Karst Trough Valley Research Articles

Differential responses in water-use strategies of evergreen (Ligustrum lucidum) and deciduous (Robinia pseudoacacia) trees to tunnel excavation in a subtropical karst trough valley

Tunnel(s) excavation could decrease groundwater levels and soil water storage and consequently change trees growth conditions in karst areas. However, the adaptation mechanisms of different tree species to altered hydrological conditions caused by tunnel(s) construction in karst areas remain unknown. Here, we investigated the water-use strategies of two different tree species (the evergreen Ligustrum lucidum and deciduous Robinia pseudoacacia) in response to tunnel excavation in a typical subtropical karst trough valley by using the sap flow density, stable isotopes (δ13C, δ2H, and δ18O) and MixSIAR models. Tunnel excavation resulted in a 3.7–6.3 % reduction in soil moisture content (SMC) in Zhongliang Mountain Karst Trough (ZMKT) Valley. Both tree species demonstrated reduced transpiration in tunnel-affected valley (TAV) compared to tunnel-unaffected valley (TUAV) (transpiration in TAV was only 77.6 % of TUAV), with evergreen trees exhibiting a smaller decline in transpiration compared to deciduous trees. Furthermore, the proportions of karst groundwater utilized by evergreen and deciduous tree species in TAV were 8.3 % and 5.0 % higher than in TUAV, respectively, with evergreen tree species exhibiting greater absorption of karst groundwater than deciduous tree species. Meanwhile, the average leaf δ13C values and WUEi (intrinsic water-use efficiency) of both tree species in TAV were 0.9 ‰ and 15.7 μmol/mol higher than those in TUAV, respectively, with evergreen trees exhibiting a smaller increase in WUEi compared to deciduous trees. These results indicate that tunneling-induced water stress events may prompt trees to adopt more conservative water-use strategies. Evergreen tree species rely on more karst groundwater absorption to sustain relatively higher transpiration rates, and deciduous trees adapt to severe water stress by increasing their WUEi or shedding leaves to minimize transpiration. Our finding contributes to the evaluation of eco-hydrological processes in karst areas affected by tunnel(s) excavation.

Differences in soil water movement between the dip and anti-dip slopes of a karst trough valley

Exploring temporal and spatial variations in soil water processes is crucial for identifying the ecological water supply capacity of soil and understanding hydrologic cycles within karst catchments. However, the existing body of knowledge related to soil water dynamics across dip and anti-dip slopes in karst landscapes remains limited. To address this gap, this study investigated soil water dynamics in a karst trough valley (KTV) in southwest China, using stable isotopes (δD and δ18O) from eight sampling sites and precipitation data from April 2017 to November 2018. Our findings suggest that seasonal variations in soil water isotopes are primarily influenced by climatic factors, with soil water lagging behind rainwater isotopes by one to four months due to differences in the residence time of precipitation in different soil layers. Spatially, our results reveal the coexistence of diffuse and preferential flows on both dip and anti-dip slopes. Specifically, on the dip slope, preferential flow dominates above 20 cm, while diffuse flow prevails below this depth. In contrast, on the anti-dip slope, preferential flow is prominent above 40 cm depth, while diffuse flow dominates beneath this threshold. These spatial patterns not only impact the ecological water and nutrient supply capacity of soil but they also contribute to distinct patterns in vegetative growth on both slopes. In turn, vegetation growth facilitates preferential flow pathways, establishing a positive feedback loop between soil water movement and vegetative cover. These findings may lead to a deeper understanding of soil water processes on the dip and anti-dip slopes of KTVs and provide crucial insights into the relationship between soil water and vegetative growth in similar karst landforms.

Soil quality assessment of karst trough valley under different bedrock strata dip and land-use types, based on a minimum data set

Typical bedrock strata dips and land use types can significantly influence the soil quality in karst trough valleys. However, data on the influence of bedrock strata dips on the assessment of soil quality based on the soil quality index in karst trough valleys are limited. Thus, the aim of this study was to investigate the impact of different land use types and bedrock strata dips on soil quality and to evaluate the suitability of soil quality assessment methods for karst trough valleys. Five land-use types were selected as research objects for each dip and anti-dip slope, and a total data set (TDS) consisting of soil physico-chemical properties and soil microbial property indices was collected. The minimum data set (MDS) was constructed using principal component analysis (PCA) and norm values, and two evaluation methods, i.e., linear (SQI-L) and non-linear (SQI-NL) were used to assess the soil quality in the study area. The results showed that the order of weight of the MDS was urease activity > protease activity > silt > soil bulk density > total phosphorus. The overall soil quality obtained from the SQI-L and SQI-NL models exhibited similar results for the dip slope; however, there were differences in the details of the anti-dip slope. The soil quality of the anti-dip slope was better than that of the dip slope (P < 0.05) and the soil quality indices of the forest and grassland were significantly higher than those of the other land use types (P < 0.05). Furthermore, the SQI-NL model can accurately replace the SQI-L model for soil quality evaluation based on its high variation interval and variable coefficients. Overall, our results suggest that the SQI-NL model is suitable for soil quality evaluation on dip and anti-dip slopes, and returning farmland to forest is an effective land-use change in the karst trough valleys.

Spatial-Temporal Heterogeneity of Ecosystem Service Value Driven by Nature-Human Activity-Policy in a Representative Fragile Karst Trough Valley, SW China

Most studies on the ecosystem service value (ESV) only focus on spatial/temporal heterogeneity or single driving effects, but little is known about the combined effects of nature-human activity-policy on ESV in the fragile karst areas. This study aimed to investigate the spatial-temporal heterogeneity of ESV between 1990 and 2020 in a representative karst trough valley in SW China. The dynamic degree of land use, the land-use transfer matrix, sensitive analyses, Geo-Detector, and Hot- and cold-spots analyses were used to determine the interactions between ESV and the natural, human activity, and policy-driving factors. The results showed that from 1990 to 2020, forestland accounted for more than 62.32% of the total area, and construction land increased continuously as a result of urbanization. The conversion from forestland to cropland dominated the land-use transfer. The ESV decreased sharply between 1990 and 2000 and slowly increased from 2000 to 2020, causing a total reduction of 562.91 million yuan in ESV, due largely to the occupation of forestland and grassland by cropland. Soil type (8.00%) had the largest explanation rate for the spatial-temporal heterogeneity of ESV, followed by population density (7.71%) and altitude (6.34%). Policy factors not only markedly influenced the ecosystem structure and function and tradeoff and synergy but also regulated their interactions with other driving factors. Our results have great significance for understanding the interaction effect of natural sources and human activities on changes in ESV in karst areas.

Effects of the angle between the rock strata and slope on flow hydraulics and sediment yield in karst trough valley: Laboratory scour experiment

The angle between the exposed bedrock and slope in a karst trough valley strongly affects overland flow behavior, thus, altering the generation of runoff and sediment. However, the directions and magnitudes of these effects remain unclear. The current study aimed to investigate the effects of the angle between the rock and slope on flow hydraulics, runoff, and sediment yield, as well as their linkages. A laboratory flume scour experiment was done under different combinations of angles between rock strata and slope (0°, 30°, 60°, 90°, 120°, and 150°), flow discharge rates (5, 7.5, and 10 L/min) and slope gradients (10°, 15°, and 20°) to simulate the field environment in a karst trough valley. The flow hydraulics (Reynolds number, flow velocity, flow depth, and Darcy–Weisbach friction factor), sediment yield, sediment concentration, and runoff rate were determined. The results showed that with increasing scour duration, the Reynolds number and flow velocity decreased but the flow depth and Darcy–Weisbach friction factor increased. The angle between the rock strata and slope significantly affected the flow velocity and Darcy–Weisbach friction factor, while slightly affecting the flow depth and Reynolds number. Over the scour duration, the runoff rate first increased and remained at a steady state but the sediment concentration and sediment yield rate first increased and then sharply decreased to a steady state. The sediment concentration and sediment yield rate both significantly decreased as the angle between the rock strata and slope increased. The runoff rate was closely linked to the flow velocity and Darcy–Weisbach friction factor as well as the sediment yield rate and sediment concentration. The current results indicate that the angle between the rock strata and the slope should be considered an important parameter when developing a soil erosion prediction model for karst trough valleys.

Dynamic assessment of restored ecosystem health based on pressure‐state‐response subsystem‐system model in a karst trough valley

AbstractLimited soil and water resources in a sloping karst trough valley have negatively affected the local population and ecosystem health. A deeper understanding of karst ecosystem health and its driving mechanism are important for ecosystem management and restoration. In recent decades, the karst trough valley has been exposed to multiple threats from various natural and anthropogenic factors. Assessment of which is essential to understand the state of the karst ecosystem and to develop a suitable, scientifically proven management strategy. Based on the pressure‐state‐response‐subsystem‐system (PSR‐SS) model, the restored ecosystem health of karst trough valley was assessed from 2000 to 2015 at the grid scale. The principal component analysis (PCA) method was adopted to explore the factors driving ecosystem health change. It is found that from the years of 2000 to 2015, there was an improvement in ecosystem health in karst trough valley ecosystem. Land use analysis maps indicate a slight decrease in woodland and grassland and increase in cropland and urban buildup, and the Land coverage increased from 84.87% to 87.64%. The area of karst rock desertification decreased by 75.3 km2 from 2000 to 2015. The AEH indices of 0.27 (2000), 0.45 (2005), 0.46 (2010) and 0.68 (2015), which indicate moderate warning, pre‐warning, early warning and generally healthy, respectively. The 17 selected driving factors were grouped into two broad categories by PCA, one is national policy, and the other one is natural condition and local activities. National policy was to be found the most important influence on ecosystem health. Our findings emphasize the key role of government in ecosystem health change, which would give directions to the sustainable development of karst trough valley areas.

Responses of soil aggregate stability and soil erosion resistance to different bedrock strata dip and land use types in the karst trough valley of Southwest China

Soil aggregate stability is an important index that reflects soil quality and anti-erosion ability and strongly affects soil processes and functions. Bedrock strata dips (dip and anti-dip slopes) and land use types primarily influence soil aggregate stability, whereas the detailed mechanisms are unclear in karst trough valley. Therefore, to explore the effects of bedrock strata dip and land use type on soil aggregate stability in karst trough valleys, soils were collected from five major land use types (abandoned land, grassland, pepper fields, corn fields and forest) on dip and anti-dip slopes. The soil was fractionated into macroaggregates and microaggrates using dry and wet sieving analysis. The soil particle size distributions in the macroaggregates and microaggregates were measured in conventional laboratories. The results showed significant differences in soil aggregate stability among different bedrock strata dips, slope positions, and land use types (P < 0.05). The variation ranges of macroaggregates and microaggregates in the pepper fields of the dip slope were higher than those on the anti-dip slope. Comparing all land use types, the forest of the anti-dip slope had >0.25 mm water-stable aggregates (85.31%) and mean weight diameter (2.67 mm) on the upper slope compared to that in the other slope positions of the dip slope. In addition, the dip slope had a higher percentage of aggregate destruction (35.57%) than the anti-dip slope (29.81%), and the soil erodibility factor value of the natural forest of the dip/anti-dip slope was significantly lower than that of the other land use types (P < 0.05). When the content of large macroaggregates was larger, the soil macroaggregate weight was greater. When the failure rate of the soil aggregates was lower, the stability of the soil structure was better. Overall, these results suggest that natural forests can significantly improve the stability of soil aggregates, thereby improving soil erosion resistance. Therefore, natural recovery measures should be implemented on dip/anti-dip slopes of karst trough valleys.

Declining trees growth and vegetation productivity resulting from decreasing soil water contents induced by tunnels excavation in karst mountain areas

Tunnel construction in karst areas greatly impacts the local hydrological conditions. The drawdown of groundwater levels and the loss of soil water storage due to tunneling limit the available water resources on the karst land surface, and this may affect the function of plants. Here, we evaluated the responses of the radial growth rates of two major tree species (Masson pine and Camphor laurel) to a tunneling induced water stress event in a typical karst trough valley. Meanwhile, to discuss the overall influence of tunnel excavation on plant growth in all vegetation cover types, the dynamics of vegetation productivity around the tunnel were evaluated using remote sensing indices, the Normalized Difference Vegetation Index (NDVI), and Net Primary Productivity (NPP). Furthermore, we estimated the spatial–temporal variations of NDVI and NPP in other tunneling regions of Southwest China. The results showed that the soil moisture contents (SMC) around the Zhongliang Mountain Railway Line 6 (ZMRL6) tunnel had dramatically declined −5.06%∼−10.53% within the areas < 1500 m. After the excavation of ZMRL6 tunnel, the basal area increment (BAI) for the tree-ring widths of Masson pine and Camphor laurel within 1500 m of the tunnel central reduced by 94.8%∼135.8% and 116.6%∼139.9%, respectively, while the trees beyond this distance range were less influenced. Meanwhile, the remote sensing indices showed that tunnel excavation can reduce 3.3%∼7.6% of the NDVI and 15.7%∼17.4% of the NPP within 1500 m of the ZMRL6 tunnel. A similar distance range of such declines in NDVI and NPP was also found in other tunneling regions throughout the karst mountain areas of Southwest China. The findings in this study suggest that tunnel excavation may cause severe ecosystem degradation within 1500 m of the tunnel center, which should be considered in ecosystem management and conservation strategies in karst mountain areas.

Effects of bedrock strata dip on soil infiltration capacity under different land use types in a karst trough valley of Southwest China

The dip of bedrock has a great influence on the infiltration of water into soil, and can control the spatial distribution of rainwater in fragile karst rocky desertification regions. Thus, it is necessary to scientifically assess the soil infiltration capacity of dip and anti-dip slopes in karst trough valleys. In this study, we comprehensively evaluated the soil infiltration capacity of dip and anti-dip slopes under different land use types using the cutting ring infiltration method. The results showed the following. i) Compared with the anti-dip slope, the dip slope had a higher bulk density, noncapillary porosity and pH and a lower soil water content, capillary porosity and soil organic carbon content (p < 0.05). In addition, the soil infiltration capacity of grassland was the highest among the studied land use types except for abandoned land. ii) Both the bulk density and the total soil porosity could be treated as indicators of the infiltration properties of dip and anti-dip slopes.iii) Based on the coefficient of determination, R2, the common experience model was the most suitable for describing the characteristics of the soil infiltration rate. iv) Based on the principal component equation, the soil infiltration capacity and organic matter of the grassland on dip slopes and anti-dip slopes were higher than those of other land use types with vegetation, but not higher than those of abandoned land without vegetation. The results of this study provide a reference for the selection of the restoration method and the assessment of the soil infiltration capacity of dip and anti-dip slopes in karst trough valleys and other karst landforms with the same geological background.

Response of Runoff Change to Extreme Climate Evolution in a Typical Watershed of Karst Trough Valley, SW China

Identifying the response of runoff changes to extreme climate evolution was of great scientific significance for the rational regulation of watershed water resources and the prevention of hydrological disasters. However, the time–frequency response relationships were not clear. The Yinjiang River watershed, a typical watershed with karst trough valley areas, was chosen to identify the impact of different climatic driving factors on runoff changes from 1984 to 2015. Continuous wavelet transform (CWT), cross-wavelet transform (XWT), and wavelet coherence transform (WTC) were performed to study the response relationship and time–frequency effect between runoff changes and extreme climate change at different time scales. The main results showed that: (1) Twelve extreme climate indices (ECIs) were detected to have a significant impact on runoff changes, mainly on a 6-year time scale; (2) The R10 and Rx1day in extreme precipitation index and SU34.4 and TNx in the extreme temperature index were the main driving factors of runoff changes, which had relatively large impacts on runoff changes in high and low energy vibration regions. However, the remaining eight ECIs that passed the 0.05 confidence level showed relatively large impacts on runoff changes only in low energy vibration regions; (3) The transition of the interaction between ECIs and runoff changes in high and low time–frequency scales was related to the abrupt change characteristics of the ECIs. The correlation of abrupt change was an important reason for the emergence of highly correlated regions that trigger high and low energy vibrations; (4) As a whole, the extreme precipitation events were ahead of runoff changes at the high time–frequency scale and exhibited small lag effects at the low time–frequency scale, while extreme temperature events were mainly ahead of runoff changes. This study has effectively revealed the impact of climate factors at different scales on runoff changes, and provides a theoretical understanding for regulating and managing water resources in karst basins.

Effects of vegetation succession on topsoil C, N, and P contents and stoichiometry following agricultural abandonment in a representative karst trough valley

Soil carbon (C), nitrogen (N), and phosphorus (P) are significantly altered by vegetation succession under execution of ecological restoration projects in China. However, the changes in soil C, N, and P contents and their molar ratios as affected by vegetation succession are elusive in fragile karst ecosystems. With space-for-time substitution, this study compared the soil C, N, and P contents and their molar ratios between different vegetation successional stages in a representative karst trough valley, Southwest China. Soil samples from four positive vegetation successional stages, including herb stage (HE), hern-shrub stage (HES), shrub stage (SH), and secondary forest stage (SF), were collected to determine the topsoil C, N, and P contents and molar ratios, and selected soil physicochemical properties. The results showed that soil C and N contents increased by 158.4% and 39.6% from HE to SF, respectively, while soil P content was not significantly affected by vegetation succession. From HE to SF, soil C:N and C:P ratios increased by 82.1% and 134.2%, but soil N:P ratio vary insignificantly, respectively. Soil bulk density (BD) and pH increased but electrical conductivity (EC) decreased markedly from HE to SF. Soil exchangeable K+, Ca2+, and Mg2+ contents did not differ among four successional stages. Soil pH, BD and exchangeable K+ were closely correlated with C and P contents, C:P, and N:P in different successional stages. Our results imply that vegetation succession driven by ecological restoration considerably increases topsoil C and N accumulations but equalizes P in the karst trough valley.

Elaborate simulations of floods in a karst trough valley basin with the simplified Karst-Liuxihe model

The karst trough valley basin in Zhongliang of Chongqing is one of the most developed karst areas in Southwest China, with unique topographic features such as karst troughs and valleys that are prone to flash floods. Accurate simulation of flood processes in these areas using hydrological models can provide references for predicting future local runoff evolution trends. The main challenge for hydrological modelling in karst areas lies in building a model with limited hydrogeological data. In this study, the Karst-Liuxihe (KL) model was simplified and improved to simulate karst flood processes in the Zhongliang karst trough valley. The KL model consists of multiple, complex structures; a large amount of data is needed to model karst areas. To overcome the modelling data requirements, we modified the model structure and its parameters to propose a simplified KL (SKL) model. Additionally, we improved the runoff generation and confluence algorithms. Compared with the simulation results for 22 flood processes obtained by using the KL model, the results for this SKL model were better with less modelling data. For example, the mean value of the Akaike information criterion (AIC) index decreasing rate was 35%, and the process relative error and the peak flow relative error decreased by 10%, indicating that the technique of simplifying and improving the KL model is effective and that the simplified SKL model can quantitatively depict karst floods in karst trough valley areas.

Analysis of the Land Use Dynamics of Different Rural Settlement Types in the Karst Trough Valleys of Southwest China

Rural settlements are the basic spatial units of rural geography research, and it is essential to explore the dynamic changes in land use on a rural settlement scale to promote the development of the rural revitalization strategy. The study took different rural settlement types in karst trough valleys as examples and applied geographic information mapping trajectory models, buffer zone spatial analysis, the nearest neighbor index, and other research methods. We explored the land use dynamic change in the buffer zone of different settlement types in the karst trough valley from 1964 to 2021 in the long time series and micro-spatial dimensions. We analyzed the homogeneity, variability, and coupling characteristics of land use evolution in typical settlements. The results indicate the following: (1) From 1964 to 2021, the karst trough valley settlements as a whole showed an aggregation state, and the settlements could be classified into four categories: expanding settlements (ES), atrophic settlements (AS), balancing rural settlements (BS), and decreasing settlements (DS) according to the settlement life cycle theory and settlement development index measurement. (2) Different expansion and shrinkage of land use buffer changes exist for different settlement types. The closer the ES is to the location of the settlement center, the richer the land use type; the further the AS from the settlement center, the richer the land use type; the BS is not affected by the distance; and the DS settlement shows dynamic changes. (3) Land use dynamic change in settlements is driven by multiple integrated factors, and there is variability in the driving factors of different settlement types. (4) In this paper, through a case study, we propose the research idea that land use change (LUCC) reflects land use transformation (LUT) in different rural settlement types from a settlement-scale perspective, and land use transformation further causes the development of rural settlement transformation (RUT). Our study revealed the LUCC—LUT—RUT interaction feedback mechanism of karst trough valley settlements in Southwest China. This study aims to enrich the theoretical research framework of rural transformation at the settlement scale, on the one hand, and to provide case studies for developing countries with karstic mountain valley landscapes, such as China, on the other.

A physically based distributed karst hydrological model (QMG model-V1.0) for flood simulations

Abstract. Karst trough and valley landforms are prone to flooding, primarily because of the unique hydrogeological features of karst landforms, which are conducive to the spread of rapid runoff. Hydrological models that represent the complicated hydrological processes in karst regions are effective for predicting karst flooding, but their application has been hampered by their complex model structures and associated parameter set, especially for distributed hydrological models, which require large amounts of hydrogeological data. Distributed hydrological models for predicting flooding are highly dependent on distributed modelling, complicated boundary parameter settings and extensive hydrogeological data processing, which consumes large amounts of both time and computational power. Proposed here is a distributed physically based karst hydrological model known as the QMG (Qingmuguan) model. The structural design of this model is relatively simple, and it is generally divided into surface and underground double-layered structures. The parameters that represent the structural functions of each layer have clear physical meanings, and fewer parameters are included in this model than in the current distributed models. This allows karst areas to be modelled with only a small amount of necessary hydrogeological data. Eighteen flood processes across the karst underground river in the Qingmuguan karst trough valley are simulated by the QMG model, and the simulated values agree well with observations: the average values of the Nash–Sutcliffe coefficient and the water balance coefficient are both 0.92, while the average relative flow process error is 10 % and the flood peak error is 11 %. A sensitivity analysis shows that the infiltration coefficient, permeability coefficient and rock porosity are the parameters that require the most attention in model calibration and optimization. The improved predictability of karst flooding enabled by the proposed QMG model promotes a better mechanistic depiction of runoff generation and confluence in karst trough valleys.

Root tensile strength of terrace hedgerow plants in the karst trough valleys of SW China: Relation with root morphology and fiber content

Root tensile strength is commonly affected by root morphology and fiber content, which combinedly affect the effectiveness of terrace hedgerow on controlling soil erosion in sloping farmland. However, the relationships between these root characteristics are still elusive. This study aimed to compare the root tensile strength, root morphologies, and root fiber contents, and quantify their differences among different plant species. Complete root systems of three terrace hedgerow plant species, i.e., O. bodinieri, V. villosa, and D. lablab L. were sampled for detecting root morphologies and fiber contents at different slope positions on a representative hillslope in karst trough valley. Single root specimens were collected to measure root tensile properties for these three plant species. Results showed that most root morphological parameters were the highest in O. bodinieri, followed by D. lablab L. Three plant species presented the same dominant diameter class as < 1.0 mm, and root volumes as < 1.50 cm3. Cellulose with contents closing to 50% was the main fiber type, and increased significantly linearly with the increasing root diameter in both O. bodinieri and V. villosa. The highest tensile strength and Young's modulus were found in D. lablab L., while the best ultimate tensile force and ultimate elongation were displayed in O. bodinieri. Root diameter showed negative power relationships with tensile strength and Young's modulus, a positive power relationship with ultimate tensile force, and a positive linear relationship with ultimate elongation. Our results will deepen the understanding of the terrace hedgerow functions for controlling soil erosion worldwide.

近60年来印江河流域极端气候演变及其对净初级生产力和归一化植被指数的影响

极端气候事件的发生改变了区域水热条件，并影响着生态环境变化。然而，目前长时间尺度上极端气候的演变规律及其对生态环境的影响尚不明晰。采用Mann-Kendall趋势及突变检验法、连续小波变换和Hurst指数法揭示了喀斯特槽谷印江河流域极端气候的变化趋势、突变时间、周期性特征和未来演变规律，并利用Lindeman-Merenda-Gold模型定量评估了极端气候溶变对生态环境变化的影响。结果表明：（1）印江河流域极端气温显著上升，降雨量增多，呈现湿热多雨的气候特征。未来极端气温事件持续等级将更高，持续强度也更强。（2）同类型极端气候具有潜在的关联性，但不同类型极端气候间的影响较小，且多呈负相关。（3）印江河流域平均净初级生产力（NPP）和归一化植被指数（NDVI）在2000-2015年间呈现相反的变化趋势，NPP平均值为598.53 g C m^-2 a^-1，平均减少速率为-3.32 g C m^-2 a^-1。NDVI平均值为0.59，平均增长速率为0.0013/a。（4）冷持续指数（CSDI）、平均温差（DTR）、系统阈值结冰日数（ID6.4）和生长期长度（GSL）对NPP的贡献较大，贡献率分别12.64%、11.50%、11.05%和7.4%。其中，CSDI、DTR、GSL对NPP变化表现为负贡献，而ID6.4则表现为正贡献。大部分极端气候指数对NPP变化的贡献都不超过5%。ID6.4对NDVI变化的影响最大，暖夜日数（TN90p）、热持续指数（WSDI）和暖昼日数（TX90p）相对次之，贡献率分别为13.67%、13.54%、12.95%和10.02%。总体上看，在2000-2015年间，印江河流域湿热多雨的气候对NDVI增长有促进作用，但是由于气温升高和降雨量增加的原因，对NPP累积产生了较大的负面影响。研究结果可为喀斯特槽谷流域气候预测、水资源管理及生态环境恢复提供参考依据。;The occurrence of extreme climate events has changed the regional hydrothermal conditions and affected the changes of ecological environment. However, at present, the evolution of extreme climate and its impact on ecological environment are unclear. Therefore, the methods of Mann-Kendall test, continuous wavelet transform, and Hurst exponent were applied to analyze the change trend, abrupt time, periodicity and future evolution of extreme climate events in Yinjiang River basin. Then, the Lindeman-Merenda-Gold model was used to quantitatively evaluate the impacts of extreme climate events to ecological environment change. The results were obtained:(1) The extreme temperature events in the Yinjiang River basin increased significantly, and the rainfall also increased, showing a hot and rainy climate. In the future, the level of extreme temperature events will be higher and the intensity will be stronger; (2) The same type of extreme climate had a potential correlation, but the influence of different types of extreme climate was small, and most of them showed negative correlation; (3) The average net primary production (NPP) and normalized difference vegetation index (NDVI) of the Yinjiang River basin showed an opposite trend from 2000 to 2015. The average NPP was 598.53 g C m^-2 a^-1 with the average reduction rate of -3.32 g C m^-2 a^-1 while the average NDVI value was 0.59 with an average growth rate of 0.0013/a; (4) The cold spell duration indicator(CSDI), diurnal temperature range (DTR), icing days defined by system threshold (ID6.4), and growing season length (GSL) contributed significantly to NPP, with contributions of 12.64%, 11.50%, 11.05%, and 7.4%, respectively. Among them, the CSDI, DTR, and GSL showed negative contribution to NPP changes, while ID6.4 showed positive contribution. Most extreme climate indices contributed less than 5% to the NPP changes. ID6.4 had the greatest impact on NDVI, followed by warm nights (TN90p), warm spell duration indicator (WSDI), and warm days (TX90p), with contribution rates of 13.67%, 13.54%, 12.95%, and 10.02%, respectively. On the whole, the study stressed that the hot and rainy climate in the Yinjiang River basin promoted the growth of NDVI during 2000-2015, but caused a significantly negative impact on the accumulation of NPP due to the increase of temperature and rainfall. The results can provide reference for climate prediction, water resources management and ecological environment restoration in a basin of karst trough valley.

Mechanism and Evolution of Soil Organic Carbon Coupling with Rocky Desertification in South China Karst

To study the spatial distribution characteristics of soil organic carbon (SOC) coupled with rocky desertification, 1212 soil samples from 152 soil profiles were sampled from different karst landforms, including karst low hills/virgin forest (KLH) in Libo County, a karst peak-cluster depression (KPCD) in Xingyi County, a karst canyon (KC) in Guanling County, a karst plateau basin (KPB) in Puding County and a karst trough valley (KTV) in Yinjiang County. The spatial distribution characteristics of the responses of SOC, SOC density (SOCD), rocky desertification and soil bulk density (SBD) to different influencing factors were analyzed. The relationships among SOC, SOCD, rocky desertification and SBD were analyzed using Pearson correlation analysis. The SOC storage capacity was characterized by using SOCD, and then the SOC storage capacity in different evolution stages of karst landforms was assessed. The SOC contents of KLH, KPCD, KC, KPB and KTV ranged from 6.16 to 38.20 g·kg−1, 7.42 to 27.08 g·kg−1, 6.28 to 35.17 g·kg−1, 4.62 to 23.79 g·kg−1 and 5.24 to 37.85 g·kg−1, respectively, and their average SOCD values (0–100 cm) were 7.37, 10.79, 7.06, 8.51 and 7.84 kg·m−2, respectively. The karst landforms as ordered by SOC storage capacity were KPCD &gt; KPB &gt; KLH &gt; KTV &gt; KC. The SOC content was negatively correlated with the SBD; light rocky desertification may lead to SOC accumulation. The rocky desertification degree and SBD were closely associated with slope position and gradient. Rocky desertification first increased, then decreased from mountain foot to summit, and increased with increasing slope gradient. However, the SBD decreased from mountain foot to summit and with increasing slope gradient. The SOC contents on the northern aspect of the mountains were generally higher than the other aspects. In summary, rock outcrops controlled the SOC contents in the studied regions. The slope position, gradient and aspect influenced the composition and distribution of vegetation, which influenced the evolution of rocky desertification. Therefore, these factors indirectly affected the SOC content. Additionally, the SOCD decreased with increasing rocky desertification. During the different evolution stages of karst landforms, the SOC storage capacity first decreases, then increases.

Changes in Soil C, N, and P Concentrations and Stoichiometry in Karst Trough Valley Area under Ecological Restoration: The Role of Slope Aspect, Land Use, and Soil Depth

This study aims to investigate the roles of slope aspect, land use and soil depth in altering the soil organic carbon (C), total nitrogen (N), and total phosphorus (P) traits in the karst trough valley area experiencing extensive ecological restoration. A total of 54 soil samples were collected at 0–10, 10–20, and 20–30 cm soil depths from secondary forest, plantation forest, and grassland on the relatively more shaded east-facing slope and the contrasting west-facing slope, respectively. The independent and interactive effects of slope aspect, land use, and soil depth on soil C, N, and P concentrations and stoichiometry were determined. The results show that soil C and N concentrations were markedly higher on the east-facing slope than on the west-facing slope, and soil P concentrations showed an opposite trend, leading to significant differences in soil C:P and N:P but not in C:N ratios between the two aspects. Soil C and N concentrations were not affected by land use, and soil P concentration was significantly higher in plantation forest than in secondary forest and grassland. Soil C and N concentrations significantly decreased with increasing soil depth, but soil P concentration presented no significant changes with soil depth. Both the land use and soil depth did not differ in terms of their elemental stoichiometry. There were no significant interactive effects of slope aspect, land use and soil depth on soil C, N, and P traits. Our results indicate that soil C, N, and P changes are more sensitive to slope aspect rather than land use and soil depth in the karst trough valley area under ecological restoration.

Different responses of ecohydrological processes in the re-vegetation area between the dip and anti-dip slope in a karst rocky desertification area in southwestern China

Rocky desertification is an economic, social, and environmental problem, which refers to the processes and human activities that transform a karst area covered by vegetation and soil into a rocky landscape. The effects of different types of soil cover on ecohydrological processes during the revegetation of areas undergoing karst rocky desertification remain unclear. Taking different measures for different ecohydrological situations will benefit re-vegetation in karst rocky desertification areas. Monthly/heavy rainfall hydrochemical data and the δ13C of dissolved inorganic carbon (DIC) in karst water samples from January 2017 to December 2019 were obtained. This revealed the controlling mechanisms on hydrochemical processes by δ13CDIC under different soil cover conditions in the dip/anti-dip slopes of a karst trough valley in Ganxi Town, Chongqing City, southwestern China. Hydrochemical variations are controlled by climate, soil, and vegetation and show clear seasonality. The δ13CDIC values (−16.88‰ to −3.49‰) in the study area were higher than the theoretical value (−14.00‰). In addition, it was observed that rain accelerated soil erosion (R2=0.7) in the karst trough valley and human activities (exogenous acid) resulted in DIC loss. The slopes, soil, and bare rock cover on the dip slope were different from that on the anti-dip slope, leading to the difference in their re-vegetation statuses. In this study, we found that different vegetation restoration schemes and treatment measures should be implemented between the dip and anti-dip slope. The protection of soil and water resources, especially soil, is the key to ecological restoration in rocky desertification areas.

Spatio-temporal distribution characteristics and driving mechanism of rural residential areas in karst trough valley area

Spatio-temporal distribution characteristics and driving mechanism of rural residential areas in karst trough valley area