured soil will necessarily promote higher values of dew, and the notion that higher amounts of dew are expected following days with low relative humidity, both of which hamper our understanding regarding dew formation. The reasons for the apparent different performance of MLs in vegetated (wet) and barren (arid) regions are discussed.

Interactions between biocrusts and herbaceous communities are divergent in dry and wet semiarid ecosystems.

Biocrusts are a prevalent form of living cover in worldwide drylands, and their presence are intimately associated with herbaceous community, forming a spatially mosaic distribution pattern in dryland ecosystems. The role of biocrusts as modulators of herbaceous community assembly is extensively studied, whereas, less is known whether their interactions are permanent or changeable with various environmental conditions. This study conducted a field survey of herbaceous community accompanied by three types of biocrusts (cyanobacterial, cyanobacterial-moss mixed, and moss crusts) in two contrasting (dry and wet) semiarid climate regions in the Chinese Loess Plateau, to explore whether or not climatic aridity gradient affects the interactions between biocrusts and herbaceous community. Our results showed that in dry semiarid climate, the biomass, species richness, and diversity of herbaceous community from biocrust plots were 89 %, 179 %, and 52 % higher than that from the uncrusted plots, respectively, while in wet semiarid climate, those herbaceous community indices from biocrust plots were 68 %, 43 %, and 23 % lower than that from the uncrusted plots, respectively. The impacts of biocrusts on herbaceous community were highly dependent on the types and coverage of biocrusts. Regardless of aridity gradient, the richness and diversity of herbaceous community were the lowest in the moss-covered plots, followed by the cyanobacteria-covered plots and the plots with a mixed cyanobacteria and moss population. Along with increasing biocrust coverage, the species richness and diversity of herbaceous plants initially increased and then decreased in dry semiarid climate, while in wet semiarid climate they decreased linearly with biocrust coverage. Structural equation modeling revealed that the factors of biocrust types and coverage affected herbaceous community indirectly through soil properties in dry semiarid climate, whereas in wet semiarid climate they directly affected herbaceous community through biotic interactions. Together, our findings indicated that cyanobacterial and moss biocrusts facilitate the development of herbaceous community in dry semiarid climate by increasing soil stability and nutrient levels, but in wet semiarid climate they restrict herbaceous plant growth through competing niche space. These results highlight the divergent relationships between biocrusts and herbaceous community across aridity gradient in dryland ecosystems, and this knowledge may be critically important in light of the projected global climate change which is going to change the aridity of global drylands.

Altitudinal Gradient and Soil Depth as Sources of Variations in Fungal Communities Revealed by Culture-Dependent and Culture-Independent Methods in the Negev Desert, Israel.

We examined fungal communities in soil profiles of 0-10 cm depth along the altitudinal gradient of 250-530-990 m.a.s.l. at the Central Negev Desert, Israel, which benefit from similar annual precipitation (95 mm). In the soil samples collected in the summer of 2020, a mycobiota accounting for 169 species was revealed by both culture-dependent and culture-independent (DNA-based) methodologies. The impact of soil depth on the variations in fungal communities was stronger than the impact of altitude. Both methodologies displayed a similar tendency in the composition of fungal communities: the prevalence of melanin-containing species with many-celled large spores (mainly Alternaria spp.) in the uppermost layers and the depth-wise increase in the proportion of light-colored species producing a high amount of small one-celled spores. The culturable and the DNA-based fungal communities had only 13 species in common. The differences were attributed to the pros and cons of each method. Nevertheless, despite the drawbacks, the employment of both methodologies has an advantage in providing a more comprehensive picture of fungal diversity in soils.

The effect of the water source on niche partioning of chlorolichens and cyanobacteria-implications for resilience?

MAIN CONCLUSION: Microclimate determines lichens and cyanobacteria distribution in the Negev, with lichens and cyanobacteria inhabit dewy and dewless habitats, respectively. Lichens experiences more frequent and extensive environmental fluctuations than cyanobacteria. The spatial partitioning of chlorolichens (eukaryotes) and cyanobacteria (prokaryotes) are intriguing, especially following recent intense search for extraterrestrial life. This is especially relevant for deserts, where both lithobionts are thought to use rain and dew but may differ in their resilience to environmental extremes and fluctuations. Following the different spatial distribution of lithobionts in a south-facing slope of the Negev Highlands (with cyanobacteria-inhabiting rocks and chlorolichen-inhabiting cobbles), measurements of temperature, non-rainfall water (NRW) and biomass were carried out within the drainage basin aiming to test the hypotheses that (i) cobble-inhabiting lichens may access more water (through NRW) and may be subjected to more extensive environmental fluctuations of temperature and water than bedrock-inhabiting cyanobacteria, and (ii) will therefore have a greater contribution to the ecosystem productivity. In contrast to cyanobacteria, cobble-inhabiting chlorolichens were found to access NRW (up to 0.20 mm of daily amounts in comparison to < 0.04 mm of the cyanobacteria) and to experience higher fluctuations of temperatures (up to 4.1 °C higher and 5.3 °C lower). With lichens and cyanobacteria inhabiting dewy and dewless habitats, respectively, NRW was found responsible for contributing 6.8-fold higher organic carbon to the lithobiontic community. At this site, chlorolichens experience more extensive environmental fluctuations than cyanobacteria, possibly indicating a higher tolerance for environmental fluctuations. These observations may assist in the interpretation of the abiotic conditions responsible for past or present lithobiontic life on Mars.

Dew and fog as possible evolutionary drivers? The expansion of crustose and fruticose lichens in the Negev is respectively mainly dictated by dew and fog.

MAIN CONCLUSION: The expansion of crustose lichens in the Negev is principally determined by dew and that of fruticose lichens by fog. Crustose and fruticose lichens are largely adapted to dew and fog, respectively. Although crustose and fruticosea lichens were shown to efficiently use dew and fog, the link between their expansion and the occurrence of dew and fog has never been shown experimentally. This is also the case for the Negev Desert Highlands, where (i) dewless habitats were not inhabited by lichens and (ii) an increase in fruticose lichens with high-altitude fog-prone areas was noted, leading us to hypothesize that the expansion of crustose and fruticose lichens is mainly linked to dew and fog, respectively. Experiments aiming to compare the non-rainfall water (NRW) were conducted. We used cloths attached to 7 cm-high cobbles to mimic crustose lichens (MCL), cloths placed horizontally aboveground to evaluate the amount of NRW without the presence of the cobble (CoP), cloths attached to a wire scaffold mimicking fruticose lichens (MFL), and cloths attached to glass plates (CPM) that served as a reference. Substrate temperatures were compared to the dew point temperature. In addition, sprinkling experiments, which mimicked fog under variable wind speeds (0.9, 1.4, 3.3 and 5.7 m s-1), were also conducted. NRW followed the pattern: MCL ≈ CPM > CoP > > MFL. While MCL yielded substantially higher amounts of NRW (0.09 mm) in comparison to MFL (0.04 mm) during dew events, similar amounts were obtained by both substrates (0.15-0.16 mm) following fog. However, fog interception increased substantially with wind speed. The findings may explain the expansion of crustose lichens in extreme deserts benefiting mainly from dew (but also fog), and the proliferation of fruticose lichens in fog-prone areas, especially when accompanied by high-speed winds. While (mainly) high proliferation of crustose lichens may serve as bioindicators for dew in extreme deserts, fruticose lichens may serve as bioindicators for fog.

Data variability or paradigm shift? Slow versus fast recovery of biological soil crusts-a review.

Biological soil crusts, known also as biocrusts, provide valuable ecosystem services, especially in arid and semiarid regions. They may affect geomorphological (stability), hydrological (infiltration, evaporation), biochemical (carbon and nitrogen fixation) and ecological (germination and growth of vascular plants) processes, and their disturbance may have important ecological consequences. The common view, as reflected in hundreds of papers, regards biocrusts as having extremely slow recovery with characteristic time of up to hundreds and even thousands of years. Long recovery time implies that disturbance or climate change may have severe long-lasting consequences even once the conditions return to their initial state, triggering ample efforts to hasten biocrust recovery by inoculation. We critically analyze available estimates of the crust recovery time and present systematic measurements and theoretical considerations that attest to relatively rapid recovery of the crusts. We conclude that the likely recovery time of cyanobacterial crusts is 5-10 years, while that of lichen- and moss-dominated crusts is 10-20 years. Subsequently, costly and potentially negative effects to the ecosystem during inoculation should be weighed against the fast natural recovery of the biocrusts.

Drought effect on biocrust resilience: High-speed winds result in crust burial and crust rupture and flaking.

Once established, biocrusts (known also as biological soil crusts or microbiotic crusts) are thought to be relatively resilient to wind erosion, with crust burial being considered as the main mechanism responsible for crust death. Thus far, to the best of our knowledge, crust flaking and rupture under natural conditions were not reported. We report herein a two-year study during two severe drought years (2010-2012) in a dunefield in the Negev Desert during which in addition to crust burial, crust rupture and flaking also took place. As for crust burial, it took place under sand sheets or coppice dunes (mounds). Subsequent removal of the coppice dunes by wind resulted in crust disintegration and erosion of the formerly buried crust and the formation of patches devoid of crusts termed herein 'erosion cirques'. As for crust flaking and rupture, it is explained by a large change in the properties of the extracellular polymeric substances (EPS) composing the crust. The EPS adherence and viscoelastic properties were monitored using a quartz crystal microbalance with dissipation monitoring (QCD-M) technology. EPS adherence and viscoelastic properties deduced from the QCM-D experiments suggest that crust coherence and elasticity, mediated by the EPS, were affected by droughts. Although crust flaking affected up to 25% of the interdunal surface, it is suggested that with continuous rain shortage, further crust flaking is likely to take place under continuous drought-driven dry surface conditions. This positive feedback mechanism, during which initially eroded crusts trigger additional crust erosion, may have severe consequences on the structure and function of drought-prone ecosystems, and may endanger the stability of dunefields, causing dust storms, triggering dune encroachment and declining air quality.