Optical properties and molecular compositions of dissolved organic matter in multiple runoff components during rainfalls on the karst hillslope.

Understanding the chemical composition, origin, and molecular structure of dissolved organic matter (DOM) in multi-interface runoff is essential for comprehending the fate of laterally transported DOM in complex soil-epikarst systems of karst hillslopes. Limited information, however, is available for the optical properties and molecular compositions of the transported OM in multiple runoff components on the karst hillslope in relation to land-uses and soil thicknesses. In this study, we conducted a study to observe the changes in the quantity and quality of DOM in multiple interface flow (surface, subsurface, and epikarst) during natural rainfall events in 2022 in karst hillslopes that are covered by different land uses (cropland and shrubland) and soil thicknesses (with mean depths of 66.0 cm for deeper soil and 35.4 cm for shallower soil) in the karst region of southwest China. chemcial compositions of runoff DOM were determined by optical analysis and microbial compositions in runoff were inferred with high-throughput sequencing. The results showed that the soil-epikarst structure was controlling the runoff DOM quantity and quality during rainfall events. A decrease in the aromaticity, humification, unsaturation, and oxidation degree and an increase in carbohydrate, aminosugars, protein, and lipid compounds were found from surface to epikarst flow, indicating that plant-and soil-derived carbon decreased, while the microbially-derived carbon increased. The results were further comfirmed by the higher bacterial richness and diversity, along with fungal diversity in the epikarst flow compared to other runoff components. The bio-labile protein materials (C2) were the most important component of runoff DOM output in karst hillslopes. In surface and subsurface flow, rainfall amount, runoff rate, and discharge significantly affected the DOM concentration and quality during rainfalls, indicating that the dynamics of DOM in runoff from karst hillslopes were predominantly influenced by hydrological processes. Furthermore, the runoff DOM quality in cropland was dominated by lower unsaturation and oxidation degrees and higher protein component, compared to those in shrubland. The compositions of DOM in runoff from hillslope plots with thicker soils were primarily characterized by microbially-derived materials. Our findings were conducive to understanding the mechanism governing the migration of DOM quality and quantity in discharge during multi-interface hydrological processes on karst hillslopes.

[Research progress on structure and hydrological processes in the karst critical zone of southwest China]. / è¥¿å—å–€æ–¯ç‰¹å ³é”®å¸¦ç»“æž„åŠå ¶æ°´æ–‡è¿‡ç¨‹ç ”ç©¶è¿›å±•.

The southwestern region of China is the largest exposed karst area in the world and serves as an important ecological security barrier for the upstream of Yangtze River and Pearl River. Different from the critical zone of non-karst areas, the epikarst, formed by an interwoven network of denudation pores, is the core area of karst critical zone. Water is the most active component that participates in internal material cycle and energy flow within the critical zone. We reviewed relevant research conducted in the southwestern region from three aspects: the characte-rization of critical zone structure, the hydrological processes of soil-epikarst system, and their model simulations. We further proposed potential research hotpots. The main approach involved multi-scale and multi-method integrated observations, as well as interdisciplinary collaboration. Precisely characterizing the eco-hydrological processes of the vegetation-soil-epikarst coupling system was a new trend in the future research. This review would provide scientific reference for further studies on hydrological processes in critical zones and regional hydrological water resource management in karst areas.

Epikarst water detection using integrated geophysical methods.

When detecting epikarst water using the self-potential method, the actual location of the anomaly center often deviates from the prospecting result due to the interference of the regional background field, which is comprised of geological noise and artificial electromagnetic fields. Ultimately, this makes it difficult to locate the detection target accurately. To address the potential offset of the anomaly center location, in this study we introduce the differential filtering method into the data processing procedure. This method has smoothing and low-pass filtering effects, facilitating the extraction of meaningful anomalies. Meanwhile, based on the anomalous features of different physical parameters, we propose an integrated method system based on differentially filtered horizontal self-potential gradient data, the composite profile method, and the high-density electrical method, which can effectively improve the accuracy of anomaly localization. This newly established method system was applied at the Xiaguantun test site in Longzhou County, Chongzuo, Guangxi Province, China, and its effectiveness and feasibility was confirmed.

Surface-subsurface hydrological processes of rainwater harvesting project in karst mountainous areas indicated by stable hydrogen and oxygen isotopes.

Rainwater harvesting (RWH) projects in a decentralized way are significant measures to deal with the water scarcity dilemma in rural areas of the karst mountains in Southwest China at present. Due to the differences in cistern construction features and geomorphological positions, the water sources of cisterns were characterized by marked spatial variability, and the recharge stability of cisterns was strongly influenced by precipitation seasonality. Nevertheless, in hydrological processes on karst hillsides, the identification of different runoff types of RWH has not been sufficiently studied. The stable isotopes of hydrogen and oxygen of eleven cisterns and epikarst springs in subtropic cockpit karst landforms were monitored from 2020 to 2021 to investigate the runoff characteristics in RWH. Evaporative fractionation in different hydrological cycles is the predominant factor regulating the stable isotopic signature of cistern water. The results indicated that the typical roles that occurred in the recharge process contributed differently to water harvesting, with surface runoff (SR) and subsurface runoff (SSR) contributing much more than rainwater (RW) and epikarst runoff (ER). Three mixing patterns were proposed by end-member analysis in which SR + SSR, ER, and RW were three end members with indicators of isotopic value and the total dissolved solids (TDS). The recharge of SR + SSR was the predominated source, which contributed to 64% of the total water resources collected through RWH in the rainy season. In addition, the influence of various runoffs on the recharge stability of the cistern can be reflected by the multiple statistical analysis of isotopic fluctuation. Poor recharge stability is caused by excessive SR + SSR, whereas a higher percentage of ER and RW leads to better recharge stability. The applied method of hydrological process analysis is significant to the cistern water resources management in rural areas of the karst mountains.

Longitudinal metabarcode analysis of karst bacterioplankton microbiomes provide evidence of epikarst to cave transport and community succession.

Caves are often assumed to be static environments separated from weather changes experienced on the surface. The high humidity and stability of these subterranean environments make them attractive to many different organisms including microbes such as bacteria and protists. Cave waters generally originate from the surface, may be filtered by overlying soils, can accumulate in interstitial epikarst zones underground, and emerge in caves as streams, pools and droplets on speleothems. Water movement is the primary architect of karst caves, and depending on the hydrologic connectivity between surface and subsurface, is the most likely medium for the introduction of microbes to caves. Recently published metabarcoding surveys of karst cave soils and speleothems have suggested that the vast majority of bacteria residing in these habitats do not occur on the surface, calling into question the role of microbial transport by surface waters. The purpose of this study was to use metabarcoding to monitor the aquatic prokaryotic microbiome of a cave for 1 year, conduct longitudinal analyses of the cave's aquatic bacterioplankton, and compare it to nearby surface water. Water samples were collected from two locations inside Panel Cave in Natural Tunnel State Park in Duffield, VA and two locations outside of the cave. Of the two cave locations, one was fed by groundwater and drip water and the other by infiltrating surface water. A total of 1,854 distinct prokaryotic ASVs were detected from cave samples and 245 (13.1%) were not found in surface samples. PCo analysis demonstrated a marginal delineation between two cave sample sites and between cave and surface microbiomes suggesting the aquatic bacterioplankton in a karst cave is much more similar to surface microbes than reported from speleothems and soils. Most surprisingly, there was a cave microbe population and diversity bloom in the fall months whereas biodiversity remained relatively steady on the surface. The cave microbiome was more similar to the surface before the bloom than during and afterwards. This event demonstrates that large influxes of bacteria and particulate organic matter can enter the cave from either the surface or interstitial zones and the divergence of the cave microbiome from the surface demonstrates movement of microbes from the epikarst zones into the cave.

[Soil-epikarst structures and their hydrological characteristics on dolomite slopes in karst region of southwest China]. / 西南喀斯特白云岩坡地土壤-表层岩溶带结构及水文特征.

Epikarst is the core area of karst critical zone, with important hydrologic regulation and storage function. However, the effects of karst development degree on hydrologic characteristics of epikasrt is still unclear. We used geophysical exploration and hydrogeological techniques, combined with the dynamic monitoring of moisture and water levels, to quantify the karst development degrees and their hydrologic characteristics on slope lands. We analyzed the responses of soil-epikarst systems to rainfall. Results showed that geophysical exploration technology could be well applied to the detection of surface-subsurface structures in the karst areas. The average thickness of soil and surface karst zone on the slope was less than 0.63 m and 2.60 m, respectively. The slopes of strong-karstification characterized by high apparent resistivity, well-developed joint fractures, and strong permeability (0.73 m·d-1). Such a result indicated that epikarst could regulate precipitation. The responses of soil moisture had a larger rainfall threshold (>20.50 mm·d-1) and the water level was determined by rainfall amount. In contrast, the slope with weak-karstification had low apparent resistivity and weak permeability (0.07 m·d-1). Moisture and water level were sensitive to rainfall. Karst channels were developed locally at 240-300 cm with a permeability coefficient of up to 432 mm·d-1. Obvious preferential flow was observed in extreme rainfall events on this slope, which could induce flood disaster in the adjacent depression. Our results would provide scientific basis for further research on water resources regulation, management, and eco-hydrology in karst areas of southwest China.

Hydrological response of karst stream to precipitation variation recognized through the quantitative separation of runoff components.

Various water transmitting media are related to highly variable water source compositions, which limit the understanding of the aquifer structure and hydrological processes in a karst catchment. This study aims to understand the variation in water contribution by matrix, fissure, and conduit flows during storm and seasonal scales based on discharge, electrical conductivity (EC), and nitrate measurements of stream water in a typical dolomite catchment during 2017-2018 and discusses the hydrological response mechanism of a karst aquifer to rainfall characteristics. Time-series analyses of discharge and EC indicated that the rapid response time (mean lag time < 1 h) was mainly controlled by rainfall intensity, and the lag time decreased significantly when the rainfall intensity was lower than 15 mm/h. However, the mean discharge was dominated by the rainfall amount and antecedent moisture state. Hydrograph separation based on nitrate indicates that the contribution of soil water was irrelevant for recharging the stream during a non-rain period, whereas epikarst water contributed more than 83.2% of the total flow during a rainfall event. As indicated by the EC frequency distribution analyses, the contribution ratios of the surface, conduit, fissure, and matrix flows were 1:1.8:2.1:7.1, 1:1.6:5.3:6.3, and 0:0:0:1 during stormy, heavy, and light rainfall events, respectively. These parameters indicate that the degree of karstification was low in the karst aquifer. Seasonal frequency distribution analyses of EC indicate that higher rainfall amounts and rainfall intensities during the wet season promoted the contribution of conduit flow to approximately 11.4% of the total flow; however, matrix flow dominated the recharge of the streamflow and its contribution was more than 55.6% during each season. Our results suggest that the permeability of the epikarst matrix dominates the storage and transfer functions in dolomite karst aquifers with low karstification.

[Challenges and probable solutions for using stable isotope techniques to identify plant water sources in karst regions: A review]. / é‡‡ç”¨ç¨³å®šåŒä½ç´ æŠ€æœ¯åˆ¤å®šå–€æ–¯ç‰¹åœ°åŒºæ¤ç‰©æ°´åˆ†æ¥æºçš„æŒ‘æˆ˜ä¸Žå¯èƒ½åº”å¯¹æ–¹æ¡ˆ.

Karst regions, which account for about 15% of the terrestrial surface area, are characte-rized by specific hydrogeological structure different from most non-karst regions. Thus, many research methods that are used in non-karst regions cannot be directly used in karst regions. This issue is especially relevant to research on plant water sources. In this paper, origins and possible solutions to the common problems associated with research on water sources used by karst plant species were reviewed. Four questions were addressed: 1) why is it important to determine plant water source in karst regions? 2) Why are stable isotopes used? 3) What are the challenges associated with using stable isotopes in karst regions? 4) What are the probable solutions for these challenges? This review emphasized the advantages of using stable isotope techniques to identify sources of water used by karst plant species and the challenges associated with satisfying the prerequisites of this method. It is suggested that sources of water used by plant species in karst regions need not to be divided into specific depths and the method of identifying sources of water used by plant species based on their hydrologic properties was much applicable.

[Impacts of Land Use on the Source of Dissolved Fatty Acids in Epikarst Spring During Rainy Season].

Fatty acids were quantitatively analyzed by gas chromatography-mass spectrometer (GC-MS). Dissolved fatty acids were collected from Nanchuan epikarst spring. The results of BQ, LQ, HQ showed that the average contents of fatty acids were 14870, 12912 and 8801 ng·L-1 from May to July, respectively, and followed the order of SFA> MUFA> BrFA> PUFA. The monomers with highest content were C16:0 and C18:0. From May to July, the fatty acid content increased in BQ , LQ, HQ. The differences in vegetation coverage and bedrock bare rate, as well as the interaction of the dilution effect and soil transfer increase, led to the changes in content and the composition of the fatty acids. Based on the molecular characteristic indices of fatty acids, like H/L, CPIh, TARFA about BQ, LQ, HQ, and contents of fatty acids which were mainly originated from bacteria, phytoplankton or terrigenous higher plant, it was indicated that fatty acids were mainly originated from higher plant in LQ in May, from phytoplankton input in HQ in July, while from bacteria in the other months in LQ, HQ and from May to July in BQ.

Simulation of the transfer and fate of γ-HCH in epikarst system.

The thin surface soil layer and karst features in karst terrains lead to poor filtration, poor pre-purification and rapid infiltration, so that karst groundwater systems are particularly vulnerable to contamination. Due to its extensive use in past, gamma-hexachlorocyclohexane (γ-HCH) is ubiquitous in various environmental compartments of China, even though it has been prohibited since 1984. However, very little is known about its movement and behavior in special karst system. In this study, a dynamic fugacity model was established for γ-HCH in epikarst system via dividing the karst soil into multiple layers coupled with the physical-chemical properties of γ-HCH. The simulated results in soil profile were in good agreement with the measured values of γ-HCH. The modeled results predict that only 18 g γ-HCH will be left in the studied area in 2020, which is only 0.4% of the largest reserves in 1983, and about 99.99% of γ-HCH will remains in soil. The concentrations of γ-HCH in air, plant and 0-20 cm layer soil in the studied area descended quickly after HCHs was prohibited in 1984, while its concentration in soil layer deeper than 20 cm (deeper soil) increased continuously till 1997. The dominant transfer process of γ-HCH between the adjacent compartments in the studied area was from 0-20 cm layer to the deeper soil. Sensitivity analysis results showed that emission rate, infiltration coefficient, total organic carbon of soil, degradation rate in soil, compartment area and volume were the top six influential parameters for predicting γ-HCH concentration.