[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.

Water source identification for plants growing on karst rock outcrops based on rainfall-exclusion experiment.

Although lack of soil coverage, rock outcrops with developed fractures in karst region can maintain water consumption of plants with different life forms. Water sources for plants on these habi-tats are unclear. Isolated rocky outcrop with relatively simple sources of water was selected for this study. We focused on typical plant species that still thrived after excluding rainfall (removing the water supply to the shallow water) over one year, compared with the same plant species living without rain shelter (always receiving rainfall supplies). Water sources of three representative tree species (Radermachera sinica, Celtis biondii, and Pittosporum tonkinense) were analyzed by using stable isotope techniques and combining with the measurement of plant water potential. The results showed that all the three species depended on deep water sources with similar isotopic values to spring water under rain-sheltered condition, during the rainy season, which explained why plants could still grow normally after rainfall-exclusion over one year. The predawn water potential of R. sinica and P. tonkinense under rain-sheltered condition was not significantly different from those living in natural conditions, which indicated both species were not under water stress. The predawn water potential of C. biondii under rain-sheltered condition was significantly lower than individuals living in natural conditions, which indicated it was under water stress. Under natural condition, water isotope values of stems of all the three species were significantly lower than that under rain-sheltered condition and were within the range of fluctuation of recent rainwater isotope values, indicating that these plants relied on shallow water sources that dominated by recent rainfall. Under both rain-sheltered and natural conditions, there was no obvious difference between the predawn water potential and the midday water potential of P. tonkinense, showing a conservative water use strategy. The midday water potential of other two species was significantly lower than the predawn water potential, showing a profligate/opportunistic water use strategy. Our results indicated that the ability to utilize shallow and deep water sources is key for the plants growing on the habitat of Karst rock outcrops where they could adapt different water environments and maintain diversified water use strategies under the condition with no soil coverage.

[Responses of sap flow to natural rainfall and continuous drought of tree species growing on bedrock outcrops]. / å‡ºéœ²åŸºå²©ç”Ÿå¢ƒå ¸åž‹æ¤ç‰©æ ‘å¹²æ¶²æµå¯¹è‡ªç„¶é™æ°´å’Œè¿žç»­å¹²æ—±çš„å“åº”ç‰¹å¾.

This study focused on bedrock outcrops, a very common habitat in karst region of southwest China. To reveal the responses of plant transpiration to natural rainfall and continuous drought, two tree species typical to this habitat, Radermachera sinica and Triadica rotundifolia, were selected as test materials. A rainout shelter was used to simulate continuous drought. The sap flow dynamics were monitored using the method of Granier's thermal dissipation probe (TDP). Our results showed that sap flow density increased to different degrees after rain in different stages of the growing season. Sap flow density of the deciduous species T. rotundifolia was always higher than that of the semi-deciduous species R. sinica. After two months without rainfall input, both species exhibited no obvious decrease in sap flow density, indicating that rainfall was not the dominant source for their water uptake, at least in the short-term. Based on the regression relationships between sap flow density and meteorological factors before and after rainfall, as well as at different stages of continuous drought, we found that the dynamics of meteorological factors contributed little to plant transpiration. The basic transpiration characteristics of both species were not changed in the circumstance of natural rainfall and short-term continuous drought, which would be closely related to the special water storage environments of bedrock outcrops and the reliance on deep water sources by tree species.

[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.

[Water use strategy of Eucalyptus urophylla ×E. grandis on karst hillslope based on isotope analysis.] / åŸºäºŽç¨³å®šåŒä½ç´ çš„å–€æ–¯ç‰¹å¡åœ°å°¾å·¨æ¡‰æ°´åˆ†åˆ©ç”¨ç‰¹å¾.

Using stable isotope techniques, water sources and water use efficiencies of Eucalyptus urophylla ×E. grandis (exotic tree species) and Liquidambar formosana (native tree species, as a reference) were studied in a typical karst artificial forest, and the risk of drought stress of the plantation was discussed. The results showed that the isotope value of shallow soil water (0-50 cm) had obvious gradient features and was similar to the recent rain, while that of deep water was more stable and different from the shallow soil water. The soil water content in wet season (May and September) was higher than in dry season (October), and was higher in upslope than in downslope segment. The main water source of L. formosana was shallow soil water, which corresponded to high water use efficiency in different seasons. The main water sources for E. urophylla ×E. grandis on the upper slope and downslope segments were both shallow soil water in wet season, however, the sum of average water using the proportion of deep soil water increased for the downslope segment in the end of wet season. In dry season, E. urophylla ×E. grandis on the downslope segment mainly used shallow soil water, while it used deeper soil water on the upslope segment when soil water was relatively low. Meanwhile, the water use efficiency of E. urophylla ×E. grandis was lower compared with L. formosana, which revealed E. urophylla ×E. grandis did not get enough water supply under drought stress. The above results indicated that E. urophylla ×E. grandis would encounter high risk of water stress if extreme drought events happened in karst regions in the coming future.

[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.