Accumulation in nutrient acquisition strategies of arbuscular mycorrhizal fungi and plant roots in poor and heterogeneous soils of karst shrub ecosystems.

BACKGROUND: Arbuscular mycorrhizal (AM) fungi and roots play important roles in plant nutrient acquisition, especially in nutrient poor and heterogeneous soils. However, whether an accumulation strategy of AM fungi and root exists in such soils of karst shrubland ecosystems remains unclear. Root traits related to nutrient acquisition (root biomass, AM colonisation, root acid phosphatase activity and N2 fixation) were measured in two N2-fixing plants (i.e. Albizia odoratissima (Linn. f.) Benth. and Cajanus cajan (Linn.) Millsp.) that were grown in heterogeneous or homogeneous nutrient (ammonium) soil with and without AM fungi inoculation. RESULTS: Both of these plants had higher AM colonisation, root biomass and relative growth rate (RGR), but lower N2 fixation and root acid phosphatase activity in the rhizosphere in the heterogeneous soil environment, than that in the homogeneous soil environment. Plants grown in the AM fungi-inoculated heterogeneous soil environment had increased root biomass and root acid phosphatase activity compared with those grown in soil without inoculation. AM colonisation was negatively correlated with the N2 fixation rate of A. odoratissima, while it was not significantly correlated with the root phosphatase activity. CONCLUSIONS: Our results indicated that enhanced AM symbiosis and root biomass increased the absorptive surfaces for nutrient acquisition, highlighting the accumulation strategies of AM and root traits for plant nutrient acquisition in nutrient poor and heterogeneous soils of the karst shrubland ecosystem.

Community structure analysis of soil ammonia oxidizers during vegetation restoration in southwest China.

Soil ammonia oxidizers play a critical role in nitrogen cycling and ecological restoration. The composition and structure of soil ammonia oxidizers and their impacting factors were studied in four typical ecosystem soils, tussock (T), shrub (S), secondary forest (SF), and primary forest (PF), during vegetation restoration in the Karst region of Southwest China. The composition and structure of the ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) communities were characterized by sequencing the amoA and arch-amoA genes, respectively. The diversity of soil ammonia oxidizers (except in S) and plant Shannon diversity index gradually increased with vegetation restoration, and the ammonia oxidizer communities differed significantly (p < 0.001). Amplicons of AOA from the Nitrososphaera cluster dominated all four ecosystem soils. AOB Nitrosospira cluster 3b only appeared in PF and SF soils, while Nitrosospira cluster 3a species were found in all soils. Changes in AOB paralleled the changes in soil ammonium content that occurred with vegetation restoration. Redundancy analysis showed that the distribution of dominant AOB species was linked to pH, soil urease activity, and soil C/N ratio, whereas the distribution of dominant AOA species was mainly influenced by litter nitrogen content and C/N ratio. These results suggested that the composition and structure of the AOB community were more sensitive to changes in vegetation and soil ammonium content, and may be an important indicator of nitrogen availability in Karst ecosystem soils.

Vegetation recovery reshapes the composition and enhances the network connectivity of phoD-harboring microorganisms to promote P availability in a karst ecosystem.

Soil phoD-harboring microorganisms can facilitate phosphorus (P) transformation and increase the available P (AP) in P-limited soils; however, the mechanism by which these microorganisms enhance AP throughout the vegetation recovery process of karst ecosystems is poorly understood. Accordingly, this study investigates the effect of vegetation recovery on soil AP and the community composition and network connectivity of phoD-harboring microorganisms to elucidate the mechanism by which phoD-harboring microorganisms enhance soil AP in the four vegetation recovery stages (i.e., grassland, shrubland, shrub-arbor forest, and arbor forest) in a karst ecosystem. Results show that soil total P, AP, and microbial biomass P concentrations, as well as alkaline phosphatase activities, litter and soil nutrients, and plant diversity indices (Shannon-Wiener and Pielou) increase with advancing vegetation recovery. Moreover, the diversity indices (Shannon-Wiener and Simpson) and network complexity of the phoD-harboring microorganisms also increase with advancing vegetation recovery, leading to distinct communities among the four recovery stages. Rhizobiales, Pseudomonadales, and Burkholderiales comprise the dominant phoD-harboring microorganism orders. The relative abundances of Pseudomonadales and Burkholderiales increase with advancing vegetation recovery; Rhizobiales is the highest in shrubland and the lowest in grassland. The structural equation model results show that advanced vegetation recovery is associated with increased plant diversity, litter nutrients, and soil nutrients. The network connectivity is enhanced with advancing vegetation recovery accompanied by increasing soil phosphatase activity and P availability. These results suggest that regulating the phoD-harboring microorganism composition and network connectivity is essential to alleviate plant P limitation in karst ecosystems.

Quantitative proteomics revealed protein biomarkers to distinguish malignant pleural effusion from benign pleural effusion.

To identify protein biomarkers capable of early prediction regarding the distinguishing malignant pleural effusion (MPE) from benign pleural effusion (BPE) in patients with lung disease. A four-dimensional data independent acquisition (4D-DIA) proteomic was performed to determine the differentially expressed proteins in samples from 20 lung adenocarcinoma MPE and 30 BPE. The significantly differential expressed proteins were selected for Gene Ontology (GO) enrichment and Kyoto Encyclopaedia of Genes and Genomes (KEGG) pathway analysis. Protein biomarkers with high capability to discriminate MPE from BPE patients were identified by Random Forest (RF) algorithm prediction model, whose diagnostic and prognostic efficacy in primary tumors were further explored in public datasets, and were validated by ELISA experiment. 50 important proteins (30 up-regulated and 20 down-regulated) were selected out as potential markers to distinguish the MPE from BPE group. GO analysis revealed that those proteins involving the most important cell component is extracellular space. KEGG analysis identified the involvement of cellular adhesion molecules pathway. Furthermore, the Area Under Curve (AUC) of these proteins were ranged from 0.717 to 1.000，with excellent diagnostic properties to distinguish the MPE. Finally, significant survival and gene and protein expression analysis demonstrated BPIFB1, DPP4, HPRT1 and ABI3BP had high discriminating values. SIGNIFICANCE: We performed a 4D-DIA proteomics to determine the differentially expressed proteins in pleural effusion samples from MPE and BPE. Some potential protein biomarkers were identified to distinguish the MPE from BPE patients., which may provide helpful diagnostic and therapeutic insights for lung cancer. This is significant because the median survival time of patients with MPE is usually 4-12 months, thus, it is particularly important to diagnose MPE early to start treatments promptly. The most common causes of MPE are lung cancers, while pneumonia and tuberculosis are the main causes of BPE. If more diagnostic markers could be identified periodically, there would be an important significance to clinical diagnose and treatment with drugs in lung cancer patients.

Integrative analyses identify opportunistic pathogens of patients with lower respiratory tract infections based on metagenomic next-generation sequencing.

Lower respiratory tract infections (LRTIs) represent some of the most globally prevalent and detrimental diseases. Metagenomic next-generation sequencing (mNGS) technology has effectively addressed the requirement for the diagnosis of clinical infectious diseases. This study aimed at identifying and classifying opportunistic pathogens from the respiratory tract-colonizing microflora in LRTI patients using data acquired from mNGS analyses. A retrospective study was performed employing the mNGS data pertaining to the respiratory samples derived from 394 LRTIs patients. Linear discriminant analysis effect size (LEfSe) analysis was conducted to discern the discriminant bacteria. Receiver operating characteristic curves (ROC) were established to demonstrate discriminant bacterial behavior to distinguish colonization from infection. A total of 443 discriminant bacteria were identified and segregated into three cohorts contingent upon their correlation profiles, detection frequency, and relative abundance in order to distinguish pathogens from colonizing microflora. Among them, 119 emerging opportunistic pathogens (cohort 2) occupied an average area under the curve (AUC) of 0.976 for exhibiting the most prominent predictability in distinguishing colonization from infection, 39 were colonizing bacteria (cohort 1, 0.961), and 285 were rare opportunistic pathogens (cohort 3, 0.887). The LTRIs patients appeared modular in the form of cohorts depicting complex microbial co-occurrence networks, reduced diversity, and a high degree of antagonistic interactions in the respiratory tract microbiome. The study findings indicate that therapeutic interventions should target interaction networks rather than individual microbes, providing an innovative perspective for comprehending and combating respiratory infections. Conclusively, this study reports a profile of LRTIs-associated bacterial colonization and opportunistic pathogens in a relatively large-scale cohort, which might serve as a reference panel for the interpretation of mNGS results in clinical practice.

Water quality and microecosystem of water tanks in karst mountainous area, Southwest China.

Karst mountainous areas in Southwest China, the world's largest bare karst area, are faced with growing water shortages. Rainwater harvesting plays an important role in alleviating water shortage. However, there remains a substantial gap in the research regarding the water quality of tanks. Water samples were seasonally collected from ten tanks to investigate the physicochemical properties, microbial communities, and their key influencing factors. The result showed that pH, turbidity, chroma, DOC, and CODMn exceeded drinking water guidelines. The alkaline pH value and the deterioration of sensory properties was the main feature of tank water, from which the over-standard rate of the uncleaned water tanks was higher. Moreover, principal component analyses suggested that tank water quality was influenced by human activities, catchment areas, and material cycling processes within the tanks, of which in-tank microbial activities were the most important driving factors in water quality variation. Proteobacteria, Actinobacteria, Bacteroidetes, Cyanobacteria, Firmicutes, and Verrucomicrobia were the predominant bacterial phyla in water tanks. Acinetobacter, Cyanobium-PCC-6307, CL500-29-marine-group, Candidatus-Aquiluna, and Exiguobacterium were the most abundant genera. The bacterial communities were significantly affected by the management practices. Higher relative abundance of Cyanobacteria and lower relative abundance of Proteobacteria was detected in the uncleaned tanks, which was a sign of tank water quality deterioration. The microbial community structure was closely related to the environmental factors. There was evidence that the water quality was affected by the existence of a microecosystem dominated by photosynthetic microorganisms in the water tanks. In addition, Acinetobacter, Enterobacter, Pseudomonas, and Legionella identified as the potential opportunistic pathogenic genera were frequently detected but the relative abundances except Acinetobacter were low in the tanks. Overall, our findings indicated that management style influences water quality and bacterial communities of tank water.

Land reclamation and short-term cultivation change soil microbial communities and bacterial metabolic profiles.

BACKGROUND: Soil microbes play an important role in many critical ecosystem processes, but little is known about the effects of land reclamation and short-term cultivation on microbial communities in red soil. In this study, soil microbial communities under five land use patterns-artificial pine forest (Fp), tussock and shrub (TS), shrubbery (Sh), sugarcane (Su) and maize and cassava rotation (Ma)-were characterised by DNA fingerprinting and metabolic profiling to reveal how land reclamation and cultivation affect the underlying diversity and function of soil microbial communities in southwestern China. RESULTS: Eight years of reclamation and cultivation significantly affected population size, composition and structure, bacterial metabolic profiles and diversity values (Shannon-Wiener index) of soil microbial communities. Soil organic carbon and pH were the most important factors shaping the underlying microbial communities; however, with significant correlations between soil carbon/nitrogen ratio and bacterial taxonomic and metabolic diversities, soil total nitrogen was a potentially important factor for soil microbial composition and function, as well as soil moisture, cation exchange capacity and physical structure to a lesser extent. In addition, the lowest pH, lower nutrient availability and the most compact soil in pine forest resulted in the lowest microbial taxonomic and metabolic diversities among the five land use patterns studied. CONCLUSION: Soil organic carbon, nitrogen and pH appeared to be the most important factors influencing microbial biomass, composition and function in red soil of southwestern China. The study suggests that measures to lessen the impact of changes in this edaphic environment should be taken to avoid an imbalance of microbial function and improve ecological sustainability in southwestern China.

Soil bacterial community changes along elevation gradients in karst graben basin of Yunnan-Kweichow Plateau.

Elevation gradients could provide natural experiments to examine geomorphological influences on biota ecology and evolution, however little is known about microbial community structures with soil depths along altitudinal gradients in karst graben basin of Yunnan-Kweichow Plateau. Here, bulk soil in A layer (0 ~ 10 cm) and B layer (10 ~ 20 cm) from two transect Mounts were analyzed by using high-throughput sequencing coupled with physicochemical analysis. It was found that the top five phyla in A layer were Proteobacteria, Acidobacteria, Actinobacteria, Bacteroidetes, and Verrucomicrobia, and the top five phyla in B layer were Proteobacteria, Acidobacteria, Actinobacteria, Verrucomicrobia, and Chloroflexi in a near-neutral environment. Edaphic parameters were different in two layers along altitudinal gradients. Besides that, soil microbial community compositions varied along altitudinal gradient, and soil organic carbon (SOC) and total nitrogen (TN) increased monotonically with increasing elevation. It was further observed that Shannon indexes with increasing altitudes in two transect Mounts decreased monotonically with significant difference (p = 0.001), however beta diversity followed U-trend with significant difference (p = 0.001). The low proportions of unique operational taxonomic units (OTUs) appeared at high altitude areas which impact the widely accepted elevation Rapoport's rules. The dominant Bradyrhizobium (alphaproteobacterial OTU 1) identified at high altitudes in two layers constitutes the important group of free-living diazotrophs and could bring fixed N into soils, which simultaneously enhances SOC and TN accumulation at high altitudes (p < 0.01). Due to different responses of bacterial community to environmental changes varying with soil depths, altitudinal gradients exerted negative effects on soil bacterial communities via soil physical properties and positive effects on soil bacterial diversities via soil chemical properties in A layer, however the results in B layer were opposite. Overall, our study is the first attempt to bring a deeper understanding of soil microbial structure patterns along altitudinal gradients at karst graben basin areas.

Clinical diagnosis and etiology of patients with Chlamydia psittaci pneumonia based on metagenomic next-generation sequencing.

The incidence of severe Chlamydia psittaci (C. psittaci) pneumonia and coinfections is increasing. Early detection of this condition is needed to prevent negative outcomes, along with detailed descriptions of its associated clinical characteristics. Our study contributes by undertaking etiological analysis of patients with C. psittaci pneumonia based on metagenomic next-generation sequencing (mNGS). A retrospective analysis of 30 patients with C. psittaci pneumonia was undertaken and confirmed by mNGS or polymerase chain reaction (PCR). Clinical manifestations of the severe and non-severe C. psittaci pneumonia groups were compared for clinical reference. Etiological analyses were also performed to comprehensively understand pathogeny and coinfection with other respiratory pathogens in C. psittaci patients. The absolute value of lymphocytes (LYM) in the severe group was lower than in the non-severe group. At the same time, neutrophil-to-lymphocyte ratio (NLR), procalcitonin (PCT), alanine aminotransferase (ALT), D-II polymer, brain natriuretic peptide (BNP), myoglobin (MYO), and cardiac troponin I (cTnI) were significantly higher (P < 0.05) in the severe group. mNGS has a broader pathogen spectrum and can more sensitively detect C. psittaci and other low-abundance pathogens with a higher positive detection rate (100%, 13/13 vs. 46%, 6/13, P <0.05) than conventional culture methods. mNGS detected the following dominant species associated with C. psittaci in patients: bacteria (53.2%, 39% gram-positive, 61% gram-negative), fungi (12.9%), and viruses (33.9%). A total of 73.3% (11/15) of patients had suspected coinfections, with a coinfection rate of 91.7% (11/12) in the severe group. No coinfection or death occurred in the non-severe group. Prognosis in the severe group was poor, with a mortality rate of 27.3% (3/11) for patients with coinfection. Eight of 11 patients with coinfections (72.7%) recovered. In conclusion, the clinical symptoms of severe C. psittaci pneumonia manifested as abnormal inflammatory indicators, impaired liver function, myocardial injury, coagulation, and relatively low immune responses. The higher proportion of patients with coinfections in our study supports the use of mNGS for comprehensive early detection of respiratory infections in patients with C. psittaci pneumonia. Simultaneous early identification of coinfections would further improve the clinical treatment of these patients.

Long-term forest restoration influences succession patterns of soil bacterial communities.

Microorganisms have a major influence on soil biogeochemical processes and vegetation establishment. However, their long-term succession patterns and short-term turnover are not well-understood in artificial forest ecosystems. The aim of the present study was to investigate the effects of stand ages and seasons on soil bacterial community in a chronosequence of Chinese Pinus massoniana plantations, in 3, 19, and 58-year-old plots. Soil physicochemical properties were measured in three stand ages between two seasons (dry-rainy). The soil bacterial community composition was determined by 16S rRNA Illumina HiSeq sequencing. The results showed that soil bacterial community diversity and structure significantly differed among three stand ages, but was not different between two seasons. The diversity of soil bacterial community increased with an increase in stand age. Proteobacteria, Acidobacteria, and Actinobacteria were the dominant phyla in the three stands. The soil bacterial community structure in all the stands was influenced by soil pH, available phosphorus content, and litter phosphorus content. With the accumulation of available phosphorus, the relative abundance of Acidobacteria decreased, while that of Proteobacteria increased. These shifts suggested that dominant microbial communities transitioned from oligotrophic to copiotrophic with increasing stand age. Extending rotation periods could increase soil bacterial diversity, and in turn help improving soil quality of P. massoniana plantations.

Alkaline Phosphomonoesterase-Harboring Microorganisms Mediate Soil Phosphorus Transformation With Stand Age in Chinese Pinus massoniana Plantations.

phoD-harboring microorganisms facilitate mineralization of organic phosphorus (P), while their role in the regulation of soil P turnover under P-limited conditions in Pinus massoniana plantations is poorly understood. The aim of the present study was to investigate the effects of stand age and season on soil P fractions and phoD-harboring microorganism communities in a chronosequence of Chinese P. massoniana plantations including 3, 19, and 58 years. The soil P fractions (i.e., CaCl2-P, citrate-P, enzyme-P, and HCl-P) varied seasonally, with the higher values observed in the rainy season. The concentrations of the fractions were higher in old plantation (OP) soils and lower in young planation (YP) soils in both seasons. The OTU abundances were negatively correlated with total available P concentration, while were positively correlated with alkaline phosphomonoesterase (ALP) activity at 0-10 cm soil depth. The results indicate that phoD-harboring microorganisms have great potential to mineralize organic P under P-poor conditions and highlights those microorganisms are indicators of P bioavailability in P. massoniana plantations.

Spatial and temporal dynamics of bacterioplankton community composition in a subtropical dammed karst river of southwestern China.

River damming influences the hydro-physicochemical variations in karst water; however, such disruption in bacterioplankton communities has seldom been studied. Here, three sampling sites (city-river section, reservoir area, and outflow area) of the Ca2+ -Mg2+ -HCO3- -SO42- water type in the dammed Liu River were selected to investigate the bacterioplankton community composition as identified by high-throughput 16S rRNA gene sequencing. In the dammed Liu River, thermal regimes have been altered, which has resulted in considerable spatial-temporal differences in total dissolved solids (TDSs), oxidation-reduction potential (Eh), dissolved oxygen (DO), and pH and in a different microenvironment for bacterioplankton. Among the dominant bacterioplankton phyla, Proteobacteria, Actinobacteria, Bacteroidetes, and Cyanobacteria account for 38.99%-87.24%, 3.75%-36.55%, 4.77%-38.90%, and 0%-14.44% of the total reads (mean relative frequency), respectively. Bacterioplankton communities are dominated by Brevundimonas, Novosphingobium, Zymomonas, the Actinobacteria hgcIclade, the CL500-29 marine group, Sediminibacterium, Flavobacterium, Pseudarcicella, Cloacibacterium, and Prochlorococcus. Their abundances covary with spatial-temporal variations in hydro-physicochemical factors, as also demonstrated by beta diversity analyses. In addition, temperature plays a pivotal role in maintaining bacterioplankton biodiversity and hydro-physicochemical variations. This result also highlights the concept that ecological niches for aquatic bacteria in dammed karst rivers do not accidentally occur but are the result of a suite of environmental forces. In addition, bacterioplankton can alter the aquatic carbon/nitrogen cycle and contribute to karst river metabolism.

[Structure Analysis of Arbuscular Mycorrhizal in Roots from Different Shrubs in Karst Regions].

To explore if there are species-preferential characteristics of arbuscular mycorrhizal (AM) and host plants in karst regions, 13 shrub plants (including leguminosae and non-leguminosae) were selected to study the AM community structure of root samples. The soil nutrients in rhizosphere soils significantly differ among shrubs; they are higher in leguminosae than in non-leguminosae. Cluster analysis shows that all 13 shrubs can be infected by AM. Significant differences of the AM community structure were observed among root samples from different shrubs, especially leguminosae and non-leguminosae. Redundancy analysis shows that soil Olsen-P, pH, and total nitrogen significantly influence the AM community structure of plant roots, although the factors affecting this fungus in leguminosae and non-leguminosae differ. These results indicate species-preferential characteristics of AM and host plants in karst regions, especially of the plant function group compared with plant species, suggesting that these characteristics should be taken into account when AM fungi are used for vegetation restoration in karst regions.

Increased associated effects of topography and litter and soil nutrients on soil enzyme activities and microbial biomass along vegetation successions in karst ecosystem, southwestern China.

Studying the influence of topography and litter and soil nutrients on soil enzymes and microbial biomass is important to the understanding of soil nutrient transformation and cycling, but these relationships in heterogeneous soils of karst ecosystem remains poorly understood. We determined environment factors influencing the urease (URS) and alkaline phosphatase (ALP) activity and microbial biomass C and N (MBC and MBN) with advancing vegetation succession. The results showed that ALP increased but URS decreased with the advancing vegetation succession. The MBC and MBN were highest in shrubland, but both were lowest in grassland. The URS was positively correlated with the surface cover of rock outcrops (SRO) but negatively correlated with litter N, and soil available N and pH. Conversely, ALP was positively correlated with litter N, soil organic carbon (SOC), and soil available N and pH, but negatively correlated with soil total N. The MBC was positively related to litter quantities and SOC but negatively related to soil pH; the MBN was positively related to slope gradient (SLG), SOC, and soil total P and available P. Additionally, the trends of the index URS/MBN were grassland > secondary forest > shrubland > primary forest, but the index ALP/MBN increased with advancing vegetation succession. It indicated that soil microorganism mainly exudate extracellular URS and ALP to soils. We also found the interactions of topography (SLG and SRO), litter (nutrients and quantity), and soil (nutrients and pH) explained 42.00, 87.00, and 66.00% of the variations in URS, ALP, and microbial biomass, respectively. Path analysis showed that the topography had a directly positive effect on litter nutrients and quantities, but not on soil nutrients; the litter nutrients and quantities had direct positive effect on soil nutrients, which had direct effect on soil enzymes and microbial biomass; the relationships (R2) between the independent variable and enzymes activities and microbial biomass increased with advancing successions. Thus, it suggested that high SLG and SRO are good for collecting litters back to soils and then the topography, litter, and soil factors increased its controlling effect on soil enzymes activities and microbial biomass with advancing successions in karst ecosystem.

Influence of Altered Microbes on Soil Organic Carbon Availability in Karst Agricultural Soils Contaminated by Pb-Zn Tailings.

Soil organic carbon (SOC) availability is determined via a complex bio-mediated process, and Pb-Zn tailings are toxic to the soil microbes that are involved in this process. Here, Pb-Zn-tailings- contaminated karst soils with different levels (paddy field > corn field > citrus field > control group) were collected to explore the intrinsic relationship between Pb-Zn tailings and microbes due to the limited microbial abundance in these soils. The SOC concentration in the paddy fields is the highest. However, based on the soil microbial diversity and sole-carbon-source utilization profiles, the rate of SOC availability, McIntosh index, Shannon-Wiener diversity index, Simpson's diversity index and species richness are the lowest in the rice paddy soils. According to the results of Illumina sequencing of the 16S rRNA gene, Acidobacteria and Proteobacteria are the dominant phyla in all samples, accounting for more than 70% of the reads, while the majority of the remaining reads belong to the phyla Verrucomicrobia, Chloroflexi, Actinobacteria, Bacteroidetes, and Nitrospirae. We also observed that their class, order, family, genus and operational taxonomic units (OTUs) were dependent on SOC availability. Pearson correlation analysis reveals that L-asparagine utilization profiles show significant positive correlation with OTUs 24, 75, and 109 (r = 0.383, 0.350, and 0.292, respectively), and malic acid utilization profiles show significant positive correlation with OTUs 4, 5, 19, 27 (Bradyrhizobium), 32 (Burkholderia), 75 and 109 (r = 0.286, 0.361, 0.387, 0.384, 0.363, 0.285, and 0.301, respectively), as also evidenced by the redundancy analysis (RDA) biplot and heat map. These results indicate that the most abundant groups of bacteria, especially the uncultured facultative Deltaproteobacteria GR-WP33-30 (OTU 24), after long-term acclimation in heavy metal-contaminated soil, are associated with the variance of labile carbon source such as L-asparagine and may have considerable control over the stability of the vast SOC pool in karst surface soils with different agricultural land-use practices. These findings can expand our understanding of global soil-carbon sequestration and storage via changes in microbial community structure of the most abundant species.

[Relationship Between the Bacterial Abundance and Production with Environmental Factors in a Subtropical Karst Reservoir].

In recent years, the increasing scarcity of water resources and eutrophication of water have become more serious. Reservoirs that are far from big cities have become important sources of drinking water and were targets of research and protection. Therefore, the abundance and production of bacteria and their correlations with environmental factors were investigated in the Dalongdong Reservoir, Shanglin County, Guangxi Province, using Quantitative Real-time PCR technology and the 14C tracer technique. The Dalongdong Reservoir is a typical subtropical karst reservoir. The bacterial abundance of the surface water decreased from the upstream to the downstream along the water flow direction and then increases. The vertical distribution of the bacterial abundance at each sampling site shows a similar trend; it is the highest in the surface water and the lowest on the bottom. The correlation analysis results show that the temperature, pH, electrical conductivity, dissolved organic carbon (DOC), chlorophyll-a, dissolved oxygen (DO), and other environmental factors significantly correlate with the bacterial abundance, indicating that these parameters are the main factors limiting the bacterial abundance in this region. The bacterial production is positively correlated with the pH, DOC, and permanganate index; negatively correlated with the conductivity and DIC; and significantly positively correlated with DO. The principal component analyses (PCA) shows that the environmental factors affecting the bacterial abundance and bacterial production can be grouped into two PCAs. PCA1 includes the temperature, pH, electrical conductivity, DIC, DO, chlorophyll-a, DOC, and permanganate index and PCA2 includes TN and TP. The bacterial abundance and production in the Dalongdong Reservoir are affected by various environmental factors and photosynthetic bacteria are the important contributors to the production of organic carbon.

[Carbon Metabolism Characteristics of the Karst Soil Microbial Community for Pb-Zn Mine Tailings].

BIOLOG and 18S rRNA PCR-DGGE methods were used to estimate the characteristics of carbon source metabolism of the soil microbial community as well as the relationship between soil fungi and soil organic carbon in different karst land use types (corn field, citrus field, and paddy field) contaminated by Pb-Zn tailings at Sidi Village, Yangshuo, Guangxi Zhuang Autonomous Region, SW China. It was found that the concentrations of Pb, Zn, Cu, and Cd were highest in the paddy field, followed by the corn field, citrus field, and control group (dry field). In addition, the geo-accumulation index indicated that the heavy metal pollution at this area was caused by Pb and Cd and that Cd was the key environmental risk factor. With the Pb-Zn tailings, there was low microbial biomass carbon, biological entropy, and microbial carbon source metabolism in the soil samples. The highest soil organic carbon and heavy metal concentrations were found in the paddy field, followed by the corn field and citrus field. According to the DDGE results, it was found that Pycnoporus sp. ZW02.30 was found in control group soils, Fusarium solani and Fusarium oxysporum were found in the corn field and citrus field, and Penicillium decumbens was found in citrus field. These were involved in the degradation of sugars, such as starch, cellulose, hemicellulose, and lignin. However, the fungi were not found in the paddy field. Based on the low functional diversity of the soil microbial community and biological entropy as well as the high soil organic carbon concentration in the paddy field, it was concluded that soil microbial carbohydrate metabolism and the mineralization rate of the soil organic carbon was controlled by soil microorganisms, especially fungi, in different land use soils in the karst area contaminated by Pb-Zn tailings.

[Effects of Lithology on the Abundance and Composition of Soil Nitrogen-fixing Bacteria and Arbuscular Mycorrhizal Fungal Communities in Karst Shrub Ecosystem].

Lithology is a key factor when used to restore vegetation in karst degraded ecosystems, and arbuscular mycorrhizal (AM) fungi and nitrogen-fixing bacteria play an important role in improving plant growth. However, little information is available regarding the effects of lithology on these two groups of microorganisms. To test whether these microbial communities are impacted by lithology, the abundance and composition of soil AM fungal and nitrogen-fixing bacteria communities were determined through terminal restriction fragment length polymorphism (T-RFLP) and real-time fluorescence-based quantitative PCR (real-time PCR). Three types of lithology (dolomite, limestone and dolomite-limestone) were selected in this study. The diversity, richness, and evenness of plant species were evaluated through field surveys and soil properties were measured. The results showed that the abundances of soil nitrogen-fixing bacteria and arbuscular mycorrhizal fungal communities were significantly influenced by lithology. The abundances of these two groups of microorganisms were the lowest in dolomite soil, inferior to dolomite-limestone soil, while highest in limestone soil. Similarly, the composition of soil nitrogen-fixing bacteria and AM fungi communities varied among lithology. A significant linear correlation was observed among soil organic carbon, available phosphorus, clay content and nitrogen-fixing bacterial abundance (P<0.05), and a significant linear correlation among total nitrogen, clay content and AM fungal abundance (P<0.05). Redundancy analysis showed that the composition of nitrogen-fixing bacterial community was closely linked to plant evenness, and the AM fungal community composition was closely linked to plant diversity (plant evenness, Shannon-wiener and richness). These results indicated that lithology influenced the abundances and compositions of soil nitrogen-fixing bacteria and arbuscular mycorrhizal (AM) fungal communities mainly through plant and soil properties.

[Various effects on the Abundance and Composition of Arbuscular Mycorrhizal Fungal Communities in Soils in Karst Shrub Ecosystems].

Slope position is a key factor used in the restoration of vegetation in degraded karst ecosystems, and arbuscular mycorrhizal fungi (AMF) play an important role in improving this plant growth. However, little information is available regarding the effects of slope position on arbuscular mycorrhizal fungi. To test whether these fungal communities are impacted by slope position, the abundance, and composition of soil, AMF communities along the slope position were analyzed through terminal restriction fragment length polymorphism (T-RFLP) and real-time fluorescence-based quantitative polymerase chain reaction (real-time PCR). The diversity, richness, and evenness of plant species were evaluated through field surveys and soil properties were also measured. The results show that content of carbon, nitrogen, and phosphorus in the soil are different along the slope, and the trends identified were that the upper slope position ≈ middle slope position > lower slope position. The trend for AMF abundance was identified as upper slope position ≈ middle slope position < lower slope position. The available phosphorus content in the soil correlated significantly with the AMF abundance. A redundancy analysis showed that the structure of soil, AMF, and plant community compositions differed along the slope. The plant evenness index was shown to significantly contribute to the distribution of the AMF community structure, while the total nitrogen and total organic carbon content of the soil had a significant effect on the plant community structure. These results indicate that the interaction effects of soil nutrients and plant community structures on the soil AMF community structures suggest micro-morphology should be taken into account when AMF is used to restore vegetation in karst regions.

Enhanced Nitrogen Availability in Karst Ecosystems by Oxalic Acid Release in the Rhizosphere.

In karst ecosystems, a high level of CaCO3 enhances the stabilization of soil organic matter (SOM) and causes nitrogen (N) and/or phosphorus (P) limitation in plants. Oxalic acid has been suggested to be involved in the nutrient-acquisition strategy of plants because its addition can temporarily relieve nutrient limitation. Therefore, understanding how oxalic acid drives N availability may help support successful vegetation restoration in the karst ecosystems of southwest China. We tested a model suggested by Clarholm et al. (2015) where oxalate reacts with Ca bridges in SOM, thus exposing previously protected areas to enzymatic attacks in a way that releases N for local uptake. We studied the effects of oxalic acid, microbial biomass carbon (MBC), and ß-1,4-N-acetylglucosaminidase (NAG) on potential N mineralization rates in rhizosphere soils of four plant species (two shrubs and two trees) in karst areas. The results showed that rhizosphere soils of shrubs grown on formerly deforested land had significantly lower oxalic acid concentrations and NAG activity than that of trees in a 200-year-old forest. The levels of MBC in rhizosphere soils of shrubs were significantly lower than those of trees in the growing season, but the measure of shrubs and trees were similar in the non-growing season; the potential N mineralization rates showed a reverse pattern. Positive relationships were found among oxalic acid, MBC, NAG activity, and potential N mineralization rates for both shrubs and trees. This indicated that oxalic acid, microbes, and NAG may enhance N availability for acquisition by plants. Path analysis showed that oxalic acid enhanced potential N mineralization rates indirectly through inducing microbes and NAG activities. We found that the exudation of oxalic acid clearly provides an important mechanism that allows plants to enhance nutrient acquisition in karst ecosystems.