•The rock-soil fabrics affects the surface evaporation.•There are significant differences in surface evaporation between different rock exposure rate after rainfall of different intensities.•The ...evaporation from rock maybe is an important part of surface evaporation.•The leakage coefficients with and without cracks varied greatly.•The amount of leakage and the time to reach the peak flow varied greatly among the different rock-soil fabrics.
Karst water is an extremely important resource and can respond to precipitation events quickly due to the high rock exposure rate, shallow and uneven soil layers, and various crack sizes. The impacts of this special rock–soil structure type on hydrological processes are difficult to quantify,mainlybecause the karst subsurface structure is very complex. In this study, lysimeters representing four rock–soil structures with two rock exposure rates (50% and 80%) and two soil thicknesses (5 cm and 20 cm) were constructed at the Puding Karst Ecosystem Research Station, Chinese Academy of Sciences. The following results were obtained by observing the evaporation and leakage from September 2019 to August 2020. 1) Over a year, the evaporation of the 80% rock exposure lysimeter was approximately 8% less than that of the 50% rock exposure lysimeter, while the two lysimeters with different soil thicknesses had only a 2% difference in evaporation. 2) The surface evaporation of different rock exposure rates varies greatly after different rainfall intensities. However, there was no significant change in surface evaporation between different soil thicknesses after rainfall of different intensities. 3) Generally, with the increase in the rock exposure rate, the infiltration coefficient nonlinearly increased. The leakage coefficients for cracks of different sizes and different soil thicknesses were approximately 0.67 and 0.57, respectively. 4) The amount of leakage and the time to reach the peak flow varied greatly among different rock–soil structures and were also affected by the antecedent rainfall. Thus, rock exposure has a great impact on surface evaporation after rainfall, and the influence of cracks on subsurface hydrological processes was greater than that of soil thickness. These findings can provide a scientific reference for water use and management practices in karst areas.
To estimate carbon sequestration potential in the karst area, soil respiration in a natural recovering karst abandoned farmland in Shawan, Puding, Guizhou, southwest China was continuously and ...automatically monitored for more than two years. The results show that the CO
2
flux of soil respiration (2.63 ± 1.89 μmol m
−2
s
−1
) is higher in the karst area than in non-karst areas under similar conditions but that regional value (1.32 μmol m
−2
s
−1
) is lower because of larger rock fragment coverage (~ 50 %). At the same time, the temperature sensitivity of soil respiration (
Q
10
) in this study area is significantly higher than that of non-karst areas under similar conditions. Soil respiration has an obvious temporal variation, which is reflected in a significant exponential relationship between soil respiration and soil temperature, but the relationship between soil respiration and soil moisture is very complex. Especially, soil respiration has an obvious spatial variation, which is likely affected by different diffusion or water–rock reaction processes.
Soil moisture is crucial to vegetation restoration in karst areas, and climate factors and vegetation restoration are key factors affecting changes in soil moisture. However, there is still much ...controversy over the long‐term changes in soil moisture during vegetation restoration. In order to reveal the changes in soil moisture during vegetation restoration, we conducted long‐term positioning monitoring of soil moisture at 0–10 and 10–20 cm on secondary forests sample plot (SF, tree land) and shrubs sample plot (SH, shrub land) in karst areas from 2013 to 2020. The results showed that the aboveground biomass of SF and SH increased by 50% and 240%, respectively, and the soil moisture of the SF and SH showed an increasing trend. When shrubs are restored to trees in karst areas, the soil moisture becomes more stable. However, the correlation coefficients (R2) between the annual rainfall and the annual average soil moisture of SF and SH are 0.84 and 0.55, respectively, indicating that soil moistures in tree land are more affected by rainfall. The soil moisture of shrubs and trees are relatively low during the months of alternating rainy and dry seasons. Rainfall has a very significant impact on the soil moisture of tree land, while air temperature and wind speed have a significant impact on the soil moisture of tree land, but the soil moistures of shrub land are very significantly affected by rainfall and relative humidity. Therefore, during the process of vegetation restoration from shrubs to trees, the main meteorological factors that affect soil moisture changes will change. The results are important for understanding the hydrological processes in the ecological restoration process of different vegetation types in karst areas.
The soil moisture of the secondary forests and shrubs showed an increasing trend. The vegetation restoration process will make soil moisture more stable. The soil moisture of shrubs and trees is relatively low during the months of alternating rainy and dry seasons. During the process of vegetation restoration from shrubs to trees, the main meteorological factors that affect soil moisture changes will change.
•Drip water δ18O, δ13CDIC, ions show interannual changes as vegetation change.•Evaporation and transpiration outside the cave affect drip water δ18O.•Vegetation uptake, transpiration, water–rock ...interaction affect drip water ions.
The transmission of stable isotopes and elements/ions from the outside to the inside of a cave and their incorporation into drip water can involve numerous biogeochemical processes. To understand how the original signals of stable isotopes and elements/ions are modified by these processes, integrated studies of the interactions between vegetation, soil, epikarst, and caves are required. We conducted a multi-year monitoring study of the vegetation biomass, tree breast-height diameter, PCO2 in soil air and cave air, δ13C in soil air, stable oxygen isotope in rainwater and drip water, and stable carbon isotope and elements/ions concentrations in drip water in Shawan Cave system, southwest China. The main results were as follows: (1) The evaporation effect weakened and the transpiration effect strengthened outsides the cave as vegetation improved, thus leading to a year-by-year increasing trend in the δ18O value of drip water. This indicates that changes in vegetation may have been another potential factor influencing the interannual variation of the δ18O value of drip water. (2) The CO2 concentration and δ13C value in soil air increased and decreased, respectively, with vegetation restoration, which caused the interannual variation in the dissolved inorganic carbon isotope (δ13CDIC) value of drip water during the autumn and winter to exhibit a year-by-year decreasing trend. (3) The variations in the elements/ions concentrations of drip water were affected by vegetation uptake, vegetation transpiration, and water–rock interactions. It is inferred that the interannual variation in the elements/ions concentrations of drip water responded to vegetation restoration. (4) A conceptual model demonstrated that the three response modes of drip water δ18O value, δ13CDIC value, and elements/ions to variations in vegetation. Overall, this study highlights the responses of the interannual changes in δ13CDIC, δ18O, and elements/ions of drip water to vegetation restoration, which contributes critical insights into the paleoenvironmental interpretation of proxies of speleothems.
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•Natural restoration can recover vegetation rapidly on abandoned farmland in the karst region of southwest China.•Carbon, water, and energy fluxes show significant variation between ...rainy season and dry season.•High carbon exchange capacity between the atmosphere and plant-soil-cave continuums.•CO2 emission through cave ventilation cannot be neglected in the carbon budget.
Extensive areas of agricultural land have been abandoned to ecological restoration in recent years in the karst region of Southwest China, which contributes to the greening of the area. However, there has yet no direct observation of carbon, water, and energy fluxes on abandoned land in the region. In addition, because of the coupling between above and below-ground processes, monitoring of the karst ecosystem needs to be conducted from a critical zone perspective. In this study, an integrated vertical observation system through air-vegetation-soil-cave continuums was constructed on abandoned farmland under natural restoration in Puding Karst Ecosystem Research Station. Preliminary results show that: First, vegetation cover restored rapidly after abandonment, and the measured net ecosystem exchange (NEE), soil CO2 efflux, actual evapotranspiration (ETa) and latent heat (LE) in the rainy season are about twice of that in the dry season, this strong seasonal variation relates to the typical subtropical monsoon climate with synchronous water and heat availability during rainy season. Second, high CO2 concentrations and significant CO2 variation in the monitored cave indicate that the exchange of underground carbon pools with the atmosphere cannot be neglected in the carbon budget of the study site.
Cave 222Rn has been a major health issue and subject of scientific debate for decades. While the basics of natural ventilation physics are well understood, it is difficult to make blind predictions ...of 222Rn concentrations in a given cave due to the complexity of cave systems. In-situ continuous observation is necessary to improve our ability to quantify radiation dose exposure and reduce radiation hazard to cave users, and trace the air exchange patterns occurring in caves. In this study, continuous monitoring using a RAD7 radon detector revealed high 222Rn concentrations and large fluctuations in 222Rn concentration in a small karst cave in southwest China, Shawan Cave. From August 2016 to July 2017, the average annual concentration was 47,419 Bqm−3 and ranged between 3720 and 123,000 Bqm−3, with lower values during summer than other seasons. Taking Shawan Cave as a case study, we suggest a framework to evaluate the potential dose exposure, allowing cave users to minimize risk of exposure to hazardous levels of 222Rn. Furthermore, we comparing results from this study with other studies in 35 caves worldwide, and conclude that there are three patterns of seasonal 222Rn variation. They were classified into five types of ventilation mode based on diversity of cave locations, geometry and connectivity of bed rock fracture networks, together with temperature differences between outside atmosphere and cave air.
•Monitored cave 222Rn concentrations as high as 123000 Bqm−3.•Dose exposure and advices were given according to ICRP recommendations.•Seasonal variations of 222Rn concentration of 35 caves were summarized.
The speleothem stable carbon isotope (δ13C) is an important proxy for paleoclimate reconstruction. Understanding the stable carbon isotope composition of dissolved inorganic carbon (δ13CDIC) in drip ...water is crucial for the interpretation of speleothem δ13C signals. In this study, we present two years of drip water δ13CDIC data and its hydrogeochemistry variables (e.g. PCO2, Ca2+, Mg/Ca, SO42−, pH, drip rate, saturation index of calcite, electrical conductivity) in the Shawan Cave. We also conducted continuous in situ monitoring of PCO2 in the atmosphere, soil air, and cave air and microenvironmental factors. The results show that cave air CO2 primarily originates from soil air CO2, but there is an unsynchronous seasonal variation between cave air CO2 and soil air CO2, which is likely caused by cave ventilation. Cave ventilation is also responsible for seasonal variations in drip water δ13CDIC and speleothem growth. Cave air exchange leads to higher drip water δ13CDIC and rapid speleothem growth in the spring and summer seasons, and cave air stagnation results in lower drip water δ13CDIC and slow speleothem growth. The impact of isotopically light CO2 of soil air on drip water δ13CDIC during spring and summer seasons is more than during autumn and winter seasons, where the more isotopically light CO2 of soil air is enriched by drip water under cave air exchange conditions. Since speleothems growth is rapid in the spring and summer seasons, the speleothems δ13C of the Shawan Cave have the potential to record climate information for the spring and summer seasons. Additionally, by comparing the speleothem δ13C values of eleven caves, we found that paleoclimate signals recorded by speleothem δ13C revealed different seasonal biases under different ventilation types. Therefore, it is necessary to determine the cave ventilation type in order to understand how speleothem δ13C responds to climate change in a ventilated cave.
•Cosmogenic nuclide burial dating of cave sediments in Liuchong River, southwestern China.•The embryonic form of modern Wujiang River was possibly presented at 0.75Ma ago.•The high river incision ...rate of northwest Guizhou Plateau is nearly ∼0.48m/ka.•Strong uplift of Tibetan Plateau in Quaternary might be the primary reason.•Others are due to the soluble carbonate bedrock and cut-through of Three Gorges.
Cosmogenic nuclides 10Be and 26Al have been analyzed for sediments from the multilevel riverside caves along the Liuchong River at the southeastern margin of the Tibetan Plateau. The Liuchong River is the northern origin of Wujiang River, which passes through the northwestern Guizhou Plateau and cuts down hundreds of meters into the bedrock, leaving behind an abundance of multilevel caves. The measured 26Al/10Be ratios produced the apparent burial ages in range of 0.49–2.85Ma. Taking into account of geomorphic and geological backgrounds, the Dashi Cave located at the highest level along the Liuchong River system formed around 0.75Ma ago, which probably suggests the initial formation age of the modern Wujiang River. The resulted incision rate of ∼480m/Ma in Guizhou in the last 0.75Ma is slightly higher than those in adjacent areas. This feature implies an intensive downcutting history of the Liuchong River during the Quaternary, which might be primarily caused by the uplift of Tibetan Plateau, cut-through of the Three Gorges and soluble carbonate bedrock.
Cave
Rn has been a major health issue and subject of scientific debate for decades. While the basics of natural ventilation physics are well understood, it is difficult to make blind predictions of
...Rn concentrations in a given cave due to the complexity of cave systems. In-situ continuous observation is necessary to improve our ability to quantify radiation dose exposure and reduce radiation hazard to cave users, and trace the air exchange patterns occurring in caves. In this study, continuous monitoring using a RAD7 radon detector revealed high
Rn concentrations and large fluctuations in
Rn concentration in a small karst cave in southwest China, Shawan Cave. From August 2016 to July 2017, the average annual concentration was 47,419 Bqm
and ranged between 3720 and 123,000 Bqm
, with lower values during summer than other seasons. Taking Shawan Cave as a case study, we suggest a framework to evaluate the potential dose exposure, allowing cave users to minimize risk of exposure to hazardous levels of
Rn. Furthermore, we comparing results from this study with other studies in 35 caves worldwide, and conclude that there are three patterns of seasonal
Rn variation. They were classified into five types of ventilation mode based on diversity of cave locations, geometry and connectivity of bed rock fracture networks, together with temperature differences between outside atmosphere and cave air.
