Microclimate stability on the critical zone of a karst hillslope in southwest China: Insights from continuous temperature observations at the air-soil-epikarst interface.

Temperature is an important near-surface microclimate parameter that plays a key role in hydrological, ecological, and biogeochemical functions. However, the spatio-temporal distribution of temperature on the invisible and inaccessible soil-weathered bedrock continuum, wherein hydrothermal processes are most active, remains poorly understood. Temperature dynamics were monitored at 5 min intervals in the air-soil-epikarst (∼3 m) system at different topographical positions of the karst peak-cluster depression in southwest China. The weathering intensity was characterized based on the physicochemical properties of samples collected through drilling. No significant difference was observed in air temperature across slope positions, which was related to the limited distance and elevation resulting in roughly consistent energy input. The control effect of air temperature on the soil-epikarst was weakened with the decrease in elevation (±0.36 to ±0.25 °C). It is attributed to the enhanced temperature regulation capacity of vegetation cover from the up slope (shrub dominant) to down slope (tree dominant) in a relatively uniform energy environment. Temperature stability is clearly distinguished in two adjacent hillslopes that were differentiated by weathering intensity. For every 1 °C change in the ambient temperature, the amplitude of soil-epikasrt temperature variation on the strongly and weakly weathered hillslopes were ±0.28 and ± 0.32 °C, respectively. The response of soil-epikarst temperature to ambient temperature was more sensitive in the wet season (±0.40 °C) than in the dry season (±0.20 °C), which was related to the cooling effect caused by abundant rainfall. The cooling effect was particularly prominent in the preferential flow development area composed of pipeline cracks, which appear in the hillslope with relatively weak weathering intensity. These demonstrate that soil-epikarst temperature responds more gently to the variability of rainfall and ambient temperature on a relatively strong weathered hillslope. Accordingly, this study highlights that the sensitivity of soil-epikarst temperature to climate change is regulated by vegetation and weathering intensity on karst hillslopes in southwest China.

Properties and the Application of Sludge-Based Biochar in the Removal of Phosphate and Methylene Blue from Water: Effects of Acid Treating.

Sludge-based biochar could be used to remove phosphate and methylene blue (MB) from water. It is a highly efficient way to treat the sludge and contaminated water synergistically. The high ash content in sludge greatly influenced the adsorption property of the resultant biochar. In this work, the influence of carbonization-activation and acid treating on the adsorption performance of the sludge-based biochar was evaluated. The composition, structure, and surface properties of biochar were improved after acid treating. The biochar was obtained in a sequence of carbonization-activation first and then acid treating, providing the optimal adsorption property. Zn550-H and Zn750-H showed excellent adsorption capacity to phosphate and MB, respectively. The adsorption process was well described by the pseudo-first-order and pseudo-second-order kinetic models. Isothermal studies implied that it was controlled by multiple processes. What is more, sludge-based biochar performed well in the adsorption of phosphate and MB from weakly acidic to alkaline conditions, which was beneficial to utilize the sludge-based biochar in water remediation practically.

Water uptake depth is coordinated with leaf water potential, water-use efficiency and drought vulnerability in karst vegetation.

Root access to bedrock water storage or groundwater is an important trait allowing plant survival in seasonally dry environments. However, the degree of coordination between water uptake depth, leaf-level water-use efficiency (WUEi) and water potential in drought-prone plant communities is not well understood. We conducted a 135-d rainfall exclusion experiment in a subtropical karst ecosystem with thin skeletal soils to evaluate the responses of 11 co-occurring woody species of contrasting life forms and leaf habits to a severe drought during the wet growing season. Marked differences in xylem water isotopic composition during drought revealed distinct ecohydrological niche separation among species. The contrasting behaviour of leaf water potential in coexisting species during drought was largely explained by differences in root access to deeper, temporally stable water sources. Smaller-diameter species with shallower water uptake, more negative water potentials and lower WUEi showed extensive drought-induced canopy defoliation and/or mortality. By contrast, larger-diameter species with deeper water uptake, higher leaf-level WUEi and more isohydric behaviour survived drought with only moderate canopy defoliation. Severe water limitation imposes strong environmental filtering and/or selective pressures resulting in tight coordination between tree diameter, water uptake depth, iso/anisohydric behaviour, WUEi and drought vulnerability in karst plant communities.

Vertical distribution of soil saturated hydraulic conductivity and its influencing factors in a small karst catchment in Southwest China.

Saturated hydraulic conductivity (Ks) is one of the most important soil hydraulic parameters influencing hydrological processes. This paper aims to investigate the vertical distribution of Ks and to analyze its influencing factors in a small karst catchment in Southwest China. Ks was measured in 23 soil profiles for six soil horizons using a constant head method. These profiles were chosen in different topographical locations (upslope, downslope, and depression) and different land-use types (forestland, shrubland, shrub-grassland, and farmland). The influencing factors of Ks, including rock fragment content (RC), bulk density (BD), capillary porosity (CP), non-capillary porosity (NCP), and soil organic carbon (SOC), were analyzed by partial correlation analysis. The mean Ks value was higher in the entire profile in the upslope and downslope, but lower value, acting as a water-resisting layer, was found in the 10-20 cm soil depth in the depression. Higher mean Ks values were found in the soil profiles in the forestland, shrubland, and shrub-grassland, but lower in the farmland. These results indicated that saturation-excess runoff could occur primarily in the hillslopes but infiltration-excess runoff in the depression. Compared with other land-use types, surface runoff is more likely to occur in the farmlands. RC had higher correlation coefficients with Ks in all categories concerned except in the forestland and farmland with little or no rock fragments, indicating that RC was the dominant influencing factor of Ks. These results suggested that the vertical distributions of Ks and RC should be considered for hydrological modeling in karst areas.

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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.

Transpiration rates decline under limited moisture supply along hillslopes in a humid karst terrain.

Topographic positions can mediate subsurface water availability, but its effects on tree transpiration are controversial. In humid karst regions, climax forests are usually not limited by moisture supply, even at the summit, through absorbing water from deep layers. However, little is known on the transpiration pattern and its limiting factor on the shrubland widely distributed along the karst hillslopes. In the current study, Rhus chinensis, a widely spread constructive species in natural restoration was selected. Meteorological factors, 0-300 cm soil-epikarst moisture, sap flow, and root water uptake were studied during an entire growing season to assess how hillslope positions affected transpiration. We found the mean water content in uphill was only around 60 % of that in downhill, indicating a contrasting water supply along the slope. However, there were no significant differences in the xylem isotopic composition and lc-excess which suggested the similar water uptake strategies in both uphill and downhill. R. chinensis primarily relied on the soil water rather than epikarst water (groundwater) along the hillslope because of the MixSIAR model results and more negative lc-excess values (-13.18 ‰). R. chinensis exhibited decreases of nearly half in the transpiration rate and amount in uphill compared to those in downhill. In downhill with sufficient water availability, transpiration followed the variation in atmospheric water demand. In uphill, a poor moisture supply limited tree transpiration and its response to atmospheric water demand. Our findings revealed that the early successional species did not entirely depend on atmospheric water demand, absorbing deep epikarst water as the mature forest. The transpiration rates of those species declined by nearly half to adapt to the water-limited environment along the hillslope in the humid karst region. This study can contribute to the evaluation of eco-hydrological functions during natural restoration.

Common Species Maintain a Large Root Radial Extent and a Stable Resource Use Status in Soil-Limited Environments: A Case Study in Subtropical China.

Coarse root systems provide a framework for water and nutrient absorption from the soil and play an important role in plant survival in harsh environments. However, the adaptions of plant roots in soil-limited environments with low water storage capacity and nutrient content needs to be better understood. The adaptation strategies of two common plant species (a deciduous tree Platycarya longipes and an evergreen shrub Tirpitzia ovoidea) from two contrasting habitats (a shallow rocky soil and a nearby deep soil) in a karst region of subtropical China were compared and analyzed. Foliar nutrient concentrations, stoichiometry, stable carbon, and oxygen isotopes were used to determine plant nutrient and water use status across these two habitats. Six indexes, including maximum root depth, maximum root radial extent, number of major roots and secondary roots, and tapering rate and curvature, were all investigated to characterize coarse root systems. Results show that both species exhibited similar nutrient and water use status in the two habitats that had contrasting water holding capacity and available nutrient content. On the other hand, although maximum root depths of the individual plants were not deeper than 33 cm, maximum radial extents were much larger when compared to rooting depths. Specifically, the ratio of radial extent to depth in the soil-limited habitat was approximately 6 and 1.5 times higher than that in the deep-soil habitat for the tree and shrub, respectively. Additionally, especially for the tree, a larger root radial extent was further accompanied by lower root tapering rate and bending levels. Our results provided evidence that plants growing in soil-limited environments maintain a stable resource use status along with large radially extended coarse root systems in humid karst regions like 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.

Analysis of soil water movement inside a footslope and a depression in a karst catchment, Southwest China.

Soil water movement is difficult to explain with event-scale approaches, especially in karst regions. This paper focuses on investigating seasonal recharge and mean residence time (MRT) of soil water based on temporal variation of stable isotopes (δD and δ18O) and a dispersion model (DM), and discussing their differences along a footslope and a depression in a small karst catchment of southwest China. Temporal variations of the stable isotopes in precipitation and soil water within 0-100 cm profiles were monitored weekly for approximately 43 and 99 weeks. Results show that the seasonal recharge of soil water inside the footslope and the depression were similar, but the vertical flow velocity was higher implying a faster hydrological process in the footslope. The MRT of soil water (2-64 weeks) increased roughly, suggesting decreasing velocity of water displacement with increasing depth. However, the MRT at 60-100 cm depths in the depression (47-64 weeks) was obviously longer than at other sites, revealing more intensive water mixing. Furthermore, a shallower isotopic damping depth was found in the depression, indicating stronger delay and attenuation effects on base flow recharge. These results provide new insights into research on hydrological processes in karst areas.

Comparison of Rooting Strategies to Explore Rock Fractures for Shallow Soil-Adapted Tree Species with Contrasting Aboveground Growth Rates: A Greenhouse Microcosm Experiment.

For tree species adapted to shallow soil environments, rooting strategies that efficiently explore rock fractures are important because soil water depletion occurs frequently. However, two questions: (a) to what extent shallow soil-adapted species rely on exploring rock fractures and (b) what outcomes result from drought stress, have rarely been tested. Therefore, based on the expectation that early development of roots into deep soil layers is at the cost of aboveground growth, seedlings of three tree species (Cyclobalanopsis glauca, Delavaya toxocarpa, and Acer cinnamomifolium) with distinct aboveground growth rates were selected from a typical shallow soil region. In a greenhouse experiment that mimics the basic features of shallow soil environments, 1-year-old seedlings were transplanted into simulated microcosms of shallow soil overlaying fractured bedrock. Root biomass allocation and leaf physiological activities, as well as leaf δ13C values were investigated and compared for two treatments: regular irrigation and repeated cycles of drought stress. Our results show that the three species differed in their rooting strategies in the context of encountering rock fractures, however, these strategies were not closely related to the aboveground growth rate. For the slowest-growing seedling, C. glauca, percentages of root mass in the fractures, as well as in the soil layer between soil and bedrock increased significantly under both treatments, indicating a specialized rooting strategy that facilitated the exploration of rock fractures. Early investment in deep root growth was likely critical to the establishment of this drought-vulnerable species. For the intermediate-growing, A. cinnamomifolium, percentages of root mass in the bedrock and interface soil layers were relatively low and exhibited no obvious change under either treatment. This limited need to explore rock fractures was compensated by a conservative water use strategy. For the fast-growing, D. toxocarpa, percentages of root mass in the bedrock and interface layers increased simultaneously under drought conditions, but not under irrigated conditions. This drought-induced rooting plasticity was associated with drought avoidance by this species. Although, root development might have been affected by the simulated microcosm, contrasting results among the three species indicated that efficient use of rock fractures is not a necessary or specialized strategy of shallow-soil adapted species. The establishment and persistence of these species relied on the mutual complementation between their species-specific rooting strategies and drought adaptations.

Seasonal recharge and mean residence times of soil and epikarst water in a small karst catchment of southwest China.

Soil and epikarst play an important role in the hydrological cycle in karst regions. This paper focuses on investigating the seasonal recharge and mean residence time (MRT) of soil water and epikarst water in a small karst catchment of southwest China. The deuterium contents in precipitation, creek, soil baseflow (direct recharge of the saturated soil water to the stream), epikarst spring, and soil waters were monitored weekly for two years, and MRT was calculated by an exponential model (EM) and a dispersion model (DM). The obvious seasonal variation of deuterium in rainfall was buffered in epikarst water, indicating sufficient water mixing. Soil baseflow contained less rainy-season rainwater than epikarst spring discharge, reflecting the retarded effect of soil thickness on rainwater recharge. MRTs of all water bodies were 41-71 weeks, and soils in the depression extended those of shallow groundwater. This demonstrated that the deep soil layer played an important role in karst hydrological processes in the study catchment. The creek was recharged mostly by rainfall through epikarst, indicating its crucial role in water circulation. These results showed epikarst had a strong water-holding capacity and also delayed water contact time with dolomite.

[Stormflow hydrochemical characteristics at different time scales in a typical karst catchment of northwest Guangxi, China].

Through in situ observation and indoor tests, the hydrochemical characteristics of a typical karst watershed at three different time scales (diurnal, single storm, and seasonal scales) from June 2013 to March 2014 were investigated, and their influencing factors were analyzed. The results showed that the diurnal variations of the hydrochemistry exhibited a regular changing pattern resulting from the shifting of the main vegetation physiological activity from photosynthesis in the day to respiration in the night. At single storm scale, however, the hydrochemical processes were mainly determined by the number of consecutive rainless days and rainfall intensity, while the diurnal scale effect was weakened. As to the seasonal scale, the overall hydrochemical processes showed quick responses to rainfall events although they responded more quickly in the rainy season than in the dry season. The temperature and the yearly rainfall distribution regime were the two main influencing factors at this scale.

[Spatial variation characteristics of surface soil water content, bulk density and saturated hydraulic conductivity on Karst slopes].

Surface soil water-physical properties play a decisive role in the dynamics of deep soil water. Knowledge of their spatial variation is helpful in understanding the processes of rainfall infiltration and runoff generation, which will contribute to the reasonable utilization of soil water resources in mountainous areas. Based on a grid sampling scheme (10 m x 10 m) and geostatistical methods, this paper aimed to study the spatial variability of surface (0-10 cm) soil water content, soil bulk density and saturated hydraulic conductivity on a typical shrub slope (90 m x 120 m, projected length) in Karst area of northwest Guangxi, southwest China. The results showed that the surface soil water content, bulk density and saturated hydraulic conductivity had different spatial dependence and spatial structure. Sample variogram of the soil water content was fitted well by Gaussian models with the nugget effect, while soil bulk density and saturated hydraulic conductivity were fitted well by exponential models with the nugget effect. Variability of soil water content showed strong spatial dependence, while the soil bulk density and saturated hydraulic conductivity showed moderate spatial dependence. The spatial ranges of the soil water content and saturated hydraulic conductivity were small, while that of the soil bulk density was much bigger. In general, the soil water content increased with the increase of altitude while it was opposite for the soil bulk densi- ty. However, the soil saturated hydraulic conductivity had a random distribution of large amounts of small patches, showing high spatial heterogeneity. Soil water content negatively (P < 0.01) correlated with the bulk density and saturated hydraulic conductivity, while there was no significant correlation between the soil bulk density and saturated hydraulic conductivity.

[137Cs profile distribution character and its implication for soil erosion on Karst slopes of northwest Guangxi].

This paper studied the profile distribution characters of 137Cs and soil organic carbon (SOC) on the Karst slopes and in the fissures in typical peak-cluster depression in Northwest Guangxi, aimed to approach the applicability of 137Cs method on Karst slopes and the implication of 37Cs for the characteristics of slope soil erosion. In all test profiles, there was a significant correlation between 137Cs and SOC, indicating that both of them might have the same loss pathway. On the slopes under secondary forests, 137Cs mainly existed within the depth 0-24 cm. On the upper middle and middle slope sites, 137Cs had an exponential decrease with depth, indicating no or slight surface erosion; while on the foot slope site, the distribution pattern of 137Cs indicated severer erosion. On the slopes with cultivated lands, 137Cs distributed uniformly within the plough layer. In the upper middle and middle slopes profiles, 137Cs mainly existed in the depth around 15 cm and far less than the background value, indicating severe soil erosion; while in foot slope profiles, 137Cs was aggraded to the depth 45 cm. A discontinuous distribution of 137Cs in the profiles was detected on the foot slopes under secondary forests, on the upper middle and foot slopes of cultivated lands, and in the fissures, indicating that the soil particles on Karst slopes had a trend of losing with rainwater to the underground, but the loss quantity was negligible, compared with surface erosion.

[Methods for determining plant water source in thin soil region: a review].

Plant water source depends on the distribution of available water in the environment, and the capability of plants in capturing water. In dry season, the soil water in thin soil region cannot satisfy the demand of normal plant growth, and whether the plants can use the water stored in weathered bedrock is the key for them to maintain their normal water consumption. This paper reviewed the research methods for determining the plant water source in thin soil region, including investigating and analyzing the characteristics of root growth and distribution, monitoring the changes of water content at various depths below ground surface, monitoring and analyzing the seasonal variation of plant water status, and identifying the plant water source by stable isotope techniques. The advantages and disadvantages of these methods and their potential applications in tracing plant water source in karst region of Southwest China were discussed.