Soil characteristics and heavy metal accumulation by native plants in a Mn mining area of Guangxi, South China.

Revegetation and ecological restoration of a Mn mineland are important concerns in southern China. To determine the major constraints for revegetation and select suitable plants for phytorestoration, pedological and botanical characteristics of a Mn mine in Guangxi, southern China were investigated. All the soils were characterized by low pH and low nitrogen and phosphorus levels except for the control soil, suggesting that soil acidity and poor nutrition were disadvantageous to plant growth. In general, the studied mine soils had normal organic matter (OM) and cation exchange capacity (CEC). However, OM (8.9 g/kg) and CEC (7.15 cmol/kg) were very low in the soils from tailing dumps. The sandy texture and nutrient deficiency made it difficult to establish vegetation on tailing dumps. Mn and Cd concentrations in all soils and Cr and Zn concentrations in three soils exceeded the pollution threshold. Soil Mn and Cd were above phytotoxic levels, indicating that they were considered to be the major constraints for phytorestoration. A botanical survey of the mineland showed that 13 plant species grew on the mineland without obvious toxicity symptoms. High Mn and Cd concentrations have been found in the aerial parts of Polygonum pubescens, Celosia argentea, Camellia oleifera, and Solanum nigrum, which would be interesting for soil phytoremediation. Miscanthus floridulus, Erigeron acer, Eleusina indica, and Kummerowia striata showed high resistance to the heavy metal and harsh condition of the soils. These species could be well suited to restore local degraded land in a phytostabilization strategy.

[Comparison of Bacterial Community Structure and Functional Groups of Paddy Soil Aggregates Between Karst and Non-karst Areas].

Studying the similarities and differences in microbial community structure and functional groups in soil aggregates between karst areas (KA) and non-karst areas (NKA) can provide a theoretical basis for revealing the interaction mechanism between soil organic microorganisms and soil aggregate structure stability in KA. The surface soils (0-20 mm) were collected from paddy fields both in KA and NKA. The aggregates were divided into macro-aggregates (Mac, 0.25-2 mm), micro-aggregates (Mic, 0.053-0.25 mm), and silt-clay fractions (SC,<0.053 mm) using the wet sieving method. The microbial community structure of soil aggregates was analyzed using high-throughput sequencing. The keystone taxa were analyzed by the co-occurrence network, and their functions were predicted by FAPROTAX. The results showed that:① the proportion of Mac, mean weight diameter, and geometric mean diameter of aggregates in KA were 30.94%, 0.41 mm, and 0.15 mm, respectively. These three factors in KA were significantly higher than those in NKA, indicating that the environment in the KA was conducive to the formation of aggregates. ② The relative abundances of Acidobacteria, Proteobacteria, Acidobacteria(Gp3, Gp4, and Gp6), and Sphingomonas in three particle size aggregates in KA were higher than those in NKA. The relative abundance of Chloroflexi (6.13%) in Mic in KA was significantly higher than that in SC (2.79%). ③ Co-occurrence network analysis showed that Proteobacteria and Acidobacteria were keystone taxa in three particle size aggregates. The positive correlation edge (65.5%) of bacteria in KA was the highest, and most of the correlation was synergistic. ④ Chemoheterotrophy (15.52%, 15.30%, and 16.89%) and aerobic chemoheterotrophy (14.62%, 14.38%, and 15.97%) were the most dominant functional groups in the three particle size aggregates in KA, and their relative abundance was significantly higher than those in NKA. The relative abundance of chitinolysis in Mac in KA (0.46%) was significantly higher than that in SC (0.39%). The results showed that the soil environment in KA can improve the stability of the aggregates, and the bacterial community structure, key groups, and functional groups in the three particle size aggregates in KA were significantly different from those in NKA.

[Comparison of Bacterial Community Structure in Soil Aggregates Between Natural Karst Wetland and Paddy Field].

Studying the impact of land-use changes in wetland systems on the community structure of bacteria in soil aggregates can provide a theoretical basis for elucidating the impact mechanisms of the stability of wetland soil aggregates. The soil samples (0-20 cm) were collected from a natural wetland and paddy field in the Huixian karst wetland. The bacterial community structure in soil aggregates (macroaggregates 0.25-2 mm, microaggregates 0.053-0.25 mm, and silt-clay<0.053 mm) were analyzed using high-throughput sequencing and ecological network analysis. The results showed that ① the proportion of microaggregates in the paddy field was 29.64%, which was significantly higher than that in the natural wetland (22.20%), and ② there were differences in the relative abundance of bacteria between the natural wetland and paddy field. The relative abundance of Chloroflexi in macroaggregates, microaggregates, and silt-clay in the paddy field were 7.97%, 8.56%, and 7.40%, respectively, which were significantly higher than those in the natural wetland (4.93%, 4.81%, and 3.76%). The relative abundance of Anaerolineales in macroaggregates, microaggregates, and silt-clay in the paddy field were 2.35%, 3.03%, and 2.65%, respectively, which were significantly higher than those in the natural wetland (0.92%, 0.91%, and 0.43%).③ Ecological network analysis showed that the number of nodes, edges, and the average numbers of neighbors in microaggregates and the silt-clay network in the paddy field were higher than those in the natural wetland, the average connectivity of the network was greater in the two components, and the characteristic path length was shorter. There was a longer characteristic path length, higher modularity, and lower average connectivity in the macroaggregate network of the paddy field. The above results indicated that the bacterial community structure in soil aggregates was altered after a natural wetland was used as a paddy field. The material circulation and information transmission efficiency of bacterial communities was higher in both microaggregates and silt-clay of the paddy field, whereas the bacterial community structure with low network tightness and high modularity in macroaggregates was more stable.

[Investigation of Soil Fungal Communities and Functionalities within Karst Paddy Fields].

Fungi are important drivers of soil biogeochemical cycles. However, the characteristics of fungal community structures and functional groups within karst area (KA) soils remain understudied. Top soil samples were collected from paddy fields within a KA and non-karst area (NKA) containing red soil, in the Maocun karst experimental site of Guilin. The fungal community structure was analyzed via high-throughput sequencing, and FUNGuild was used to predict the function of fungi. The average relative abundance of Mucormycota in KA was 4.87%, which was significantly lower than that in NKA (29.92%); The average relative abundance of Mortierellomycetes in KA was 3.36%, which was significantly lower than that in NKA (29.15%). However, in KA, the average relative abundances of Glomeromycetes, Chytridiomycetes, and Exobasidiomycetes were 0.91%, 0.98%, and 0.23%, respectively, significantly higher than those in NKA (0.47%, 0.28%, and 0.04%). In KA, the average relative abundances of Ramophialophora and Emericellopsis were 2.39% and 1.25%, respectively, significantly higher than those in NKA (0.05% and 0.09%). However, the average relative abundance of Mortierella was 3.04% in KA, which was lower than that in NKA (28.34%). KA contained 32 dominant OTUs, including OTU141, 99, and 192. There was more connectivity between OTU69 (Emericellopsis terricola) and OTU138 (Westerdykella globosa) with the cation exchange capacity (CEC), exchangeable Ca2+, and total phosphorus (TP) in the correlation network. In KA, the average abundances of symbiotroph and pathotroph-saprotroph fungi were 1.29% and 1.50%, respectively, significantly higher than those in NKA (0.08% and 0.09%). The average abundance of the saprotroph-symbiotroph fungi in KA was 10.81%, which was significantly lower than that in NKA (63.69%). In KA, dung saprotroph-wood saprotroph fungi were dominant, with an abundance of 9.73%, whereas in NKA, endophyte-litter saprotroph-soil saprotroph-undefined saprotroph fungi were dominant, with an abundance of 45.93%. The above results suggest that the soil factors of KA, such as CEC, exchangeable Ca2+, and TP, alter the structures and functions of fungi.

[Effect of Land-use on Soil Fungal Community Structure and Associated Functional Group in Huixian Karst Wetland].

Studying the impact of land-use on fungal communities and their functional groups in wetland soil can provide a theoretical basis for the protection of wetlands. The top soil (0-20 cm) samples were collected from the wetlands with Phragmites communis (PCW), wetlands with Cladium chinense (CCW), abandoned paddy fields (APF), paddy fields (PF), and corn fields (CF) in the Huixian Karst Wetland. The fungal community structure and its functional groups were analyzed using high-throughput sequencing methods and the FUNGuild database, respectively. The results showed that the Simpson and Shannon index in PF and CF were significantly higher than those in PCW and CCW. Ascomyceta was the most dominant phylum in five land-use types with the abundance of 70.60%-87.02%, followed by Rozellomycota in PCW with the abundance of 7.14% and Basidiomycota in CCW, APF, PF, and CF with the abundance of 9.70%, 5.19%, 8.13%, and 7.50%, respectively. Pleosporales was the most dominant order in PCW with the abundance of 16.47%, while Hypocreales was the dominant one in CCW, APF, PF, and CF with the abundance of 22.52%, 23.50%, 17.60, and 23.80%, respectively. Ascobolus and Archaeorhizomyces were the most dominant genera in PCW and CCW with the abundance of 6.65% and 13.44%, respectively, and Fusarium was the most dominant genus in APF, PF, and CF with the abundance of 10.22%, 10.51%, and 11.12%, respectively. Saprotroph was the main trophic mode in the Huixian wetland with the abundance of 48.67%-80.13%. The abundance of pathotroph in CF (5.39%) was higher than that in PCW (2.34%) and CCW (1.53%). Dung saprotroph-wood saprotroph and soil saprotroph were the most dominant functional groups in PCW and CCW, respectively, while animal pathogen-endophyte-lichen parasite-plant pathogen-soil saprotroph-wood saprotroph was the most dominant functional group in APF, PF, and CF. Redundancy analysis showed that both soil water content and the ratio of carbon-to-nitrogen were the main factors affecting fungal communities, and available nitrogen was the main factor affecting the functional groups. Overall, the results indicated that land-use has changed the soil fungal diversity and community structure, complicated the functional groups, and increased the risk of corn disease in the Huixian Karst wetland.

[Community Structure of CO2-fixing Soil Bacteria from Different Land Use Types in Karst Areas].

Carbon dioxide (CO2)-fixing bacteria are important microbial communities of the soil carbon cycle. It is important to study their community structure characteristics in karst areas to understand the carbon-sequestration mechanism of the soil ecosystem. The top soil samples of paddy fields, maize fields, and citrus orchards were collected in a karst area, mixed zone, and non-karst area at the Maocun karst experimental site in Guilin.The community abundance, composition, and diversity of CO2-fixing bacteria were analyzed using high-throughput sequencing technology with cbbLR as indicating gene. The results show that most of the CO2-fixing bacteria can only be classified as the shallow taxonomic group including bacteria and actinomycetes. The α-Proteobacteria of Proteobacteria was the dominant class in the three areas. Facultative autotrophic bacteria dominated by rhizobia were the main CO2-fixing bacteria. The abundances of Burkholderiales, Rhodopseudomonas, Azospirillum, Sinorhizobium fredii HH103, Rhizobium leguminosarum bv. trifolii were higher in the karst area than in the other two areas. However, Bradyrhizobium is the dominant species in the mixing and non-karst areas. The redundancy analysis shows that the pH, soil organic carbon, soluble organic carbon, cation exchange capacity, and total nitrogen are the main ecological factors affecting the community structure of CO2-fixing bacteria. The above-mentioned results show that the soil properties in the karst area can significantly affect the community structure of CO2-fixing bacteria.

[Comparison of Soil Bacterial Community Structure Between Paddy Fields and Dry Land in the Huixian Karst Wetland, China].

In order to explore the effect of land-use change on soil bacteria in wetland systems, the topsoil (0-20 cm) of a natural wetland (NW), paddy field (PF), and dry land (DL) were collected in the Huixian karst wetland. The α-diversity, species composition, and abundance of soil bacterial communities were analyzed using high-throughput sequencing. The effect of environmental factors on bacterial community structure was also examined. The results showed that the soil bacteria in the Huixian karst wetland can be divided into 49 phyla and 145 classes. The Shannon index of bacteria in the PF was significantly higher, and the Simpson index of bacteria in the NW is significantly lower, than in the other two land-use types. The dominant phyla (operational taxonomic units, OTUs>1%) in the NW were Proteobacteria (52.15%), Actinobacteria (15.16%), and Acidobacteria (8.80%); the dominant phyla in the PF were Proteobacteria (45.79%), Acidobacteria (17.20%), and Chloroflexi (11.75%); the dominant phyla in the DL were Proteus (51.42%), Acidobacteria (15.51%), and Chloroflexi (7.43%). The dominant classes (OTUs>1%) in the NW were α-Proteobacteria (17.98%), ß-Proteobacteria (13.72%), and Actinobacteria (13.13%); the dominant classes in the PF were Acidobacteria (14.35%), ß-Proteobacteria (13.37%), and δ-Proteobacteria (12.02%); the dominant classes in the DL were α-Proteobacteria (19.44%), Formobacteria (13.30%), and Acidobacteria (13.03%). Among the dominant OTUs (>0.3%), the dominant genera of in the NW were Sphingomonas (OTU2, 59), Micromonospora (OTU5, 24 and 50487), Gemmatimonas (OTU1), and Tenotrophomonas (OTU8); the dominant genera in the PF were Lysobacter (OTU4 and 115) and Aquabacterium (OTU33); the dominant genera in the DL were Sphingomonas (OTU85, 157 and 2916), Rhodanobacter (OTU19 and 52), and Penlobacterium (OTU60). A heatmap showed that there were significant differences in soil bacterial community structure among the three land-use types. Redundancy analysis showed that pH, soil organic carbon (SOC), total nitrogen (TN), alkali-hydrolyzable nitrogen (AN), exchangeable Mg2+, exchangeable Ca2+, soluble organic carbon (DOC), and available phosphorus (AP) were the main factors that affected the bacterial community structure in the Huixian karst wetland. These results indicate that changes in land-use types have significantly shaped the structure of soil bacterial communities in this area.

[Spatial-Temporal Variations of CO2 and CH4 Flux Through a Water-air Interface Under the Effect of Primary Productivity in Wulixia Reservoir].

In recent years, the effect of greenhouse gas has been a focus. In order to reveal the spatial-temporal variations of CO2 and CH4 flux through a water-air interface, Wulixia reservoir was selected as a typical case for measuring CO2and CH4 flux using a floating chamber during four study periods in 2016.The primary productivity of the water was also studied. The results indicate that surface-layer water in Wulixia reservoir is a CO2 sink in summer, varying between -30.14 and -3.47 mg·(m2·h)-1. However, it is the source of CO2 in autumn and winter, varying between 15.57 mg·(m2·h)-1 and 115.06 mg·(m2·h)-1. The variation of methane flux is obvious in summer, but it is stable in autumn and winter. The spatial distribution of CO2 and CH4 show that they are higher in the typical bay area and lower in the tailing dam as well as in the drawdown zone of the reservoir. In addition, the spatial and temporal variation of CO2 and CH4 are negatively and positively correlated with primary productivity (r -0.477 and 0.771), respectively. Due to the high activity of photosynthetic micro- organisms in summer, CO2 can be sequestered, allowing the surface-layer water in Wulixia reservoir to be a CO2 sink. In addition, methanogenic bacteria can oxidize organic carbon produced by photosynthetic micro-organisms. Therefore, CH4 has a negative correlation with primary productivity. The results provide important information for the study of microorganism functions in karst water systems.

[Water Chemistry and Characteristics of Dissolved Organic Carbon During the Wet Season in Wulixia Reservoir, SW China].

In recent years, scientists have focused on the karst carbon cycle. To better understand the hydrochemical characteristics and the physical-chemical properties of DOC in the wet season in karst areas, the water chemistry and DOC distribution characteristics in Wulixia reservoir were analyzed. The molecular weight of the water organic matter was analyzed based on the UV absorption spectrum. The results showed that the water chemistry of Wulixia reservoir was HCO3-Ca ·Mg, the nutrient status of the water body was of the medium nutrient type, and the water maintained a good quality. The DOC mass concentration was lower in the wet season than in the dry season, and DOC was the main component of TOC. The DOC mass concentration tended to decrease from the surface to the bottom in a vertical direction. Chl-a and DIC were the main factors affecting the DOC vertical distribution. The organic matter in the reservoir area related to the absorption spectrum parameters of S275~295, M, SUVA254, and E253/E203 showed that the water was dominated by constituents with small molecular weight, a high proportion of fulvic acid, low proportion of humic acid, and weak aroma. The results showed that the organic matter in the reservoir area was readily consumed by microbial bioactivity and that it played an active role in the carbon cycle of the reservoir. It also showed that the DOC endogenous characteristics of the reservoir were strong and provided a sufficient carbon source for heterotrophic microbes.

[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.

[Distinguishing the Properties and Sources of the Dissolved Organic Matter in Karst Reservoir Water During Winter Using Three-Dimensional Fluorescence Spectrum Technology:A Case Study in Wulixia Reservoir of Guangxi Province].

In recent years, the fluorescence spectrum technology has been widely applied to study the physical and chemical properties of dissolved organic matter (DOM) in natural water. In order to reveal the properties of the dissolved organic matter in reservoir water, the three-dimensional fluorescence spectrum and the parallel factor analysis were used to determine the properties of DOM and its sources in the Wulixia reservoir during the winter of 2016. Moreover, the chemical characteristics of water and the relative proportion of DOM fluorescent components were estimated. Results show that:① Ca2+ and HCO3- are the main cations and anions in the Wulixia reservoir during winter. The water is mainly controlled by the carbonate balance system and dissolved organic carbon (DOC) accounted for about 91% of total organic carbon (TOC). ③ DOM from the Wulixia reservoir can be divided into three components:fulvic acid, humic acid, and tryptophan. ③ The parameters, fluorescence index, humification index, biological index, and freshness index show the existence of DOM in the Wulixia reservoir. Microorganisms have a significant effect on DOM composition.

[Comparisons of Microbial Numbers, Biomasses and Soil Enzyme Activities Between Paddy Field and Drvland Origins in Karst Cave Wetland].

The purpose of this study is to compare microbial number, microbial biomass as well as soil enzyme activity between paddy field and dryland originated karst wetland ecosystems. The soil samples (0-20 cm) of uncultivated wetland, paddy field and dryland were collected in Huixian karst cave wetland, Guilin, China. Microbial numbers and biomass were detected using dilute plate incubation counting and chloroform fumigation-extraction, respectively. Microbial DNA was extracted according to the manufacturer's instructions of the kit. Microbial activity was examined using soil enzyme assays as well. The result showed that the bacteria number in paddy filed was (4.36 +/- 2.25) x 10(7) CFU x g(-1), which was significantly higher than those in wetland and dryland. Fungi numbers were (6.41 +/- 2.16) x 10(4) CFU x g(-1) in rice paddy and (6.52 +/- 1.55) x 10(4) CFU x g(-1) in wetland, which were higher than that in dryland. Actinomycetes number was (2.65 +/- 0.72) x 10(6) CFU x g(-1) in dryland, which was higher than that in wetland. Microbial DNA concentration in rice paddy was (11.92 +/- 3.69) microg x g(-1), which was higher than that in dryland. Invertase activity was (66.87 +/- 18.61) mg x (g x 24 h)(-1) in rice paddy and alkaline phosphatase activity was (2.07 +/- 0.99) mg x (g x 2 h)(-1) in wetland, both of which were higher than those in dryland. Statistical analysis showed there was a significant positive correlation of microbial DNA content, alkaline phosphatase activity and microbial carbon with soil pH, soil organic carbon (SOC), total nitrogen, alkali-hydrolyzable nitrogen, soil moisture, exchangeable Ca2+ and exchangeable Mg2+, as well as a significant positive correlation of intervase activity with the former three microbial factors. The above results indicated that microbial biomass and function responded much more sensitively to land-use change than microbial number in karst cave wetland system. Soil moisture, SOC and some factors induced by land-use change could affect mainly microbiological characteristics. We suggest that rice paddy, a kind of constructed wetland, should be kept and protected in experimental area or buffer area of Huixian karst cave wetland in the light of its similar property with the natural wetland.

[Effects of Bisphenol A on Characteristics of Paddy Soil Microbial Community Under Different Cultural Conditions].

Bisphenol A (BPA) is widely known as a typical synthetic environmental hormone. Effects of BPA concentrations and aerations on soil microbial communities were rarely reported. This paper presented the studies on effects of various concentrations of BPA (0, 0.25 mg·kg-1, 0.50 mg·kg-1, 1.00 mg·kg-1, 2.00 mg·kg-1) and soil aertaiton (aerobic and anaerobic) on characteristics of paddy soil microbial communites by technology of qPCR (fluorescence quantitative PCR) and PCR-DGGE (polymerase chain reaction-denaturing gradient gel electrophoresis). The results lined out that: ① the microbial abundance index was significantly different among different BPA concentrations under the same condition of soil aeration (anaerobic or aerobic). However, the index of microbial evenness, Shannon-Wiener-diversity and evenness-indices were insignificantly different under these conditions. At a concentration of 0.50 mg·kg-1, the microbial abundance index reached a maximum value under anaerobic conditions; while under aerobic conditions the opposite result was found, the microbial abundance index dropped to a minimum value. ② The two-way analysis of variance (ANOVA) showed that: the concentration of BPA, soil aeration and their interaction significantly affected the abundance of bacteria, whereas the abundance of fungi was only affected by soil aeration. The study results showed that: the abundance index was a sensitive indicator for the variation of soil microbial diversity; it was a critical value for the change of soil microbial abundance when the BPA concentration was 0.50 mg·kg-1; as for the abundance of fungi, the response of bacteria abundance was more sensitive to BPA and soil aeration conditions.

[Microbial community abundance and diversity in typical karst ecosystem to indicate soil carbon cycle].

The soil microbial characteristics were detected to clarify their indications in organic carbon cycle in karst system. Soil samples from three karst types (saddle, depression and slop) at 0-10 cm, 10-20 cm and 20-30 cm layers were collected in the Yaji Karst Experimental Site, a typical karst ecosystem. The microbial diversity and abundance were assayed using polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) and fluorescence quantitative PCR. The data showed that the highest abundance of 16S rRNA and 18S rRNA were in depression with 1.32 x 10(11) copies x g(-1) and in saddle with 1.12 x 10(10) copies x g(-1), respectively. The abundance of 16S rRNA in saddle and depression decreased from top to bottom, while that of 18S rRNA in three karst forms decreased, which showed that the abundance changed consistently with soil organic carbon (SOC). The 3 diversity indices of 16S rRNA and 6 diversity indices of 18S rRNA increased from top to bottom in soil profiles of three karst forms. These results showed that microbial diversity changed conversely with the abundance and SOC in soil profile. It can be concluded that the abundance was more important than the diversity index for soil carbon cycle in karst system.

[Spatial difference of microbial community structure and enzyme activity in constructed wetlands receiving wastewater containing heavy metals].

The microbial community structure and enzyme activity were investigated in the three-stage wavy subsurface constructed wetland (W-SFCW) receiving wastewater containing heavy metals (Cr, Cu and Ni). Colony forming units (CFU) abundance of microbes was calculated by using plate culture. The microbial community structure was investigated by using PCR-DGGE and sequencing. Enzyme activity was detected by using colorimetry. The results showed that the CFU of bacteria ranged from (4.10 +/- 0.72) x 10(6) to (1.61 +/- 0.10) x 10(7), fungi ranged from (7.21 +/- 1.60) x 10(4) to (1.29 +/- 0.02) x 10(5) and actinomycetes ranged from (1.41 +/- 0.27) x 10(6) to (3.38 +/- 0.11) x 10(6). All of 8 bands retrieved from DGGE gels were unique 16S rRNA or 18S rRNA sequences and revealed that the microbial community structures had changed spatially. Maximum urease [(0.67 +/- 0.2) mg/(g x 24 h)], invertase [(40.15 +/- 0.14) mg/(g x 24 h)] and alkaline phosphatase [(1.03 +/- 0.16) mg/(g x 2 h)] activities were at first stage, third stage and first stage, respectively. A negative correlation between fungi CFU abundance and the total content of Cr, available Cr content and activation rate of Cr was detected, a negative correlation was also found between the invertase activity and total Cu and Ni contents and the Ni activation rate.