Vertical migration from surface soils to groundwater and source appointment of polycyclic aromatic hydrocarbons in epikarst spring systems, southwest China

Understanding the transfer process of polycyclic aromatic hydrocarbons (PAHs) in the karst terrain is of great importance to their ecological risk assessments, however, the impact of the vertical transfer of the soil PAHs on the underground water is largely unknown in the karst system. Here, the vertical distribution and the seasonal variation of 16 PAHs in the soils and the water of 4 epikarst spring catchments in Southwest China were investigated. The total concentration of the PAHs ranged within 61-3285 ng g in the soils, and 341-4969 ng L in the spring water. The vertical distribution of the PAHs in soils varied with ring numbers and altitude of the catchment. PAHs concentrations were linearly related with the total organic carbon (TOC) at different depths in the catchments 563-783 m above the sea level (A.S.L.). However, no correlation with TOC was observed in the catchment of a high altitude (2090 m A.S.L.), because the large water flux led to the fast migration of the 2-3 rings PAHs in soils. The PAHs in soils and springs were mainly derived from the combustion of grass/wood/coal, closely related with the primary fossil fuels used in this area. This study demonstrate that the groundwater was heavily polluted by PAHs in the karst terrains of Southwest China, due to the vertical transfer of PAHs from the surface soils, and effective protection was urgently needed

Earth's critical zone and karst critical zone: Structure, characteristic and bottom boundary

Critical zone science has become an important research area in Surface-Earth system science, representing a future new concept and developing trend in Earth system science. The karst critical zone, a unique type, covers approximately 15.2% of the global ice-free continental land, which is a typical case and represents the Earth's critical zone. However, there is still a lack of a unified understanding of the scientific connotation of the karst critical zone (KCZ) and a lack of related discussions on the structure, characteristics and lower boundary of the KCZ at present. On the basis of reviewing the history of the scientific development of the Earth's critical zone and summarizing its main characteristics, this study sorts out the background and development processes of the concept and summarizes the characteristics of the KCZ. The bottom boundary of the KCZ, which is still controversial, is analyzed and discussed, and the further development direction of the KCZ is also analyzed. These results show that the KCZ is a unique type of Earth's critical zone shaped by material circulation and energy flow at the interface of the lithosphere, hydrosphere, atmosphere, biosphere as well as pedosphere in soluble rock areas. The spatial span of the KCZ ranges from the vegetation canopy to the lower boundary of the karst aquifer in the soluble rock area, including the vegetation layer, soil layer, epikarst, vadose zone and saturated zone, which are sensitive to environmental changes and have surface and subsurface double geological structures and special geochemical processes characterized by coupling cycles of carbon, water and calcium. By comparison, the KCZ has eight following characteristics: active involvement of carbonate rock in the material cycle, sensitive response to external environmental changes, multilayer hydrogeological structures, strong horizontal spatial heterogeneity, large underground space network, large-span biota structure, unique ecohydrological processes with carbonate rock water-vegetation interaction as well as varying lateral boundary controlled by subsurface divide change. The bottom boundary of the KCZ is the depth at which the groundwater recharged by precipitation has no influence on the dissolution of carbonate minerals (calcite, dolomite, etc.) and has no ability to further dissolve carbonate minerals (calcite, dolomite, etc.) in a certain range below the surface. According to the development trend of the disciplines and national strategic demands, further attention should be paid to the following four directions: the structure, formation and evolution mechanisms of different types of the KCZ; the ecological service function and regional sustainable development in degraded karst ecological zones; the impacts of engineering activities on the structure, attributes and evolution process of the KCZ; and the coupling process between climate change and the change in the structure, function and attributes of the KCZ

Trade-off and synergy analysis among ecosystem services in a karst watershed

The underground river watersheds in karst areas are undergoing the degradation of ecosystem functions, thus hindering the development of benefits to humans. Taking the Nandong underground river watershed (NURW) as a test case in southern China, this study evaluated net primary productivity (NPP), water yield (WY), and soil retention (SR) and the relationships between the three factors from 2000 to 2018. The results showed that (1) NPP exhibited a continuously rising trend from 2000 to 2018, and the WY and SR also increased in general; (2) land use was an important influencing factor for ecosystem services, and the synergy between the two improved since 2006; (3) the ecosystem synergy was poor in the NURW, and the conflicts among ecosystem services were especially severe in areas with a high incidence of rocky desertification. This study will provide a reference for regional governments by clarifying the possible future state of the ecosystem

Response of carbon- and water-use efficiency to climate change and human activities in China

Carbon-use efficiency (CUE) and water-use efficiency (WUE) are critical parameters for evaluating the exchange of carbon and water in ecosystems, influenced by climate change and human activities. Quantifying the relative contributions of climate change and human activity to CUE and WUE is essential in the context of global warming and rapid land use transformation. This study assessed spatiotemporal variations in CUE and WUE and evaluated their interconnections with climate factors through comprehensive trend and partial correlation analyses. Furthermore, it explored the responses of CUE and WUE variations to climate change and human activities across distinct geographical zones in China from 2001 to 2019, employing multiple regression residual trend analysis. Our analysis suggested that annual average CUE was 0.57 and annual average WUE was 1.47 g C kg−1H2O. Spatially, CUE demonstrated an increase in northern China but a decline in the southern regions, while WUE exhibited a contrasting trend. Generally, CUE exhibited a positive correlation with increased precipitation (PRE) and solar radiation (RAD) but displayed a negative association with elevated air temperature (TEM). Conversely, WUE showed an increase with greater PRE and TEM but a decrease with heightened RAD. Notably, it was revealed that human activities exerted a more significant impact on CUE and WUE in China than climate change. However, climate change played an important role in some areas with low populations, such as the Qinghai–Tibet Plateau. The results will improve the understanding of the carbon and water coupled process in terrestrial ecosystems, which is helpful to optimize the allocation of ecological resources and maintain ecosystem balance

Stable isotope and aquatic geochemistry of a typical subtropical karst subterranean stream in southwest China

A hydrogeochemical and isotopic study was conducted on the subterranean karst stream, namely the Guancun subterranean stream (GSS). The hydrogeochemical processes of the GSS were controlled through calcite dissolution and precipitation and were driven by the concentration of CO2, which controlled changes in the pH and of PCO2 in the water. The δ18O and δD values of the GSS were within the global meteoric water line and the local meteoric water line, thereby indicating that the water of the GSS comes from precipitation. Certain abnormal δ18O and δD values suggest the effect of evaporation on the GSS given its use in a particular irrigation system, wherein the GSS in transformed into a surface stream and flows for a relatively long time on the surface during the wet season. The δ13CDIC values of the GSS range from −13.5 to −11.3‰ in the dry season and from −13.9 to −9.5‰ in the wet season, thereby indicating that the GSS belongs to a semi-open system. The δ13CDIC values in the GSS were formed by the δ13CDIC values of the soil CO2 and carbonate dissolution at different proportions. According to the simplified mass balance formula, the contributions of carbonate dissolution to the dissolved inorganic carbon (DIC) of the GSS were calculated to be 50.2–58.3% and 48.7–64.7% in the dry and wet seasons, respectively, thereby indicating a less than 50% carbonate dissolution contribution during the formation of DIC in karst groundwater. Moreover, sulfuric acid and nitric acid were observed to participate in karst processes

Source, Distribution and Transformation of Organic Matter in a Subtropical Karst Reservoir

In order to improve the understanding of the global carbon cycle and the stability of karst carbon sinks, it is necessary to better understand the source, distribution and transformation characteristics of organic matter (OM) in aquatic ecosystems. Here, stable isotope ratios (δ13C and δ15N), elemental analysis (C/N ratios), and lipid biomarkers were analyzed for dissolved organic matter (DOM) (0.7 μm) of water, and organic matter from sediment cores (SCOM) to identify the sources, distribution, and transformation of OM in a subtropical karst reservoir. The results showed that short-chain (C14–20) n-alkyl lipids were more abundant than long-chain (C21–34) n-alkyl lipids in both the DOM and SCOM samples, indicating that bacteria were the primary sources of these lipids, while terrestrial organic matter (OM) made only a minor contribution to the n-alkyl lipid pool, and aquatic plants (macrophytes) OM contributed major contribution to the n-alkyl lipid pool in POM. Microbial activity and lipid degradation were more pronounced in the DOM. Furthermore, terrigenous and macrophyte-derived lipids were found to be more abundant in POM than in DOM and SCOM, suggesting that they are relatively resistant to degradation compared with phytoplankton-derived OM

In-stream metabolism and atmospheric carbon sequestration in a groundwater-fed karst stream

Atmospheric carbon sequestered in karst systems through dissolution of carbonate minerals is considered to have no net effect on long-term regional and global carbon budgets because precipitation of dissolved carbonate minerals emits CO2 back to the atmosphere. Even though recent studies have implied that rapid kinetics of carbonate dissolution coupled with the aquatic photosynthetic uptake of dissolve inorganic carbon (DIC) could facilitate a stable atmospheric C sink in karst rivers and streams, little is known about the magnitudes and long-term stability of this C sink. To assess in-stream biogeochemical processes and their role on stream C cycling, we measured diel cycles of water characteristics and chemical composition (temperature, pH, DO, SpC, DIC, Ca2 +, δ13CDIC) in a groundwater-fed karst stream in southwest China. Our results show no diel variations at the groundwater discharge point (CK site) due to the absence of a sub-aquatic community (SAC). However, all hydrochemical parameters show significant diel cycle 1.3 km downstream (LY site). Diel variations in pH, DO, and δ13CDIC were inversely related to diel changes in SpC, DIC, Ca2 + and pCO2. This result indicates that in-stream metabolism (photosynthesis and respiration) of SAC controls diel variations in stream water chemistry. Significant diel cycles of net ecosystem production (NEP) influences in-stream diel fluctuation of pH, DO, SIc, DIC, pCO2, Ca2 + and δ13CDIC, with gross primary production (GPP) dominating in day and ecosystem respiration (ER) dominating at the night. Absence of in-stream metabolism at CK enhances CO2 degassing from stream to the atmosphere, which is estimated to be 3– 5 times higher than at LY. We estimate the carbon sink through in-stream metabolism of SAC to be 73 t C km− 2 a− 1, which is around half the rate of the oceanic biological pump. These results imply in-stream photosynthesis sequesters DIC originating from karst weathering and controls CO2 evasion

River sequesters atmospheric carbon and limits the CO2 degassing in karst area, southwest China

CO2 fluxes across water-air interfaces of river systems play important roles in regulating the regional and global carbon cycle. However, great uncertainty remains as to the contribution of these inland water bodies to the global carbon budget. Part of the uncertainty stems from limited understanding of the CO2 fluxes at diurnal and seasonal frequencies caused by aquatic metabolism. Here, we measured surface water characteristics (temperature, pH, and DO, DIC, Ca2 + concentrations) and CO2 fluxes across the air-water interface at two transects of Guijiang River, southwest China to assess the seasonal and diurnal dynamics of fluvial carbon cycling and its potential role in regional and global carbon budgets. The two transects had differing bedrock; DM transect is underlain by carbonate and detrital rock and PY is underlain by pure carbonate. Our results show that the river water both degasses CO2 to and absorbs CO2 from the atmosphere in both summer and winter, but the degassing and absorption varied between the two transects. Further, CO2 fluxes evolve through diurnal cycles. At DM, the river evaded CO2 from early morning through noon and absorbed CO2 from afternoon through early morning. At PY in summer, the CO2 evasion decreased during the daytime and increased at night while in winter at night, CO2 uptake increased in the morning and decreased in the afternoon but remained relatively stable at night. Although the river is a net source of carbon to the atmosphere (~ 15 mM m− 2 day− 1), the evasion rate is the smallest of all reported world\u27s inland water bodies reflecting sequestration of atmospheric carbon through the carbonate dissolution and high primary productivity. These results emphasize the need of seasonal and diurnal monitoring of CO2 fluxes across water-air interface, particularly in highly productive rivers, to reduce uncertainty in current estimates of global riverine CO2 emission