Bare Sand Research Articles

Ecophysiological responses of bromelias in the restinga in simulated climate change scenarios.

We investigated the ecophysiological responses of the bromeliads Aechmea nudicaulis and Vriesea procera, seeking to assess their suitability to survive in a climate change scenario (optimistic scenario RCP 2.6 of the IPCC, 2021) in a Restinga environment. To carry out this investigation, we used open-top chambers (OTC). During a period of nine months (June 2022 to February 2023), the bromeliads A. nudicaulis and V. procera were subjected to the following treatments: treatment T: plants transplanted to the environmental conditions of the bare sand of the restinga and subjected to the microclimatic conditions of the OTC's; control C: plants transplanted to the environmental conditions of the bare sand of the restinga. The ecophysiological variables height, rosette diameter, relative water content, specific leaf area and total weight of the plants were evaluated. In addition, dead plants were counted. The OTC's showed an average increase in temperature and VPD (Vapor Pressure Deficit) of 1.6°C and 0.5 Kpa, respectively, and an average reduction in RH (relative humidity) of 5.3%. The results of this study indicated that the increase in local temperature that occurred between the sixth and seventh months evaluated (November and December) created limiting conditions that exceeded the tolerance capacity of the bromeliads studied. Furthermore, the climatic conditions of the OTCs intensified the damage that occurred in the plants, verified here by the reductions in the values ​​of the ecophysiological attributes evaluated in the bromeliads studied. In addition, the high mortality rate (above 50%) reinforces the idea that the climatic conditions of the OTC's induced the bromeliads studied to a senescence process. Therefore, these results are important, as they indicate that even the most optimistic climate change scenario (IPCC 2021 RCP 2.6) can harm the growth and development of these bromeliads, which are essential for the structure and functioning of Restinga communities.

Hiding from heat: The transcriptomic response of two clam species is modulated by behaviour and habitat

Rising occurrence of extreme warming events are profoundly impacting ecosystems, altering their functioning and services with significant socio-economic consequences. Particularly susceptible to heatwaves are intertidal shellfish beds, located in estuarine areas already stressed by factors such as rainfall events, red tides, eutrophication, and pollution. In Galicia, Northwestern Spain, these beds support vital shellfisheries, featuring the native clam Ruditapes decussatus and the non-indigenous R. philippinarum. Over recent decades, these populations have experienced notable abundance shifts due to various anthropogenic impacts, including climate change. In this habitat, patches of the seagrass Zostera noltei that coexist with bare sand can act as thermal refuges for benthic organisms such as clams. To assess the impact of heatwaves on these ecosystems, a mesocosm experiment was conducted. Juveniles of both clam species in two habitat types—bare sand and sand with Z. noltei—were exposed to simulated atmospheric heatwaves during diurnal low tide for four consecutive days. Subsequent transcriptomic analysis revealed that high temperatures had a more pronounced impact on the transcriptome of R. philippinarum compared to R. decussatus. The habitat type played a crucial role in mitigating heat stress in R. philippinarum, with the presence of Z. noltei notably ameliorating the transcriptomic response. These findings have direct applications in shellfishery management, emphasizing the importance of preserving undisturbed patches of Z. noltei as thermal refuges, contributing to the mitigation of heatwave effects on shellfish populations.

Do topographic changes tell us about variability in aeolian sediment transport and dune mobility? Analysis of monthly to decadal surface changes in a partially vegetated and biocrust covered dunefield

Vegetation and biological soil crust (biocrust) cover can have a stabilizing effect on dunes by fixing sediment in-place and increasing surface roughness, thus limiting dune mobility, sediment transport, and erosion. These biological effects influence rates of aeolian activity and thus surficial changes, though variability in wind and sediment supply may obscure these topographic effects. In this study, we compare monthly measures of sediment transport and decadal estimates of dune mobility to repeat topographic changes measured as a net volume change in sediment storage (difference in volume between all positive and negative topographic changes) and total volume change (absolute summed volume of all positive and negative changes) for areas of bare, vegetated, and biocrusted sand within a dunefield with limited sediment supply and unimodal winds. We found that monthly net volume changes normalized by area were similar between bare sand and sand with at least 20 % vegetation cover. However, total volume change was significantly greater for bare sand and correlated with monthly sediment flux estimates (R2 = 0.46), though the relationship was significantly improved by including monthly changes in surface roughness (R2 = 0.8). Longer-term decadal trends in topographic change showed larger total volume changes with the greatest decreases in vegetation canopy cover. Additionally, decadal total volume changes strongly correlated with estimates of dune mobility (R2 = 0.99). We also found that increased total volume changes did not necessarily signal increased net volume changes for all land cover types. Specifically, increases in total volume change for bare sand resulted in near equal or lower net volume changes, as both positive (deposition) and negative (erosion) change increased with sediment transport. Conversely, less mobile land covers, such as biocrust covered sand, increased in erosion without significant increases in total volume change, demonstrating that more stable surfaces might exhibit a larger topographic change imbalance than mobile sediment surfaces under the same conditions. This study highlights the importance of considering multiple measures of topographic change for interpreting sediment mobility, transport, and availability. Additionally, we hypothesize a novel framework for remote sensing-based empirical studies aimed at interpreting aeolian landscape evolution resulting from climate change effects on weather as well as biological controls such as vegetation and biocrusts.

Moss crusts mitigate the negative impacts of shrub mortality on the nutrient multifunctionality of desert soils

AbstractThe distribution of biological soil crusts (BSCs) and shrubs in temperate deserts often forms a common landscape surface feature. As climate change continues, desert shrubs experience varying rates of mortality, which can have severe negative impacts on soil structure and function. However, it remains uncertain whether moss crusts, prevalent beneath shrub canopies, can mitigate the effects of shrub mortality on soil nutrient environments. Therefore, this study focuses on the Gurbantunggut Desert, a typical temperate desert in northern China, with a primary focus on the dominant shrubs, Ephedra przewalskii, and the advanced stage of moss crust development within BSCs. We collected soil samples from bare sand and moss crusts under living shrubs and dead shrubs and analyzed them for their carbon, nitrogen, phosphorus, and potassium contents. Additionally, we calculated soil nutrient multifunctionality, which measures a soil's ability to sustain multiple ecosystem services simultaneously, to provide a comprehensive assessment of the effects of shrub mortality on soil nutrient function. Our results indicate that shrub mortality led to reductions in soil moisture, pH, electrical conductivity, and levels of carbon, nitrogen, phosphorus, and potassium in exposed sand compared to the sand under living shrubs. However, the presence of moss crusts significantly alleviated the adverse effects of shrub mortality on soil carbon, nitrogen, phosphorus, and potassium levels. The nutrient multifunctionality index of the moss crust only decreased by 4%, while bare sand experienced a 67% reduction following shrub mortality. Standard error of the mean analysis results revealed that when shrubs and crusts coexisted, the impact of shrubs on soil nutrient multifunctionality was much stronger than that of the moss crust. Specifically, total nutrient content was the most influential factor driving changes in soil nutrient multifunctionality. In conclusion, in desert ecosystems with declining shrubs, moss crusts can mitigate the reduction in soil nutrient contents caused by shrub degradation, thereby maintaining soil stability and nutrient multifunctionality as a viable substitute.

An Auto-Detection and classification algorithm for identification of sand dunes based on remote sensing images

Dunes are among the most abundant aeolian landforms found on Earth and other planets. The automatic mapping of dunes over large-scale areas is crucial for predicting, monitoring, and managing lands threatened by sand encroachment. However, the spectral similarity between dune and inter-dune areas, as well as the diverse presentations of dunes of the same type, have made it challenging to automatically classify and map dunes. In this study, we propose a two-step 'detect-then-classify' framework that enables the automatic identification of heterogeneous dune types. The first step involves dune detection. We introduce a newly developed convolutional neural network called SandUnet, which is adapted from Attention U-Net. This network is designed to preserve uncompressed input signals ensuring that minor differences in the color and texture of the dunes are retained. The second step focuses on dune-type classification. To classify dunes, a fine-tuned MobileNet was constructed to integrate the wealth of knowledge ingrained in MobileNet's pre-trained layers with the adaptations tailored specifically for the sand dune images. Subsequently, each dune image is automatically classified into six different types using this framework. By applying this framework to all the bare sand areas of the Taklamakan (or Taklimakan) Desert using Landsat-8 imagery, the final classification of dune types of the desert was achieved. Testing on randomly selected 10% image patches of whole desert, the overall accuracy of the dune-type classification is 70%. This study presents an automatic and economical solution for identifying dunes in large-scale areas.

The implication of seaweed farming on seagrass and seagrass-associated communities in Pemba, Zanzibar

The effect of seaweed farming on seagrass and its associated community was investigated on Pemba Island. Seagrass biometrics and macrofauna diversity were evaluated in seaweed plots (SP), seagrass meadows, Thalassia hemprichii (Th), mixed seaweed plots and seagrass (ThSP+) and bare sand (BS). Results revealed significantly higher (p = 0.001) seagrass biometric characteristics in Th compared to ThSP+ . Kruskal-Wallis test (χ2 _54.24 p = 0.001) showed a significant difference between habitats, with higher relative abundance in Th. Macrofauna diversity was significantly higher (p = 0.001) in Th compared to other habitats. Shannon-Weiner and Simpson's indexes ranked all habitats as the most diverse except SP and BS. The findings showed the negative effects of seaweed farming on seagrass and its associated community. This could be due to frequent trampling and boat mooring. Although seaweed farming is an economic liberation to coastal communities in Pemba, it should be readdressed to balance its economic values and services from seagrass.

Vegetation-cover control of between-site soil temperature evolution in a sandy desertland

Vegetation has an important influence on soil temperature (ST). However, the possible effects of surface vegetation on ST and their feedback on microclimate remain uncertain due to the lack of in-situ and long-term environmental records, especially for arid and semiarid regions of the world. A continuous, two-year study was implemented over a bare sand dune (BF) and two scrub-vegetation sites of variable cover in the Mu Us Desert of northwest China. Surface vegetation at the two non-bare sites varied from about 40% (moderate cover, MF) and 80 % (high cover, HF) of their respective surface area. Depiction of the vertical ST-profile was based on an array of field-based measurements taken within the uppermost 180 cm of the soil complex at each site. Compared with the BF site, mean ST at MF and HF decreased by 1.2 and 1.6 °C during the uniform thaw period and increased by 0.1 and 1 °C during uniform freezing. Amplitude of seasonal variation in ST for both vegetated sites, i.e., MF and HF, was reduced by 2.4 and 4.9 °C, respectively. As soil cooling during the uniform thaw period was greater than soil warming during uniform freezing, annual mean ST decreased at both vegetated sites by 1.6 and 1.2 °C (for MF and HF, respectively) compared to ST at BF. Differences in ST among the three sites during the uniform freeze and thaw periods were exponentially correlated with the extent of site vegetation cover, leaf area index, aboveground biomass, and on-the-ground litter thickness. Vegetation cover was shown to reduce the depth of the frost layer by 30 cm and prolonged the uniform thaw period by 1–35 days at the HF site. Mean daily STs at the center of each soil layer at the three sites were simulated with a two-equation model developed for this study, yielding a coefficient of determination (R2) of 0.91 when modeled STs were compared against their corresponding field observations. Increases in winter ST has potential to safeguard ground-dwelling grubs and other agriculturally harmful insects from freezing and dying. Likewise, decreases in annual ST could help promote decreases in litter decomposition, potentially lessening the effects of wind erosion.

Establishment of an Efficient and Rapid Regeneration System for a Rare Shrubby Desert Legume Eremosparton songoricum.

Eremosparton songoricum (Litv.) Vass. is a rare and extremely drought-tolerant legume shrub that is distributed in Central Asia. E. songoricum naturally grows on bare sand and can tolerate multiple extreme environmental conditions. It is a valuable and important plant resource for desertification prevention and environmental protection, as well as a good material for the exploration of stress tolerance mechanisms and excellent tolerant gene mining. However, the regeneration system for E. songoricum has not yet been established, which markedly limits the conservation and utilization of this endangered and valuable desert legume. Assimilated branches derived from seedlings were cultured on several MS mediums supplemented with various concentrations of TDZ or 6-BA in different combinations with NAA. The results showed that the most efficient multiplication medium was MS medium supplemented with 0.4 mg/L 6-BA and 0.1 mg/L NAA. The most efficient rooting medium was WPM + 25 g/L sucrose. The highest survival rate (77.8%) of transplantation was achieved when the ratio of sand to vermiculite was 1:1. In addition, the optimal callus induction medium was MS + 30 g/L sucrose + 2 mg/L TDZ + 0.5 mg/L NAA in darkness. The E. songoricum callus treated with 100 mM NaCl and 300 mM mannitol on MS medium could be used in proper salt and drought stress treatments in subsequent gene function tests. A rapid and efficient regeneration system for E. songoricum that allowed regeneration within 3 months was developed. The protocol will contribute to the conservation and utilization of this rare and endangered desert stress-tolerant species and also provide a fundamental basis for gene functional analysis in E. songoricum.

Assessment of Diversity of Marine Organisms among Natural and Transplanted Seagrass Meadows

Seagrass ecosystems have been declining, and restorations are conducted in many parts of the world to compensate for habitat loss and restore the ecosystem services seagrasses provide. Assessment of transplantation success requires the monitoring of the level of biodiversity between the donor and transplanted sites. In this study, we assessed a seagrass ecosystem after restoration in terms of the diversity of marine organisms using environmental DNA (eDNA) to compare four sites: (1) bare sand, (2) a natural meadow of Cymodocea serrulata, (3) a natural meadow of Halophila ovalis, and (4) a transplanted seagrass meadow. The results showed the presence of 3 domains, 34 phyla, 59 classes, 92 orders, 155 families, 156 genera, and 121 species. Proteobacteria, Actinobacteria, Cyanobacteria, and Bacteroidetes were the dominant bacterial phyla. Among eukaryotes, Phragmoplastophyta/Charophyta (epiphytes), Ascomycota (fungi), Cnidaria (jelly fish), and Arthropoda (Crabs and bivalves) were the dominant phyla. Dugong tails and commercial species (sea cucumber, dog conch, and swimming crab) have been observed in both the natural and transplanted meadows. Relative abundance among the four sites was significantly different. There were no differences in species richness and evenness between the four sites and no differences in species richness and evenness between the natural meadows and the transplanted seagrass meadow. It is possible that transplanted seagrass meadow can be successfully restored and established and can provide habitat for fauna and microbes. Additionally, fauna are not limited in their capacity to move between the natural and transplanted habitats. This study provides an assessment of biodiversity of restored seagrass patches and a better understanding of a seagrass ecosystem after restoration. However, to assess seagrass ecosystem services after restoration and the success of restoration actions, long-term monitoring of marine organism diversity and additional assessments are needed.

Observations of the association by early-juvenile western rock lobsterPanulirus cygnuswith seagrass assemblages (Decapoda: Achelata: Palinuridae)

AbstractThe fishery of the western rock lobster, Panulirus cygnusGeorge, 1962, is Australia’s most valuable wild-caught single-species fishery. Recruitment in some regions of the fishery was observed to be significantly lower than expected after the 2010/2011 West Australian marine heatwave that caused extensive disturbance of dominant coastal habitats. This event generated interest in the study of the factors influencing survival and recruitment of post-larval benthic P. cygnus after settlement. The habitat associations of the highly cryptic post-settlement early-juveniles were previously unknown, with only anecdotal observations of individuals within limestone crevices in nearshore habitats. Our study used early-juveniles derived from ongoing monitoring of puerulus settlement to examine their habitat association mechanism in mesocosm experiments. Comparison of common nearshore habitat assemblages (bare sand, limestone crevices, and seagrasses (Posidonia and Amphibolis) at varying seagrass densities) found that most early-juveniles associated strongly with Amphibolis assemblages at high stem densities (~2,100 stems m–2). A shift in association between Amphibolis fronds and stems at high stem density to Amphibolis-shaded sand and leaf debris at low stem density indicated active habitat selection by early-juveniles. Habitat choices were tested with the scents of prey items and habitat types within Amphibolis assemblages using Y-maze bioassays. No significant olfactory choices were found, suggesting that habitat associations may be driven by multiple cues. Our study provides new laboratory-based insights into the habitat association of early-juvenile P. cygnus and suggests changes in seagrass assemblage identity and density are likely to be important. Further experimentation is needed to define the cues driving these patterns. The impact of habitat change on recruitment in this important fishery remains unknown and should be an objective of future research.

Dune slope, not wind speed, best predicts bare sand in vegetated coastal dunes

Globally vegetation cover on coastal sand dunes has increased since at least the 1950s. With the aim of restoring or increasing biodiversity, land managers in several countries have removed vegetation and/or reprofiled dune slopes to reinvigorate geomorphic activity. However, the longevity of these interventions can be relatively short (on the order of 5 to 10 years), and further active management is required. Hypotheses for controls on geomorphic activity on dunes have frequently suggested that wind speedis the most important controlling factor. Here we show dune slope to be the best predictor of bare sand at four predominantly vegetated coastal sand dunes in England and Wales. We suggest that bare sand on steep dune slopes is maintained by three important factors: (1) Wind erosion, due to topographic acceleration (2) Granular avalanches of unconsolidated sediment and (3) Rotational slumping of unstable slopes. Our results indicate that where land managers wish to ‘rejuvenate’ areas of bare sand, efforts should focus on steep windward dune slopes and reprofiling of the dune slope should mimic the concave profiles of active slope faces on active parabolic dunes with an overall slope angle of between 18° and 23° from the dune toe to the crest.

Factors impacting bat activity and species richness in protected parks in the oak openings region of Northwest Ohio.

Protected areas are important for wildlife, especially in heavily developed areas. Bats are one group utilizing protected areas, but it is unclear what makes an ideal place for bats to live in parks, especially since preferences vary between open and forest foraging species and at different scales. The main objective of this study was to determine the landscape and vegetation factors at multiple scales most associated with higher bat activity and species richness in protected parks. Total bat activity, species richness, and activity for open and forested foraging species were compared to small-scale data vegetation structure collected in the field and larger-scale landscape data calculated in ArcGIS and FRAGSTATS. Bat activity and species richness increased with higher percentages of dry and open land cover types such as sand barrens, savanna, cropland, and upland prairie and decreased with higher percentages of forest and wet prairies. Patch richness, understory height, and clutter at the 3-6.5 m level were negatively associated with total bat activity. The most important variables for bats differed depending on spatial scale measured and if species were open or forest adapted. When managing for bats in parks, it would be advantageous to restore open land cover types such as savanna and mid-level clutter, and excessive fragmentation. Whether species are open or forest adapted and scale-specific differences should also be considered.

Skimming Flow and Sand Transport Within and Above Ammophila (Marram) Grass on a Foredune

AbstractAn investigation of wind flow and sand transport over a low to high density Ammophila dominated foredune on the Łeba barrier, Poland, utilizing multiple anemometers and sand traps is presented. The study provides the first high resolution data on modified saltation and suspended sand transport over a foredune under conditions of both canopy and skimming flow. Topographic acceleration of flow and topographic steering over the foredune were observed during oblique incident above threshold flow conditions. The concentration of sand in aeolian transport decreases with height to a minimum that is reached at different elevations above the ground, depending on Ammophila grass density. Of the three Ammophila (marram) grass densities investigated, nearly half (46%) of the total sand‐transport rate occurred above the lower density Ammophila grass, whereas up to 70% of the total transport occurred above the higher density marram grass. This likely results from both elastic collisions of sand grains with grass leaves over the Ammophila canopy and the different positions of the canopy rebounding surface, which is sensitive to even slight changes in wind speed. In addition, as plant density increases, there is less flow penetration into the canopy, vegetative forcing (similar to topographic forcing) of vertical flow is greater, and sand in both modified saltation and suspension is forced higher above the canopy in skimming flow. The maximum sand transport above the vegetation canopy increased in elevation from 32 to 48 to 53 cm as the vegetation density increased from low to moderate to high, respectively. The study also demonstrates that in skimming flow, relatively more sand is transported at higher elevations above a canopy surface than above a bare sand surface.

A deep learning model for measuring coral reef halos globally from multispectral satellite imagery

Reef halos are rings of bare sand that surround coral reef patches. Halo formation is likely to be the indirectly result of interactions between relatively healthy predator and herbivore populations. To reduce the risk of predation, herbivores preferentially graze close to the safety of the reef, potentially affecting the presence and size of the halo. Reef halos are readily visible in remotely sensed imagery, and monitoring their presence and changes in size may therefore offer clues as to how predator and herbivore populations are faring. However, manually identifying and measuring halos is slow and limits the spatial and temporal scope of studies. There are currently no existing tools to automatically identify single reef halos and measure their size to speed up their identification and improve our ability to quantify their variability over space and time. Here we present a set of convolutional neural networks aimed at identifying and measuring reef halos from very high-resolution satellite imagery (i.e., ∼0.6 m spatial resolution). We show that deep learning algorithms can successfully detect and measure reef halos with a high degree of accuracy (F1 = 0.824), thereby enabling faster, more accurate spatio-temporal monitoring of halo size. This tool will aid in the global study of reef halos, and potentially coral reef ecosystem monitoring, by facilitating our discovery of the ecological dynamics underlying reef halo presence and variability.

Do wave exposure and drifting algae drive the functional diversity of fishes in tropical ocean-exposed sandy beaches?

Tropical ocean-exposed sandy beaches have an essential ecological function for marine fish species, which use these turbulent areas as nurseries. These habitats are characterized as homogeneous environments, but previous studies report the influence of drifting algae in providing physical structure, which increases habitat complexity and heterogeneity. This additional habitat complexity is expected to have a crucial impact on fish assemblages. Here, we analyzed the influences of wave exposure and drifting algae volume on abundance and functional diversity on two beach types: wave-dominated and tide-modified. Beach seine samplings were carried out in the rainy and dry seasons to collect the fish at each beach. Four volume categories of drifting algae were selected: bare sand, low, mid and high. A total of 97 fish species were captured. The community attributes of fishes (abundance, richness and biomass), as well as algal volume, were higher in tide-modified beaches. For abundance, three distinct patterns emerged: 1) there was a trend of decreasing herrings and anchovies compared to other species, from the bare to high-volume drifting algae in tide-modified beaches, with the opposite trend for the wave-dominated beaches; 2) L. breviceps, M. littoralis, S. naso, S. rastrifer and S. stellifer increased in abundance from the bare to high drifting algae in wave-dominated beaches; 3) P. virginicus was abundant in both beach types. These trends were accompanied by functional diversity, principally by responses in the two types of traits: diet and habitat use. The differences in habitat characteristics (wave exposure and drifting algae) among beaches affected the composition and functional diversity and consequently supported distinct fish assemblages. The presence of drifting algae in this habitat favors the attractiveness for fishes, such as larvae and juveniles, and seems to drive habitat use by tropical fishes.

A legacy of invasive sun corals: Distinct mobile invertebrate assemblages at near-reef coral-dominated rubble

Fast-growing and reproducing sun corals have successfully invaded rocky reefs around the Atlantic Ocean, markedly reducing the diversity of fouling invertebrates and macroalgae, and profoundly changing the composition of reef-associated mobile invertebrates. Here, we address sun-coral rubble depositions and report, for the first time, the effects of sun corals on near-reef soft-bottom invertebrate assemblages. Abundance, richness and diversity were higher at rubble habitats compared to bare sandy grounds, which could be a positive effect of substrate complexity. All those parameters were also higher at rubble patches dominated by sun-coral fragments compared to rubble patches dominated by pebbles or shell fragments, also suggesting possible additive effects of coral-borne chemical attraction (sun-coral specific, as inputs of other coral species were virtually absent). Different epifaunal groups were exclusive of rubble habitats and a subset of those exclusive of sun-coral rubble, explaining the incremental richness across habitats. The relative abundance of the two dominant groups – polychaetes (p) and amphipods (a) – contributed the most to the observed contrasts on community structure, as their proportion (p:a) changed from 10:1 in bare sand to nearly co-dominance in coral rubble. While previous research suggested that spreading sun corals reduce prey supply for fish foraging on reef walls, our results suggest they may increase prey abundance and diversity at the adjacent non-consolidated habitat, possibly reshaping trophic pathways connecting the benthic and the pelagic environment.

Experiments and COMSOL simulations: A comparative study of the heat flux plate method and the gradient method for soil heat flux measurements in barren sand

Soil heat flux (G) is important for studying the energy balance at the soil-atmosphere interface. In most cases, a reference flux (true value) is not available for soil heat flux. The heat flux plate method and the heat pulse sensor gradient method are two commonly used methods to measure soil heat flux. Both methods have inherent shortcomings that lead to unavoidable measurement errors. There is a need for a theoretical analysis to identify the origin of errors associated with the gradient method. In this study, by introducing a plane heater of known heating strength to generate a one-dimensional heat flux, we compared the measurements of soil heat flux by both methods at different soil moisture contents for laboratory (indoor) and field (outdoor) experiments, as well as computer simulations for a sandy soil. Differences between the COMSOL simulation results and the indoor measurements were less than 15%, indicating that the COMSOL simulated soil heat flux results were reliable. The indoor experiments provided close agreement between the heat flux plate method and the gradient method (Pearson's correlation coefficient r ≥ 0.993). The results of the field experiments showed that the correlation coefficient of the two methods decreased by at least 9.3%. Our field measurements indicated that the gradient method is not suitable for measuring G in the presence of alternating freezing-thawing soil conditions. Although heat flux plates have several short-comings, they may perform better than the heat pulse sensor gradient method in partially frozen soils. The finite difference approximation of nonlinear thermal gradients causes errors in the gradient method heat flux values. However, this gradient method error could be eliminated by altering the heat pulse probe positioning with depth to provide finer scale measurements of temperature with depth.

Biological soil crust development affects bacterial communities in the Caragana microphylla community in alpine sandy areas.

Biological soil crusts (BSCs) constitute a substantial portion of primary production in dryland ecosystems. They successionally mature to deliver a series of ecosystem services. Bacteria, as an important community in BSCs, play critical roles in maintaining the structure and functions of BSCs. However, the process by which bacterial diversity and community are altered with BSC development is not fully understood. In this study, amplicons sequencing was used to investigate bacterial diversity and community compositions across five developmental stages of BSCs (bare sand, microbial crusts, algae crusts, lichen crusts, and moss crusts) and their relationship with environmental variables in the Gonghe basin sandy land in Qinghai-Tibet Plateau, northwestern China. The results showed that Proteobacteria, Actinobacteria, Cyanobacteria, Acidobacteria, Bacteroidetes, and Firmicutes were predominant in different developmental stages of BSCs, accounting for more than 77% of the total relative abundance. The phyla of Acidobacteria and Bacteroidetes were abundant in this region. With BSC development, bacterial diversity significantly increased, and the taxonomic community composition significantly altered. The relative abundance of copiotrophic bacteria, such as Actinobacteria, Acidobacteria, Bacteroidetes, Verrucomicrobia, Planctomycetes, and Gemmatimonadetes significantly increased, whereas the relative abundance of oligotrophic bacteria, such as Proteobacteria and Firmicutes significantly decreased. The relative abundance of Cyanobacteria in the algae crusts was significantly higher than that in the other developmental stages (p < 0.05). Variations in bacterial composition suggested that the potential ecological functions of the bacterial community were altered with BSC development. The functions varied from enhancing soil surface stability by promoting soil particle cementation in the early stages to promoting material circulation of the ecosystem by fixing carbon and nitrogen and decomposing litter in the later stages of BSC development. Bacterial community is a sensitive index of water and nutrient alterations during BSC development. SWC, pH value, TC, TOC, TN, NO3 -, TP and soil texture were the primary environmental variables that promoted changes in the bacterial community composition of BSCs.

Stimulating effects of snow cover on gaseous nitrogen emissions are intensified by biological soil crusts

Winter weather patterns are changing due to global climate change, which impacts the soil nitrogen cycle and greenhouse gas flux in these ecosystems, particularly the snow cover in temperate deserts. Biological soil crusts are important soil communities of the desert ecosystem and are sensitive to environmental changes. However, knowledge of the responses of gaseous nitrogen emissions from biological soil crusts to changes in snow cover is limited. In this study, the effects of snow cover on gaseous nitrogen emissions and the abundance of nitrifier (amoA) and denitrifier (narG, nirS, nirK, nosZ) genes were investigated in three snow cover treatments (snow removal, ambient snow, and increased snow depth). Snow cover significantly affected N2O and NO emissions from bare sand and biological soil crusts in winter. The abundance of denitrifier genes in bare sand and biological soil crusts and the abundance of nitrifier genes in moss crust increased with the depths of snow addition. N2O emissions increased with the successional stages of biological soil crusts during the winter. Both N2O and NO emissions from bare sand and biological soil crusts peaked in the freeze–thaw period (March) and were lowest in mid-winter (January). The emissions of N2O increased with increasing the abundance of amoA, nirK and nosZ genes. Our findings show that N2O emissions generally increased with increasing snow cover through their effects on nitrogen cycle related functional microbial groups. These effects were regulated by the types of biological soil crust, which can increase the effect of snow cover in winter.

The role of livestock grazing in long-term vegetation changes in coastal dunes: a case study from the Netherlands

The vegetation of coastal sand dunes is characterized by high species diversity and comprises some of the rarest vegetation types in North-Western Europe. Among them are dune grassland communities whose species richness relies on grazing. Those communities are assessed as a priority habitat type under the Natura 2000 legislation. In autumn 1990, Galloway cows and Nordic Fjord horses were introduced in the coastal dunes of Meijendel near The Hague (52°7'N, 4°20'E), The Netherlands, to reduce encroachment of tall grasses and shrubs, to develop bare sand patches, and as such facilitating diverse vegetation structures in the dune grasslands. In the 1950s, decades before the introduction of livestock, 41 permanent plots were installed. On average, they were examined every four years. Our study hypothesised that the livestock grazing in the set densities would halt progressive succession and facilitate regressive succession. Up to 1990, we observed an equilibrium between progressive and regressive succession. After 1990, however, our data showed a pronounced progressive succession contradicting the hypothesized effect of the livestock grazing. We relate the main observed patterns with two factors linked to rabbit populations: (i) the myxomatosis outbreak in 1954 and (ii) the rabbit Viral Haemorrhagic Disease (rVHD-1) outbreak in 1989. In addition to livestock grazing, rabbits block progressive succession by feeding on seedlings of shrub and tree species and digging burrows, creating small-scale mosaics of bare sand and initiate blowout development when collapsing. We state that the substantial decrease in rabbit numbers due to the viral diseases likely caused the observed increase of shrubs and trees in the study area's permanent plots. Climate change might have contributed to the observed increase in autonomous blowout development since 2001, as well as a decrease in atmospheric nitrogen deposition since 1990, after a strong increase the decades before.