[Effect of Land Use on the Stability of Soil Organic Carbon in a Karst Region].

The composition of soil organic carbon and its stability mechanism are the key to understanding the terrestrial carbon sink capacity. The stability of soil organic carbon in a karst ecosystem greatly affects the soil carbon fixation capacity. In order to understand the impact of human activities on the stability of soil organic carbon in karst areas, the karst valley area of Zhongliang Mountain in Chongqing was selected as an example, and soil samples of four typical land use modes (mixed forest, bamboo forest, grassland, and cultivated land) were collected in layers to analyze the total organic carbon (TOC) and heavy fraction organic carbon (HFOC). The distribution characteristics of light fraction organic carbon (LFOC), labile organic carbon (LOC), and recalcitrant organic carbon (ROC) were analyzed quantitatively by using a structural equation model to provide basic data for soil carbon sink assessment and soil quality protection in karst areas. The results showed that the organic carbon components under different land use patterns in karst areas had obvious surface accumulation, and the content of organic carbon components in the surface layer was 1.2 times that in the bottom layer. Except for LFOC, the content of other organic carbon components was the highest in the mixed forest, followed by that in the bamboo forest and wasteland, with the lowest in cultivated land. Mixed forest ω(TOC) content was the highest, 42.5 g·kg-1, followed by that of bamboo forest (36.6 g·kg-1) and grassland (18.7 g·kg-1), and cultivated land content was the lowest, 13.4 g·kg-1. The soil organic carbon content of cultivated land was 68.5%, 63.5%, and 28.3% lower than that of mixed forest, bamboo forest, and grassland, respectively. Mixed forest had the highest content of ω(HFOC), 21 g·kg-1, followed by those of bamboo forest (20.9 g·kg-1), grassland (18.2 g·kg-1), and cultivated land (13.5 g·kg-1). The mixed forest ω(LOC) content was the highest, 16.3 g·kg-1, followed by those of bamboo forest (14.9 g·kg-1), grassland (11.5 g·kg-1), and cultivated land (5.3 g·kg-1). Mixed forest ω (ROC) content was the highest, 25.7 g·kg-1, followed by those of bamboo forest (21.6 g·kg-1), grassland (15.9 g·kg-1), and cultivated land (10.3 g·kg-1). The bamboo forest land ω(LFOC) content was 15.9 g·kg-1, followed by those of mixed forest (13.9 g·kg-1), grassland (7.3 g·kg-1), and cultivated land (4.9 g·kg-1). The recalcitrant organic carbon index (ROCI) was used to indicate the stability of soil organic carbon. The variation range of ROCI was 33.9%-64.5%, of which the highest was mixed forest (64.5%-66.3%), and the lowest was cultivated land (33.8%-39.6%). The ROCI of mixed forest, bamboo forest, and grassland were 1.8 times, 1.6 times, and 1.4 times that of cultivated land, respectively. Karst area ω (inert organic carbon) content and ROCI showed that human agricultural activities caused the reduction in soil organic carbon content and the destruction of soil physical structure, resulting in the accelerated decomposition and turnover rate of soil organic matter. The most important factor affecting soil stability in karst areas was soil pH. Tillage activities caused soil pH to rise, reduced soil microbial activity, and were not conducive to the accumulation of the inert organic carbon and soil organic carbon pool in the soil.

[Effects of Photosynthesis of Submerged Aquatic Plants on CDOM in a Karst Water System: A Case Study from Xueyu Cave, Chongqing, China].

Dissolved organic matter (DOM) in karst water is one of the most important carbon sink components, whose origins, distributions, and transport processes are of significance to carbon sink studies. Chromophoric dissolved organic matter (CDOM) can be utilized to express the composition and structural properties of DOM. In this paper, water samples were collected monthly from Xueyu Cave in a karst underground river from both inside and outside the cave. The changes in hydrochemistry and CDOM spectral characteristics as well as the influencing factors for CDOM changes were studied, with the aim of providing experimental support for carbon sink fluxes in karst processes. The main results were as follows. ① The dominant type of CDOM in Xueyu Cave karst water consisted of small molecular weight organic matter that was autochthonously derived and easily bio-degraded, and the tryptophan-like and tyrosine-like components accounted for more than 60% of the total CDOM. ② The most effective influencing factor inside the cave was microbes, which degraded the small molecular DOM in groundwater and induced slight decreases in the TOC and DOC concentrations; they also increased the humic index (HIX). ③ However, the dominant factor outside the cave was submerged aquatic plant photosynthesis, which induced significant increases in the TOC and DOC concentrations and autochthonous small molecular weight DOM contents.

[Impact of Tourism on Bacterial Communities of Karst Underground River: A Case Study from Two Caves in Fengdu, Chongqing].

In this research, the bacterial community compositions of underground water in a tourist and pristine cave were studied. Xueyu Cave and Shuiming Cave are tourist and pristine caves, respectively, in the same karst cave system located in Chongqing, southwest China. To understand the impact of tourism on bacterial community compositions in underground water that flows through the caves, filtered materials from water were collected, and 16S rDNA gene sequences were obtained by high-throughput sequencing. The Shuiming Cave (the pristine cave) had less diversity than Xueyu Cave (the tourist cave) based on the Shannon's diversity index according to Illumina operational taxonomic units (OTUs). Proteobacteria, represented mostly by γ-Proteobacterium and Bacteroidetes, dominated both systems. OTUs from Shuiming Cave were dominated by 38% Proteobacteria, 24% Chlorobi, and 19% Bacteroidetes. In the Xueyu Cave, OTUs from upstream samples were comprised of 62% Proteobacteria but comprised 64% in the downstream samples. In the Xueyu Cave, Bacteroidetes accounted for 11% of the total OTUs in the upstream sample and 16% in the downstream. Among the γ-Proteobacterium and Bacteroidetes, Acinetobacter, Pseudomonas spp., and Flavobacteriaceae, which are related to potentially pathogenic species, were prevalent in the Xueyu Cave, while Methylococcaceae-uncultured, Methylomonas spp., and Methylobacter, all methane-oxidizing bacteria, had high relative abundances in the Shuiming Cave. These results revealed that potentially more pathogenic bacteria are present in the stream waters from the tourist cave, which has important implications for the protection of tourist caves. The RDA analysis of the environmental factor and bacteria community in groundwater showed that the distribution of pathogenic bacteria was positively correlated with the cave air CO2, and the Spearman correlation analysis of the two environmental factors indicated that the influence of the number of tourists on the structure of the bacterial community in the groundwater was more obvious and led to the disappearance of a large number of native bacteria. We proposed that tourist caves control the number of daily tourists and that they enter in batches and increase the import and export of closed devices to avoid the cave air exchange inside and outside. In addition, it was recommended that they increase the import and export of sterilization devices to reduce tourists with bacteria and organic matter, and avoid leaving garbage in the hole to avoid cave microbial exchange inside and outside. A reduction in the fixed lighting inside caves should be required to reduce long exposure, since the tourists can bring individual source lighting.

[Distinguishing the Compositions and Sources of the Chromophoric Dissolved Organic Matter in a Typical Karst River During the Dry Season: A Case Study in Bitan River, Jinfo Mountain].

Since resistant dissolved organic matter (RDOM) plays a critically important role in a karst carbon sink, one of the most important continental carbon sinks, research focusing on the origination, transportation, and translation of RDOM in a karst water system is important. Currently, 3D-fluorescence EEMs are used to detect the composition and origination of chromophoric dissolved organic matter (CDOM), an important part of RDOM. This is a very fast and efficient method for CDOM analysis. In this study, 3D-fluorencence EEMs combined with UV-visible absorption spectrum were used to analyze the composition and origination of CDOM in the Bitan River at Jinfo Mountain. Samples were collected from nine sampling sites from January to March 2017 and analyzed with CDOM EEMs and UV-visible absorption spectrums. In addition hydrochemical characteristics were determined and then samples were stimulated with PARAFAC to detect the chromophoric fluorescent groups and indexes. The PARAFAC stimulation revealed three chromophoric fluorescent groups in which fulvic acid was the largest component, accounting for about 44%, with a humic acid content of about 32% and tyrosine-like acid content of about 24%. Four indexes: FI, BIX, HIX, and ß∶α, were calculated, and the mean values were 2.06, 0.87, 4.35 and 0.69, which showed relatively high FI, BIX, and ß∶α values and a low HIX value, implying that the CDOM was autochthonous and originated from microbes and aquatic plants in the dry season. The spatial dynamic of the index revealed an increased BIX and decreased HIX from the upstream area to the downstream area, implying the impact of land-use and human activities. The forest soil input more humic acid and agriculture input more N and P resulting in flourishing aquatic plants and microbes. Moreover, the correlation coefficients of DIC and humic acid, tyrosine-like acid were 0.515 (P<0.05) and 0.644 (P<0.01), from which it could be inferred that DIC contributed to CDOM formation. The conclusions of this study revealed that DIC would be fixed by karst water aquatic plants and microbes and then sink as autochthonous CDOM and become part of karst water carbon sink.

[Tracing the Fecal Contamination Sources Based on Bacteroides 16S rRNA PCR- DGGE in Karst Groundwater: Taking Laolongdong Underground River System, Nanshan, Chongqing as an Example].

Microbial contamination in karst groundwater continually increases and tracing the source researches has become a hot topic for international researchers. In this study, Laolongdong underground river at Nanshan, Chongqing was chosen as an example to adopt filter membrane methods to monitor the fecal microbial contaminations including the total bacterial concentration (TB), the total E. coli concentration (TE), the total fecal coliform (FC) and the total fecal Streptocoocci (FS). Bacteriodes was used as an indicator and PCR-DGGE analysis was used to trace fecal contamination sources in karst groundwater. The results suggested that groundwater in this area was seriously polluted by microbes from feces. The concentrations of microbial parameters exceeded limited levels greatly and the total bacterial amounts ranged 10-2.9 x 107 CFU · mL⁻¹, the concentrations of E. coli were between 4.3-4.0 x 105 CFU · mL⁻¹, the max concentration of FC was 1.1 x 106 CFU · (100 mL)⁻¹ and the max concentration of FS was 1.1 x 105 CFU · (100 mL)⁻¹. The FC/FS ratios were mostly over 2 which suggested that the main fecal source in groundwater was human feces. In addition, PCR-DGGE contrastive analysis of Bacteroides communities showed that the similarities between groundwater samples and human feces were in range of 7. 1% -69. 1% , and the similarity of the groundwater sample from Laolongdong underground river outlet was 69.1% . Bacteroides community similarities between groundwater samples and swine feces were in range of 1.1%-53.4%, and the similarity of Laolongdong underground river outlet was merely 1.5%. The similarity data implied that groundwater contamination resulted mainly from human feces, swine feces contamination composed part of animals' fecal contamination, and other animals' feces participated too. Furthermore, sequencing results of PCR-DGGE products revealed that most Bacteroides in groundwater originated from human intestinal tract and human feces.

[Characteristics and Transport Patterns of Ammonia, Nitrites, Nitrates and Inorganic Nitrogen Flux at Epikarst Springs and a Subterranean Stream in Nanshan, Chongqing].

In a karst groundwater system, it develops complex multiple flows because of its special geological structure and unique physical patterns of aquifers. In order to investigate the characteristics and transport patterns of ammonia, nitrite and nitrate in epikarst water and subterranean stream, the water samples were collected monthly in a fast-urbanizing karst region. The results showed distinctive characteristics of three forms of inorganic nitrogen. The concentration of inorganic nitrogen was stable in the epikarst water while it was fluctuant in the subterranean stream. Epikarst water was less affected by rainfall and sewage compared with subterranean stream. In epikarst water, the nitrate concentration was much higher than the ammonia concentration. Dissolved inorganic nitrogen, mainly from non-point source pollution related to agricultural activities, passed in and out of the epikarst water based on a series of physical; chemical and biological processes in the epikarst zone, such as ammonification, adsorption and nitrification. On the contrary, subterranean stream showed a result of NH4⁺-N > NO3⁻-N in dry seasons and NO3⁻-N > NH4⁺-N in rainy seasons. This can be due to the fact that sanitary and industrial sewage flowed into subterranean river through sinkholes, fissures and grikes in dry season. Dissolved inorganic nitrogen in subterranean river was mainly from the non-point source pollution in wet season. Non-point source pollutants entered into subterranean water by two transport ways, one by penetration along with vadose flow through fissures and grikes, and the other by conduit flow through sinkholes from the surface runoff, soil water flow and epikarst flow. The export flux of DIN was 56.05 kg · (hm² · a)⁻¹, and NH4⁺-N and NO3⁻-N accounted for 46.03% and 52.51%, respectively. The contributions of point-source pollution and non point-source pollution to the export flux of DIN were 25.08% and 74.92%, respectively, based on run-off division method.

[Hydrogeochemical characteristics of a typical karst groundwater system in Chongqing].

The two-year hydrologic process, hydrochemistry, and a portion of deltaD, delta18O of both the surface water at the inlet and the groundwater at the outlet, were investigated to identify the spatial and temporal variations of hydrogeochemistry in the Qingmuguan karst groundwater system. Research results show that there are wet and dry periods in the groundwater system owing to the striking influence of seasonal rainfall. The evolution of the chemical compositions in the groundwater is significantly influenced by the water and rock interaction, anthropogenic activities and rainwater dilution. The variations of the chemical compositions in the groundwater exhibit obvious spatiality and temporality. The deltaD and delta18O of the surface water beneath the local Meteoric Water Line of Chonqing indicate that the surface water is strongly evaporated. Furthermore, the deltaD and delta18O of the surface water are more positive in the dry period than in the wet period, showing a distinct seasonal effect. The deltaD and delta18O of the groundwater are quite stable and much negative compared with those of the surface water, which suggests that the rainwater recharge the groundwater via two pathways, one directly through sinkholes and the other via the vadose zone.

[Nitrate storage and transport within a typical karst aquifer system in the paralleled ridge-valley of east Sichuan].

In order to investigate the nitrate storage and transport in the karst aquifer system, the hydrochemical dynamics of Qingmuguan underground river system was monitored online by achieving high-resolution data during storm events and monthly data in normal weather. The principal component analysis was employed to analyze the karst water geochemistry. Results showed that nitrate in Jiangjia spring did not share the same source with soluble iron, manganese and aluminum, and exhibited different geochemical behaviors. Nitrate was derived from land surface and infiltrated together with soil water, which was mainly stored in fissure, pore and solution crack of karst unsaturated zone, whereas soluble iron, manganese and aluminum were derived from soil erosion and directly recharged the underground river through sinkholes and shafts. Nitrate transport in the karst aquifer system could be ideally divided into three phases, including input storage, fast output and re-inputting storage. Under similar external conditions, the karstification intensity of vadose zone was the key factor to determine the dynamics of nitrate concentrations in the groundwater during storm events. Nitrate stored in the karst vadose zone was easily released, which would impair the aquatic ecosystem and pose seriously threats to the local health. Thus, to strengthen the management of ecological system, changing the land-use patterns and scientifically applying fertilizer could effectively make a contribution to controlling mass nutrient input from the surface.

[Transportation and sources of the suspended particle in a karst spring during a storm event].

Storm periods are the crucial stage to reveal input and outlet of material and energy in groundwater system. Jiangjia spring, the outlet of Qingmuguan groundwater system, was taken as an example. Distribution of suspended particle in the Jiangjia spring was continuously monitored. Supported by hydrochemical data, characteristics and sources of the suspended particle in the karst groundwater system were investigated. Results show that the number of suspended particle in size of 0-11 microm abruptly increased in the channel of Jiangjia spring at the beginning of rainfall occurred. It indicates the suspended particle was derived from allochthonous material. While the groundwater was recharged by karst fissure, the number of suspended particle in size of 0-11 microm sharply raised, which shows that the suspended particle was autochthonous. Finally, the number of suspended particle in size of 0-4 microm elevated due to the entire groundwater watershed recharged by rainfall and dilution effect occurring again, suggesting the suspended particle was derived from allochthonous material. Owing to the intrinsic hydrogeological settings and rainfall intensity, the threshold of size for the suspended particle was 4 pm in Qingmuguan groundwater system. It probably was a high practical significance to indicate the groundwater was whether polluted by microbe or not.

[High-resolution research on the NO3- changes of karst groundwater and its responses to the outside environmental variations].

In order to thoroughly reveal the karst groundwater system in response to the external environment, multi-index high-resolution auto-monitoring instruments were used to research the hydrogeochemistry variations of Qingmuguan subterranean stream. The monitoring indicators were pH value, electrical conductivity (EC), water level, rainfall and NO3-. In the observed 6 times rainfall events, the pH value was mainly controlled by acid rain and EC was impacted by the rain chemistry, physical dilution effect of rainfall and agricultural wastewater. NO3- derived from agricultural activities was less impacted by rain chemistry; and its variations were mainly affected by physical dilution effect of rainfall and agricultural wastewater. Under the influences of the R1 rainfall, the rapid changes of EC and NO3- were contrary in generally, but pH value decreased due to the acid rain. In the R2, R3, R4 and R6 rain events, the water level was positively response to the rainfall and the pH value decreased due to the acid rain; however both of the EC and NO3- decreased due to the dilution effect. With the afflux of agricultural wastewater, both of the EC and NO3- were rapidly changed in synchronization within the shortest of 5 h of and the longest of only 27 h and the pH value showed accelerated decline. During the R5 torrential rain events, water level abruptly increased and all of the pH value, EC and NO3- sharply decreased as a result of dilution effect. The groundwater quality was changed because the afflux of agricultural wastewaters in the entire rainfall periods. According to the National Groundwater Quality Standard, P. R. China (GB/T 14848-9), the groundwater quality of the QSS showed the following order in the entire rainfall events: Grade III --> Grade IV --> Grade V --> Grade 1V --> Grade V --> Grade LV --> Grade V --> Grade IV --> Grade III. As a important drinking water source around local inhabitants, the groundwater quality of Qingmuguan subterranean stream was deteriorated to Grade V in rain-fertilize periods. However, the sampling methods in field did show a simple variation curve. Consequently,the results from the research illustrate the need to redesign hydrochemical sampling methods for karat hydrology system, especially under the impact of human activities.

[Substances transport in an underground river of typical karst watershed during storm events].

Hydrologic process, turbidity, suspended particles matters (SPM), major cations and TOC concentrations during two storm events in late April 2008 were monitored at Jiangjia Spring which is the outlet of Qingmu Guan underground river system. Scanning electron microscopy (SEM) and energy disperse spectroscopy (EDS) analyses of SPM were also performed in order to investigate the transport characteristics of substances, such as SPM, turbidity and major cations in the underground river of typical karst watershed. The results show that at a single and well-developed karst conduit of Jiangjia Spring, discharge, turbidity, and concentrations of SPM, major cations and TOC respond promptly to the rainfall. The carbonate-derived cations including Ca2+, Mg2+ and Sr2+ are subject to dilution effect during the rising limb of discharge. The elevation in turbidity and SPM concentration is a result of the gradual increase of allochthonous substances (soil) flux input from the surface. Al3+, Fe, Mn, Ba2+ and TOC are concomitant substances of SPM. And their concentrations are ascending with turbid rise. The flux of SPM in diameter > 0.45 microm in the underground river is about 9.7 tons during the events. The bad water quality suggests us that the spring water is unfit to drink without purification during the period of rising and recession time of discharge at Jiangjia Spring. Thus, soil erosion and nutrient losing not only strongly destroy the fragile karst ecological environment, but also lead to non-point source pollution, and seriously threaten the drinking water safety of locals.

[Temporal and spatial variations of the nitrate-nitrogen sources in an underground river using 15N isotope technique].

Groundwater in Qingmuguan underground river was monitored using hydro-chemical and 15N isotope techniques to investigate temporal and spatial variations of nitrate-nitrogen and its possible sources from October, 2007 to October, 2008. The results show that nitrate concentrations are 3.20 mg/L of the inlet (D1) and 20.35 mg/L of the outlet (S2) of the underground river. Affected by the fertilizers in agricultural field and the rainfall flush and dilution,nitrate concentrations of groundwater are higher but less stable from April to July, 2008 than that from October, 2007 to March, 2008. They are elevated during August and September, 2008 due to the residual fertilizers and decreased rainfall events. According to the NO3- -delta 15N values of -0.857% per hundred +/- 2.01% per hundred (n=9), the sources of nitrate-nitrogen of D1 are dominated by residual fertilizers from the paddy fields from October, 2007 to March, 2008 and from July to October, 2008, while the NO3- -delta 15N values of 2.50% per hundred +/- 0.29% per hundred (n=3) demonstrate that the mixture of soil organic nitrogen and fertilizers are the sources from April to middle and late June, 2008. And the NO3- -delta 15N values of -3.74% per hundred in late May and 0.52% per hundred in early June indicate that the nitrate-nitrogen comes from fertilizers applied in the paddy fields in 2008. The nitrate-nitrogen of S2 is partly from fertilizers in the upper basin, and also from fertilizers and soil organic nitrogen carried by lateral fissure and soil permeation water of forest and farm land in the middle and lower basin. The NO3- -delta 15N values of 4.77% per hundred (n=9) show nitrate-nitrogen of S2 mainly originates from fertilizers from October, 2007 to March, 2008 and from July to October, 2008, while NO3- -delta 15N values of 3.16% per hundred +/- 0.39% per hundred (n=5) explain that the nitrate-nitrogen derives from the mixture of soil organic nitrogen and fertilizers from April to June, 2008.