Genesis of saltpeter deposits in karst caves in Southwest China

Karst cavity mapping is attracting great interest from engineering geologists because of its relation to the dangerous geohazards faced during engineering construction. Ground-based geophysical methods still face challenges in karst mapping, and concealed karst cavities potentially pose threats to tunnel construction in southwest China. Given the significant contrast in electrical resistivity between karst cavities and their host rocks, geoelectrical methods are widely used for mapping these cavities. We have developed a successful case history of mapping karst cavities on a planned railway route using controlled-source audio magnetotellurics (CSAMT). Scalar CSAMT, with frequencies ranging from 0.5 to 8192 Hz, was used for field data acquisition. A full-frequency domain apparent resistivity correction method was used for near-field corrections. Electromagnetic array profiling (EMAP) filtering was used for topographic and static shift corrections, and the Bostick conversion was used for data interpretation. Our study indicated that the results of the Bostick conversion with EMAP filtering were more acceptable than the results of rapid relaxation inversion and nonlinear conjugate gradient inversion in this case. The G Tunnel is a key tunnel along the Gui-Guang high-speed railway in southwest China. Initial geophysical and engineering geologic results suggest that the bedrock of the survey section of the G Tunnel route is sandstone. A CSAMT survey with three inline sections and three crossline sections over the tunnel route was conducted in two phases to verify the rock conditions of the tunnel route. A concealed karst cavity with a low-resistivity anomaly was found on the tunnel route and was verified by the borehole. Data from the CSAMT survey significantly refined our understanding of the subsurface engineering geologic conditions along the tunnel route.

For the sake of studying the catastrophe mechanism of stress-fissure coupling field in mining close distance seams in southwest China, a test working face in Guizhou province in southwest China is adopted and researched by the methods of numerical calculation and similar experiment. When the working face advances to 180 m in 4# coal seam in a similar experiment, the overlying rock breaks to the central base plate of Yulongshan limestone, and the conductive fractures run through the Changxing limestone karst cave and Yulongshan limestone karst cave. When the 1402 working face advances to 350 m, the top of vertical karst caves in the middle of the model produces extrusion damage, forming a penetrating water inrush passage. When the 1402 working face advances to 480 m, the top slab of the working face comes under periodic weighting with the short step. Besides, the mining of 9# coal seam starts after 4# coal seam in mining close distance seams. When the working face in 9# coal seam advances to 340 m in numerical simulation, the maximum opening of the overburden fractures is 51.16 mm. The fractures in the roof are mainly caused by the periodic breaking and falling of the basic roof, connected with the floor fractures of 4# coal seam. When the working face in 9# coal seam advances to 500 m, the maximum opening of the overburden fracture is 93.09 mm. Specifically, as the working face advances, the opening of fracture in the roof after collapse of the basic roof periodically is mainly greater than 5 mm, and the compaction closure is mainly 1 mm-5 mm. The fractures in the gob floor are mainly 0.1 mm-1 mm, and the fracture opening of the collapsed rock mass in the gob is mainly 1 mm-5 mm and greater than 5 mm. The karst caves in the overburden reduce the periodic weighting step of working face and play a guiding role in the direction of fracture development and water inflow passage formation. The karst caves are connected to surface waterfall holes and trap pits, and atmospheric precipitation recharges the water in the caves. The research results can be treated as an important basis for the prevention and treatment for water inrush disaster in mining close distance seams in the karst area of southwest China.

Karst caves are widely distributed subsurface systems, and the microbiomes therein are proposed to be the driving force for cave evolution and biogeochemical cycling. In past years, culture-independent studies on the microbiomes of cave systems have been conducted, yet intensive microbial cultivation is still needed to validate the sequence-derived hypothesis and to disclose the microbial functions in cave ecosystems. In this study, the microbiomes of two karst caves in Guizhou Province in southwest China were examined. A total of 3,562 bacterial strains were cultivated from rock, water, and sediment samples, and 329 species (including 14 newly described species) of 102 genera were found. We created a cave bacterial genome collection of 218 bacterial genomes from a karst cave microbiome through the extraction of 204 database-derived genomes and de novo sequencing of 14 new bacterial genomes. The cultivated genome collection obtained in this study and the metagenome data from previous studies were used to investigate the bacterial metabolism and potential involvement in the carbon, nitrogen, and sulfur biogeochemical cycles in the cave ecosystem. New N2-fixing Azospirillum and alkane-oxidizing Oleomonas species were documented in the karst cave microbiome. Two pcaIJ clusters of the β-ketoadipate pathway that were abundant in both the cultivated microbiomes and the metagenomic data were identified, and their representatives from the cultivated bacterial genomes were functionally demonstrated. This large-scale cultivation of a cave microbiome represents the most intensive collection of cave bacterial resources to date and provides valuable information and diverse microbial resources for future cave biogeochemical research.IMPORTANCE Karst caves are oligotrophic environments that are dark and humid and have a relatively stable annual temperature. The diversity of bacteria and their metabolisms are crucial for understanding the biogeochemical cycling in cave ecosystems. We integrated large-scale bacterial cultivation with metagenomic data mining to explore the compositions and metabolisms of the microbiomes in two karst cave systems. Our results reveal the presence of a highly diversified cave bacterial community, and 14 new bacterial species were described and their genomes sequenced. In this study, we obtained the most intensive collection of cultivated microbial resources from karst caves to date and predicted the various important routes for the biogeochemical cycling of elements in cave ecosystems.

Coal is the major energy source in China. Many coal mines in Southwest China have arranged the excavation roadway system in the Maokou limestone in coal seam floor because of its special hydrogeological conditions. Since karst is widely distributed in Maokou limestone and water-filled karst caves are also developed commonly there, once a strongly water-filled karst cave is exposed during roadway excavation, the water inrush disaster occurs easily. In order to reveal the phenomenon of water inrush disasters from covered karst caves more effectively, the water inrush that occurred during the roadway excavation process of Jiyuan coal mine in northern Guizhou was taken as a research object in this study. Based on the regional geological structure, the hydrogeological condition and the fluid-structure interaction theory, a fluid-structure interaction model of water inrush in the roadway excavation was established, and variation laws of stress field, displacement field and seepage field of surrounding rock during the process of water inrush from karst cave were analyzed. Besides, the influence of different geological conditions on water inrush from karst cave was discussed from aspects of the size of karst cave, the water pressure of karst cave and the thickness of aquiclude.

AimPrevious studies primarily based on the culture method have revealed that caves host a high fungal diversity and that exogenous carbon sources influence fungal communities. Here, we assess the fungal diversity and distribution patterns in karst caves in Southwest China and reveal the major factors that shape the fungal communities.LocationSouthwest China.MethodsSamples of air, rock, sediment and water were collected from eight caves in Southwest China. Using high‐throughput sequencing of the internal transcribed spacer 1 region of fungal rDNA, we examined the cave fungal communities in Southwest China and studied their correlations with potentially influential factors, such as substrate, cave, location and environmental variables.ResultsApproximately 10,000 fungal operational taxonomic units (OTUs) spanning 15 phyla were obtained. The highest diversity genus was Penicillium, while Mortierella had the highest relative abundance. Compared with air and rock samples, sediment and water samples harboured higher diversity and distinct communities. Substrate and cave were revealed to be the key determinants of the fungal communities in caves. Sample location and sampling distance from the entrance had more significant impacts on rock and sediment communities than on air and water assemblages. The fungal assemblages in air and water samples from the cave and outside cave environments were similar, while those of rock and sediment samples were significantly different.Main conclusionsOur results indicate that sediment and water harboured higher fungal diversity than air and rock, and substrate and spatial variables are key determinants of fungal communities in caves.

Caves are typified by their permanent darkness and a shortage of nutrients. Consequently, bacteria play an important role in sustaining such subsurface ecosystems by dominating primary production and fueling biogeochemical cycles. China has one of the world’s largest areas of karst topography in the Yunnan-Guizhou Plateau, yet the bacteriomes in these karst caves remain unexplored. In this study, bacteriomes of eight karst caves in southwest China were examined, and co-occurrence networks of cave bacterial communities were constructed. Results revealed abundant and diversified bacterial communities in karst caves, with Proteobacteria, Actinobacteria, and Firmicutes being the most abundant phyla. Statistical analysis revealed no significant difference in bacteriomes among the eight caves. However, a PCoA plot did show that the bacterial communities of 128 cave samples clustered into groups corresponding to sampling types (air, water, rock, and sediment). These results suggest that the distribution of bacterial communities is driven more by sample types than the separate caves from which samples were collected. Further community-level composition analysis indicated that Proteobacteria were most dominant in water and air samples, while Actinobacteria dominated the sediment and rock samples. Co-occurrence analysis revealed highly modularized assembly patterns of the cave bacterial community, with Nitrosococcaceae wb1-P19, an uncultured group in Rokubacteriales, and an uncultured group in Gaiellales, being the top-three keystone members. These results not only expand our understanding of cave bacteriomes but also inspires functional exploration of bacterial strains in karst caves.

Karst caves are characterized by darkness, low temperature, high humidity, and oligotrophic organisms due to its relatively closed and strongly zonal environments. Up to now, 1626 species in 644 genera of fungi have been reported from caves and mines worldwide. In this study, we investigated the culturable mycobiota in karst caves in southwest China. In total, 251 samples from thirteen caves were collected and 2344 fungal strains were isolated using dilution plate method. Preliminary ITS analyses showed that these strains belonged to 610 species in 253 genera. Among these species, 88.0% belonged to Ascomycota, 8.0% Basidiomycota, 1.9% Mortierellomycota, 1.9% Mucoromycota, and 0.2% Glomeromycota. The majority of these species have been previously known from other environments, and some of them are known as mycorrhizal or pathogenic fungi. About 52.8% of these species were discovered for the first time in karst caves. Based on morphological and phylogenetic distinctions, 33 new species were identified and described in this paper. Meanwhile, one new genus of Cordycipitaceae, Gamszarea, and five new combinations are established. This work further demonstrated that Karst caves encompass a high fungal diversity, including a number of previously unknown species. Taxonomic novelties: New genus: Gamszarea Z.F. Zhang & L. Cai; Novel species: Amphichorda cavernicola, Aspergillus limoniformis, Aspergillus phialiformis, Aspergillus phialosimplex, Auxarthron chinense, Auxarthron guangxiense, Auxarthronopsis globiasca, Auxarthronopsis pedicellaris, Auxarthronopsis pulverea, Auxarthronopsis stercicola, Chrysosporium pallidum, Gamszarea humicola, Gamszarea lunata, Gamszarea microspora, Gymnoascus flavus, Jattaea reniformis, Lecanicillium magnisporum, Microascus collaris, Microascus levis, Microascus sparsimycelialis, Microascus superficialis, Microascus trigonus, Nigrospora globosa, Paracremonium apiculatum, Paracremonium ellipsoideum, Paraphaeosphaeria hydei, Pseudoscopulariopsis asperispora, Setophaeosphaeria microspora, Simplicillium album, Simplicillium humicola, Wardomycopsis dolichi, Wardomycopsis ellipsoconidiophora, Wardomycopsis fusca; New combinations: Gamszarea indonesiaca (Kurihara & Sukarno) Z.F. Zhang & L. Cai, Gamszarea kalimantanensis (Kurihara & Sukarno) Z.F. Zhang & L. Cai, Gamszarea restricta (Hubka, Kubatova, Nonaka, Cmokova & Řehulka) Z.F. Zhang & L. Cai, Gamszarea testudinea (Hubka, Kubatova, Nonaka, Cmokova & Řehulka) Z.F. Zhang & L. Cai, Gamszarea wallacei (H.C. Evans) Z.F. Zhang & L. Cai.

Karst formations in Southwest China are significantly developed. With the increase in tunnel construction year by year, constructing tunnels in areas of karst geology is inevitable. There are great safety risks associated with exposed karst caves, and the water inside the caves can seriously impact the tunnel structure and its filling materials. Traditional cave disposal technology mainly focuses on safe crossing treatment at the construction stage; however, problems such as backfill collapse and floor floating caused by karst water erosion during the service period are not considered. Therefore, proposals for a new construction scheme to ensure the stability and safety of the tunnel during the service period are urgently needed. Using Huangchongyan Tunnel as an example, we propose a safe crossing scheme of ‘plate–pile–bedrock’ for karst caves, based on a comparison of karst cave treatment schemes at home and abroad. In addition, considering the impact of karst water on the tunnel, we developed a ‘bottom to top’ reverse drainage structure, which solved the problem of floating during tunnel service. In our study, we developed a full life cycle disposal scheme to enable safe passage through tunnels in karst caves, providing a reference for the design and construction of similar projects.

As a unique ecosystem, Karst caves harbor an impressive diversity of specific fungi. However, the factors and mechanisms that shape fungal biodiversity in caves remain elusive. In this study, we explored the assembly patterns of fungal communities based on our previous research in eight representative Karst caves in Southwest China. Our results indicated that dispersal limitation plays a crucial role in shaping the overall fungal community as well as specific communities in rock, sediment, and water samples. However, "Undominated" processes contributed more than dispersal limitation in air samples. Interestingly, the dominant assembly processes varied between caves. Consistently, environmental selection had a minor impact on the assembly of fungal communities. Among the examined spatial and environmental variables, latitude, longitude, altitude, and temperature were found to significantly influence fungal communities irrespective of substrate type. These findings provide valuable insights into the ecological factors governing fungal community assembly in Karst caves.

Radon-222 in karst caves of Southwestern China: Sources, seasonal dynamics, and human exposure risks.

Karst caves are important tourism and natural heritage destinations. Within dry cave passages otherwise notable for their rich flowstone decoration, areas with a characteristic black color can sometimes be observed on cave walls and floors and on speleothems. To determine the origin of these black deposits, mineralogical and chemical analyses were carried out in two caves (Postojna Cave and Crna Jama (Kocevje), Slovenia). Qualitative chemical composition was obtained by scanning electron microscopy in conjunction with energy dispersive X-ray spectrometry (SEM/EDS). Organic matter in black deposits was confirmed by the Walkley–Black (wet burning) and loss-on-ignition (LOI, dry burning) methods. Results showed that black deposits from the caves studied could be attributed to natural and anthropogenic causes. Natural causes are related to Mn–Fe oxide precipitation and are probably dependent on microorganism activity. Anthropogenic sources are related to historic (or/and prehistoric) human activities such as cave visitors using torches and fires, events outside the caves such as forest and settlement fires, and recent air pollution. Black deposits show the effect of long-term environmental impact on karst caves and could be included in guided tours of show caves along with a scientific explanation of their causes and origin.

<p indent=0mm>Although karst systems comprise approximately 5%−20% of the Earth's ice-free land surface, only 10% of them have so far been explored. Karst caves are important components of karst systems and represent one of the most unique and poorly understood ecosystems on Earth. They are extreme environments in terms of nutrition deficiency, permanent darkness, and isolation from other ecosystems, and serve as a natural laboratory to study the deep biosphere. Caves are also ideal places to discover microbial dark matter such as microbial biodiversity and novel metabolic pathways and metabolites. Microorganisms in caves have been demonstrated to be involved in the dissolution and formation of minerals, biogeochemical cycles, and degradation of organic matter and thus have a fundamental role in sustaining the cave ecosystems. However, knowledge about microbial distribution, diversity, potential functions, and interactions in subsurface caves has remained limited to date particularly in different niches, which limits the utilization of microbial resources and hampers our understanding of biogeochemical processes in the deep biosphere. To address these issues, bacterial communities from seven different niches including cave wall, weathered crust, loose sediments, biofilms from the wet surface of stalagmite, biofilms from the dry surface of stalagmite, microbial mat on rock surface, and overlying soils in the Xincuntun Cave, Southwest China, were investigated via high-throughput sequencing and bioinformatics. Alpha diversities of bacterial communities significantly differed from one niche to the other. Specifically, bacterial diversity was the highest in the overlying soil, followed by those in weathering crust, sediments, wet surface of stalagmite, cave wall, dry surface of stalagmite, and microbial mat. Bacterial communities were dominated by <italic>Actinobacteria</italic> and <italic>Proteobacteria</italic> and showed high specificity for their niches as indicated by principal coordinates analysis (PCoA) and bacterial indicator groups identified by the linear discriminant analysis effect size (LEfSe) method. <italic>Nitrospira </italic>and <italic>Polycyclovoran </italic>were indicator genera on the dry surface of stalagmite. <italic>Candidatus </italic>Udaeobacter, <italic>Vicinamibacter</italic>, <italic>Bacillus</italic>, and <italic>Longimicrobium </italic>were indicator genera in overlying soils, microbial mat, sediments, and weathering crust, respectively. Redundancy analysis was conducted to investigate the relationship between environmental factors and bacterial communities. Results showed that pH, TOC, and Mg/Si (chemical weathering index) were of significance to construct bacterial communities. The pH had significantly positive association with<italic> Gammaproteobacteria</italic>,<italic> Rokubacteria</italic>, and <italic>Nitrospirae</italic>, whereas it was negatively related with<italic> Actinobacteria</italic>. TOC had a significantly positive correlation with <italic>Alphaproteobacteria</italic>,<italic> Gammaproteobacteria</italic>,<italic> </italic>and <italic>Planctomycetes</italic> and negative correlation with <italic>Acidobacteria</italic>. Bacterial interactions among different groups were analyzed via co-occurrence networks. The results indicated a high-modularity network with <italic>Gammaproteobacteria</italic>, <italic>Candidatus</italic> Udaeobacter, and <italic>Thermoleophilia</italic>, being the top-three keystone members, which may contribute to the stability of microbial network. The high proportion of positive links between nodes inferred fundamental cooperation among different microbial groups surviving under the extreme conditions in karst caves. Our results revealed habitat specificity of bacterial communities, microbial interactions, and environmental driving forces for bacterial communities in the Xincuntun Cave, enhancing our understanding of the deep terrestrial biosphere.

Microorganisms, sensitive to the surrounding environment changes, show how the cave environment can be impacted by human activities. Zhijin Cave, featured with the most well-developed karst landform in China, has been open to tourists for more than 30 years. This study explored the microbial diversity in a karst cave and the impacts of tourism activities on the microbial communities and the community structures of bacteria and archaea in three niches in Zhijin Cave, including the mixture of bacteria and cyanobacteria on the rock wall, the aquatic sediments, and the surface sediments, using 16S rRNA high-throughput sequencing technology. It was found that Actinobacteriota and Proteobacteria were the dominant bacteria in the cave and Crenarchaeota and Thermoplasmatota were the dominant archaea. The correlation between microorganisms and environmental variables in the cave showed that archaea were more affected by pH and ORP than bacteria and F-, Cl-, NO3-, and SO42- were all positively relevant to the distribution of most bacteria and archaea in the cave. PICRUSt's prediction of microbial functions also indicated that abundance of the bacteria's functions was higher than that of the archaea. The intention of this study was to improve the understanding, development, and protection of microbial resources in caves.

Natural dissolved organic matter (DOM) is ubiquitous in aquatic environments and is an essential component in the carbon cycle in karst areas. To improve understanding of the carbon cycle in karst caves with heterogeneous hydrological processes, we examined the spatiotemporal variability of DOM composition and further uncovered its source and fate. Results may also provide insights into the feedbacks of organic carbon to carbon sinks in karst regions. In this study, concentrations and compositions of DOM, partial pressure of aqueous carbon dioxide (pCO2), dissolved inorganic carbon, and other physicochemical parameters were investigated in a karst cave at Mahuang, Southwest China. Ultraviolet-visible absorption spectroscopy was coupled with multiple statistical analyses to identify the compositional variations and potential fates of DOM in cave waters. The results showed that DOM dynamics were regulated by both terrigenous and biogenic drivers under the control of meteorological conditions. With higher air temperature, precipitation, and microbial activity, fulvic fractions were consumed to generate CO2, leading to the accumulation of refractory DOM in cave waters and changing the hydrochemical features. When temperature and precipitation decreased, DOM was dominated by lignin fractions, which served as an indicator of terrestrial inputs and vascular plants, suggesting variation in the preferential fraction of biological consumption. In addition, different hydrological path patterns influenced DOM properties in cave waters due to differences in recharging, the leaching process, and subsurface reworking. Thus, hydrology could serve as an important constraint on the coupling between dissolved organic and inorganic carbon.

Elevated mercury accumulation and inefficient biomagnification in subterranean streams: Insights from karst cave ecosystems.