Karst Region Of Southwestern China Research Articles

Vegetation cover change and its response to human activities in the southwestern karst region of China

Vegetation to some extent can reflect the overall state of the ecological environment, and increasing vegetation cover can improve the quality of the ecological environment. The southwest karst region of China is a typical ecologically fragile area and an important ecological barrier. Against the backdrop of intensified human activities, the vegetation dynamics in the karst region have attracted widespread attention. Analyzing vegetation cover changes and their responses to human activities is important for regional ecological development. This study is based on SPOT-VGT NDVI data from 1998 to 2020 and employs trend analysis, “baseline” determination, multiple regression, residual analysis, and principal component analysis to investigate the characteristics of vegetation cover changes in the southwest karst region of China. Additionally, it quantifies the specific impacts of various human activities on these changes. The research findings indicate that (1) over the past 23 years, the interannual variability and seasonal differences in the NDVI of the southwest karst region have been significant, exhibiting an overall increasing trend. Spatially, the vegetation in this region demonstrates a decreasing trend from south to north and southwest to northeast. (2) Over time, the areas where vegetation has improved gradually expand, and the effectiveness of vegetation restoration increases with time. (3) Overall, human activities influence vegetation growth in the southwest karst landscape region of China. (4) The socioeconomic factors affecting vegetation cover changes in the southwest karst region of China primarily include economic development level, population and labor migration, urbanization development, and vegetation construction. These results provide a scientific basis for formulating reasonable ecological environment protection strategies by humans.

Estimating Forest Aboveground Biomass Combining Pléiades Satellite Imagery and Field Inventory Data in the Peak–Cluster Karst Region of Southwestern China

The mountainous region of southwest China has the largest karst geomorphology in China and in the world. Quantifying the forest aboveground biomass in this karst region is of great significance for the investigation of carbon storage and carbon cycling in terrestrial ecosystems. In this study, the actual measured aboveground biomass was calculated based on the allometric functions of 106 quadrats from 2012 to 2015. A backpropagation artificial neural network (BPANN) inversion model was constructed by combining very high-resolution satellite imagery, field inventory data, and land use/land cover data to estimate the forest aboveground biomass in the Banzhai watershed, a typical peak–cluster karst basin in southern Guizhou Province. We used 70% of the actual measured aboveground biomass for training the BPANN model, 20% for accuracy verification, and 10% to prevent overtraining. The results show that the absolute root mean square error of the BPANN model was 11.80 t/ha, which accounted for 9.92% of the mean value of aboveground biomass. Based on the BPANN inversion model, the average value of the forests’ aboveground biomass was 135.63 t/ha. The results showed that our study presented a quick, easy, and relatively high-precision method for estimating forest aboveground biomass in the Banzhai watershed. This indicates that the Pléiades image-based BPANN model displayed satisfactory results for estimating the forests’ aboveground biomass in a typical peak–cluster karst basin. This method can be applied to the estimation of forest AGB in the karst mountainous areas of southwest China.

Spatiotemporal pattern of vegetation water use efficiency between 2003 and 2017 and its coupling relationship with artificial carbon sequestration in the karst region of Southwestern China

After extensive ecological restoration in the karst region of Southwestern China, a potential zone for achieving “carbon neutrality” has emerged, despite facing water scarcity. We analyzed the dynamics of water use efficiency (WUE) and its correlations with soil moisture (SM) and leaf area index (LAI) from 2003 to 2017 using PML-V2 data and multiple datasets for SM and LAI. Advantageous areas for artificial carbon sequestration (CS) were also identified. Key findings are as follows: (1) Temporally, WUE exhibited fluctuating growth at an annual rate of 0.01 gCmm−1H2O (P < 0.05). The advantage areas for CS accounted for 15.96%, with over 31.74% of regions needing management intervention. (2) Monthly WUE was highest in the peak forest plain (PFP) landform (2.88 gCmm−1H2O), while PFP experiencing an annual decrease of −0.0021 gCmm−1H2O. (3) Monthly WUE was highest in forests (2.88 gCmm−1H2O), followed by shrubs (2.49 gCmm−1H2O), farmland (2.32 gCmm−1H2O) and grassland (1.93 gCmm−1H2O), while farmland showing the highest rate of increase (0.02). (4) WUE and SM exhibited both positive (14.26%∼26.02%) and negative (14.19%∼30.98%) correlations. In positive areas, WUE decreased as the drought stress index (DSI) increased for all vegetation types. Clear DSI threshold observed for farmland (0.29 to 0.42) and grassland (0.19 to 0.30). However, the peak values for forests and shrubs were less pronounced. There was a transitional point in the negative impact of DSI on WUE, which increased at a faster rate for all vegetation types when DSI exceeded 0.53. (5) WUE and LAI exhibited both positive (27.09%∼30.25%) and negative (23.37%∼34.57%) correlations. In positive areas, WUE increased as LAI increased for all vegetation types. However, forests exhibited threshold LAI values of 1.85 based on MODIS data, 2.71 based on GLASS data, and 2.59 based on GEOV2 data, shrubs had values of 1.69, 2.66, and 2.23, respectively. While grassland displayed LAI values of 0.79, 0.70, and 0.72, respectively. In negative areas, WUE decreased as LAI increased for all vegetation types. The minimum WUE values were observed when the LAI of farmland was 3.14 based on MODIS data, 4.05 based on GLASS data, and for grassland, it was 1.04, 1.97, and 1.76 based on the GEOV2 data, respectively. This study helps us to identify areas with potential for enhancing CS. It also assists in making informed decisions regarding the implementation of ecological restoration initiatives by considering SM as the limiting factor for adjustment measures and utilizing LAI as the reference standard.

Ecological Stoichiometry and Stock Distribution of C, N, and P in Three Forest Types in a Karst Region of China.

Ecological stoichiometry plays important roles in understanding the nutrient constraints on tree growth and development, as well in maintaining ecosystem services in forests, yet the characteristics of carbon:nitrogen:phosphorous (C:N:P) stoichiometry in forests under karst environment have not been sufficiently evaluated. In this study, concentration, distribution, stocks of Nitrogen (N) and Phosphorous (P), and ecological stoichiometry were studied in three common forest types: Masson pine natural forests (MPNF), Masson pine plantation forests (MPPF), and Slash pine plantation forests (SPPF) in a karst region of southwestern China. Results showed that N concentrations were higher in overstory than in understory and litter in the studied forests. However, P concentration was relatively low in overstory component of the forested ecosystems. Meanwhile, the N and P concentrations were higher in SPPF in the stem and litter, while these contents were higher in MPPF and MPNP in the overstory and understory. The N and P stocks ranged from 5.7-6.2 t ha-1, and 0.5-0.6 t ha-1 in the examined forests. The ecological stoichiometry of C:N:P in the three forest types was similar in litter (46-49:2:1), and relatively steady in soil (250-320:13-16:1) and tree leaf (100-200:14-20:1). Soil P status was the primary limiting factor in affecting tree growth in MPPF and SPPF (N:P ratio > 16), while both N and P conditions were the main restrictive factors in MPNP (N:P ratio = 15) in the study area. Our study provides scientific references and useful datasets of C:N:P stoichiometry for sustainable management of forest ecosystems in karst regions.

Continuous Cropping Alters Soil Hydraulic and Physicochemical Properties in the Karst Region of Southwestern China

Continuous cropping causes soil degradation and decreases crop yield in the karst region of southwestern China. However, the relationship between continuous cropping systems and soil hydraulic and physicochemical properties remains incompletely elucidated. In this study, we performed a comparative investigation on the soil physicochemical properties and soil-water-characteristic-curve-derived parameters from sites subjected to 3, 5, or 7 years of continuous cropping (CC3, CC5, and CC7) and cropping rotation (CC0). Soil organic matter content, clay content, and pH were significantly greater in soils under CC0 and short-term cropping (CC3) than in soils under long-term cropping (CC5 and CC7). This finding illustrated that continuous cropping reduced soil organic matter content, clay content, and pH. Across all continuous cropping durations, soil water holding capacity at 40~60 cm was greater than the 20~40 cm and 0~20 cm layers. The significantly greater soil water characteristics (except saturated moisture) in CC0 and CC3 soils than in CC5 or CC7 soils at all soil depths demonstrated that soil water characteristics deteriorated with the prolongation of cropping duration. The same soil water characteristics were positively correlated with soil organic matter content, clay content, and pH. These correlations, when viewed within the context of continuous cropping, can inform the development of more sustainable cropping systems in similar karst regions.

桢楠幼苗适应喀斯特岩溶裂隙生境及降雨时间格局变化的方式

PDF HTML阅读 XML下载 导出引用 引用提醒 桢楠幼苗适应喀斯特岩溶裂隙生境及降雨时间格局变化的方式 DOI: 10.5846/stxb202112183590 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 重庆市自然科学基金面上项目（cstc2020jcyj-msxmX0244）；中央高校基本科研业务费专项（XDJK2020B037）；国家自然科学基金项目（31500399） Phoebe zhennan S. Lee seedlings adjust the biomass allocation and root distribution to adapt to the karst fissure habitat and rainfall temporal pattern Author: Affiliation: Fund Project: General Project of Chongqing Natural Science Foundation(cstc2020jcyj-msxmX0244);Special Funds for Fundamental Scientific Research Operation of Central Universities(XDJK2020B037);National Natural Science Foundation of China(31500399) 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:喀斯特生态系统是全球陆地生态系统的重要组成部分，生态环境极为脆弱。由于碳酸盐岩长期强烈的化学溶蚀作用，其基本特征体现为地表土壤和地下岩溶裂隙及洞穴的二元结构。近年来，在全球气候变化下，我国西南地区降雨格局呈现降雨频次减少且单次降雨量增加的趋势。因此，岩溶裂隙和区域降雨时间格局改变将对喀斯特地区的植物生长产生重要影响。通过模拟不同岩溶裂隙生境（S0：24 cm土壤；S1/2：12 cm土壤层+12 cm裂隙层；S3/4：6 cm土壤层+18 cm裂隙层）和不同降雨时间格局（I2d：2 d降雨间隔；I19d：19 d降雨间隔），探究二年生桢楠（Phoebe zhennan S.Lee）幼苗是否通过生物量分配及根系分布的调整来适应变化环境。结果显示：（1）短时间降雨格局下，相比全土生境，少量岩溶裂隙存在并不影响桢楠幼苗生物量的积累，然而随着岩溶裂隙层进一步增厚和降雨时间间隔延长，桢楠降低了总生物量，减少了茎且增大了根和叶的生物量分配。（2）桢楠幼苗的根系垂直分布随着深度增加而下降，且无论在何种降雨时间格局下，两种岩溶裂隙生境下桢楠均增加了岩溶裂隙之上土壤层的根系生物量分配比例。研究表明：赋存有土壤的岩溶裂隙能成为植物幼苗赖以生存的生境，但这种提供生境的能力随岩溶裂隙层增厚以及降雨时间间隔延长而减弱。桢楠幼苗以牺牲对茎生物量的分配投资为代价，提高对根系或者叶片的生物量投资，同时增大界面土层中的根系分布，来保证最大化地利用有限的土壤资源，从而适应喀斯特的岩溶裂隙（及干旱）生境。 Abstract:Karst is one of the most important fragile ecological environments in globally terrestrial ecosystems. It is basically characterized by the dual structure of surface soil and underground karst fissures and caves, due to the long-term strong chemical dissolution of carbonate rocks. In recent years, under global climate change, the rainfall pattern in southwestern China has shown a trend of decreasing rainfall frequency and greater sub-rainfall. The karst region in southwestern China, which belongs to the subtropical monsoon climate, often suffers from more obvious alternating wet and dry conditions. The underground karst fissures and variation in the temporal pattern of regional rainfall will have a significant impact on plant growth and development in this region. In this study, in order to explore whether the seedlings of Phoebe zhennan S. Lee adjust biomass allocation and root distribution to adapt to the changing environment, we simulated different karst fissure habitats (S0:24 cm soil; S1/2:12 cm soil layer+12 cm karst fractured layer; S3/4:6 cm soil layer+18 cm karst fractured layer) and different rainfall temporal patterns (I2d:rainfall interval of 2 days; I19d:rainfall interval of 19 days), and checked the plants biomass accumulation and allocation, root vertical distribution. The results showed that:(1) under the short-term rainfall pattern, compared with the whole soil habitat, the habitat with a small proportion of karst fissures did not affect the biomass accumulation of P. zhennan seedlings. With the thickening of karst fractured layer and the extension of the rainfall interval, P. zhennan reduced the total biomass, allocating less biomass to stems and more to roots and leaves. (2) The vertical distribution of root system of P. zhennan seedlings decreased with the deepen of soil layer, but increased the root biomass allocation ratio in the soil layer above the karst fissures, regardless of the rainfall temporal pattern. The results show that the fissures hosted soil can be a certain habitat for plants. However, with the thickening of fractured layer and extension of the rainfall interval, the fissures weaken the ability of the habitat. P. zhennan seedlings increase investment to root and leaf at the expense of stem biomass, and increase the root distribution in the interface soil layer above the fissures to ensure the maximum use of limited soil resources so as to adapt to the fissure (and arid) habitat in karst. 参考文献 相似文献 引证文献

Spatial and temporal dynamics of rainfall erosivity in the karst region of southwest China: Interannual and seasonal changes

Global warming changes global and regional patterns of precipitation, which will inevitably lead to more severe soil erosion risks in fragile karst ecosystems. Understanding the variation pattern of rainfall erosivity is vital for preventing and controlling soil erosion in the karst region of southwestern (SW) China. In this study, the spatiotemporal dynamic evolution pattern of the rainfall erosivity throughout the year in the karst region of SW China was explored using regression analysis and the Mann–Kendall test based on the monthly rainfall data for 1901–2020 in the CRU_TS 4.05 dataset. The results indicate that: (1) The rainfall erosivity in the karst region of SW China varies with latitude and is more significant in the south, and its interannual variations are linear and positive. (2) The interannual linear increase in rainfall erosivity was the greatest in the peak forest-plain rocky desertification control zone (FLPY). (3) The rainfall erosivity exhibited an increasing trend in spring, summer, and winter, with the most significant increase in January. (4) More variability was observed on the monthly scale. In summary, future investigations and control of soil erosion in the karst region of SW China under climate change should focus on the critical times when (e.g., January) and areas where (e.g., peak forest-plain rocky desertification control zone) rainfall erosivity changes significantly. Identifying the shift in the rainfall erosivity patterns caused by climate change is critical for assessing soil erosion risks in karst areas and for formulating countermeasures.

Quantifying production-living-ecology functions with spatial detail using big data fusion and mining approaches: A case study of a typical karst region in Southwest China

Demands for increasing production and living functions often threaten ecological functions. Reliable quantification of land use multifunctionality (LUMF) contributes to identify and coordinate these conflicts. However, the comprehensive LUMF indicator system and fine spatial resolutions restricted each other. Previous assessments were limited to coarse grids or administrative units, thus failing to provide detailed spatial information and causing scale biases, particularly in landscapes with strong heterogeneity. Combining big data mining and fusion techniques, this study built a comprehensive and quantitative evaluation scheme of production-living-ecology (PLE) land functions with spatial detail. The scheme was applied in the karst region of southwestern China as a case study. Data on socioeconomic status, remote sensing, and location points were fused using a variable-boundary spatialized approach to quantify human activity extent and degree of convenience. Specific environmental characteristics, such as topography and karst rocky desertification, were integrated into the spatialized PLE functions. The results showed that the global distribution of the production and living functions presented some similarity, yet the high production and living functions were not completely aligned. Also, the spatial patterns of the living and production functions usually showed opposite formations to the ecology function. Unlike the global relationship, the enlarged PLE relationships in the study regions validated the importance and necessity of fine spatial information. Local relationships between the production-living and production-ecology functions were significantly different in areas with various economic development levels, particularly in built-up areas. Regions with strong synergistic relationships of PLE functions were rare and were usually found in sightseeing areas of ecology and agriculture. These results demonstrated the effectiveness of our proposed method of spatializing PLE functions with spatial detail. The findings provide detailed spatial information on local anthropogenic and physical differences and support for balancing PLE functions and policy-making about land use optimization in practice.

New strategy for exploring the accumulation of heavy metals in soils derived from different parent materials in the karst region of southwestern China

Heavy metals with high average background values in soils from karst regions of China have been reported previously. However, the accumulation mechanism of heavy metals in karst soils derived from different parent materials related to the molecular soil chemistry is still unclear. In this study, six soil cores (0–100 cm) from three locations were collected from Hengxian County, a typical karst basin of Guangxi, among which soils of Maling (ML1 and ML2) were derived from the weathering and leaching residues of carbonate rocks, and soils of Yunbiao (YB1 and YB2) and Dengxu (DX1 and DX2) were mainly developed from deposits of Quaternary formations. Soil heavy metal quantification, X-ray diffraction (XRD) analysis, synchrotron radiation-based Fourier transform infrared (SR-FTIR) spectroscopy, and two-dimensional correlation spectroscopy (2DCOS) analysis were combined to study the contents of heavy metals, mineral compositions, and functional assignments in the karst soils. The results showed that the total concentrations of As, Cr, Cd, Cu, Ni, Pb, and Zn in the soils from ML exceeded the risk screening values from the agricultural soil quality standard in China (GB15618-2018), while the contents in the soils from YB and DX were below the risk screening values. The mineral components were significantly different in soils and their clay minerals. ML soils contained more Fe and Mn oxides than YB and DX soils. Furthermore, it was interesting to find extensive existence of chlorite in ML soils (36.73%-39.07%) and their clay minerals (90.49%-90.97%). While illite was the dominant mineral in YB and DX clay fractions (75.08%-100%) and its percentages decreased to 6.06%-35.23% in the soil samples. In addition, ML soils contained the hydroxyl groups with hydrogen bonds (vO-H: 3523, 3450, and 3378 cm−1) and the hydroxyl groups without hydrogen bonds (vO-H: 3698, 3623 cm−1), while YB and DX soils only had the hydroxyl groups without hydrogen bonds (vO-H: 3695, 3622 cm−1). Our study indicated that the accumulation of heavy metals in karst soils from different parent materials was significantly attributed to their molecular soil chemistry. For instance, the higher contents of Fe-Mn oxides and more reactive vO-H (3523, 3450, and 3378 cm−1) and vSi-O (1000 cm−1) assignments could contribute to the higher contents of heavy metals in ML soils compared to the soils from YB and DX.

Contrasting Hydraulic Efficiency and Photosynthesis Strategy in Differential Successional Stages of a Subtropical Forest in a Karst Region.

Understanding the successional process from a disturbed forest to a mature forest is essential for species recovery and conservation initiatives. The resource acquisition and drought tolerance of plants can be instructive to predictions of species abundance and distribution for different forests. However, they have not been adequately tested at different successional stages in karst regions. Here, we selected seven dominant species in an early-succession forest and 17 species in a late-succession forest in a karst region of southwestern China. Resource acquisition-related traits such as hydraulic conductivity and photosynthetic rate, and drought tolerance-related traits, including turgor loss point and wood density, were measured. We found that species in the early-succession forest had a higher hydraulic conductance and photosynthetic rate than those in the late-succession forest, while leaf water potential at turgor loss point and wood density showed nonsignificant differences between the two forests. In addition, we observed a significant negative relationship between photosynthetic rate and drought tolerance in the early-succession forest, which was not identified in late-succession forests. Our study indicates that resource acquisition rather than drought tolerance was the key factor explaining plant distributions in forests at different successional stages in karst regions. We also suggest that the resource acquisition and drought tolerance trade-off hypothesis is not always supported for karst region species. Our study could inform about the design of species replacements in successional forests and provide forest management and restoration guidelines for karst regions.

Evaluation of Cadmium Transfer from Soil to the Human Body Through Maize Consumption in a Cadmium Anomaly Area of Southwestern China.

Evaluating the bioavailability, bioaccessibility, and transferability of cadmium (Cd) in soil-grain-human systems is essential in areas with a Cd anomaly in the karst region of southwestern China. In the present study, the main controlling factors and prediction models for Cd transfer in a soil-grain-human system were investigated in a typical area where natural processes and anthropogenic activities interact in the karst region of southwestern China. The environmental availability of Cd (diethylenetriaminepentaacetic acid- and CaCl2 -extractable Cd [ ]) in the soil varies significantly because of the diversity of soil properties. However, Cd concentrations in the maize grain were significantly related only to the concentrations in the soil (r = 0.595, p < 0.01), indicating that soil is a good indicator for evaluating Cd uptake by maize grain. Of all the measured soil properties, the soil cation exchange capacity (CEC) and the soil calcium (Casoil ) were the most important factors influencing Cd accumulation in the soil-maize grain system. A transfer model combining , soil CEC, and Casoil was sufficiently reliable for predicting Cd accumulation in the maize grain (R2  = 0.505). Although there is room for improvement regarding the prediction performance of the chain model combining soil with Casoil to predict the bioaccessible Cd concentration in maize grain (R2  = 0.344 for the gastric phase and R2  = 0.356 for the gastrointestinal phase), our findings provide a useful reference to further explore a model that can be used for a relatively rapid and reliable estimation of dietary Cd exposure for specific regions prior to crop harvest. Environ Toxicol Chem 2021;40:2923-2934. © 2021 SETAC.

Dynamics of soil nitrogen availability following vegetation restoration along a climatic gradient of a subtropical karst region in China

Soil nitrogen (N) availability plays an important role in determining ecosystem structure and function. However, the variation patterns of soil N availability under different vegetation restoration strategies along climatic gradients in the ecologically fragile karst region of China have been poorly understood. Soil N variables, including total N (TN), inorganic N [ammonium (NH4+) and nitrate (NO3-)], and rates of net N mineralization and nitrification were measured following managed (plantation forest) and natural (spontaneous regeneration) vegetation restoration along a climatic gradient in the karst region of southwestern China. Compared to that in the cropland, soil TN and NH4+ concentrations significantly increased in both vegetation restoration strategies after 15 years of cropland abandonment. The net N mineralization and nitrification rates were lower in both vegetation restoration strategies compared with those in cropland and secondary forest. There were no significant differences in the soil N variables between the two vegetation restoration strategies. Furthermore, soil NO3− was positively correlated with mean annual temperature (MAT) following vegetation restoration, whereas the net nitrification rate was negatively correlated with MAT. Generally, soil pH was the main factor affecting soil inorganic N, whereas elevation, soil microbial abundance, SOC or C:N, and climatic factors mainly affected soil net N mineralization and nitrification rates. Vegetation restoration was beneficial to soil N accumulation and is likely to decrease the risk of N loss in warmer regions given the reduced potential nitrification rate with increased MAT in the karst region of Southwest China.

Three new species of Liparis s.l. (Orchidaceae: Malaxideae) from Southwest China based on morphological characters and phylogenetic evidence

Liparis aureolabella and L. mengziensis, two new species from the karst region of southwestern China, and L. bingzhongluoensis, a new species from montane region in Yunnan, are described and illustrated. L. aureolabella is easily distinguished from its relatives by having abaxially purple leave with purple reticulate veins prominent adaxially, a lip auriculate at base, and falcate-lanceolate pollinia. Liparis mengziensis is closely related to L. petiolata and L. auriculata, but differs from them by having an ovate to broadly ovate leaf, purple lip and apex connate along the margins. Liparis bingzhongluoensis is similar to Liparis nanlingensis, but the new species is characterized by having a lip with two transparent ridges on its disc, longitudinally concave basal callus and triangular column wings. Phylogenetic analyses based on nuclear ribosomal ITS and plastid matK sequences showed that L. aureolabella and L. mengziensis are nested with L. petiolata or L. auriculata in a monophyletic clade. L. bingzhongluoensis is sister to a clade formed by L. nanlingensis, L. tsii, L. sasakii and L. krameri. Moreover, morphological comparisons strongly support that the three species as separated species newly to science.

Interaction of Ca2+ and soil humic acid characterized by a joint experimental platform of potentiometric titration, UV–visible spectroscopy, and fluorescence spectroscopy

Rocky desertification has become a major environmental issue in the karst region of southwestern China. Karst rocky desertification was more severe in regions of limestone soil than in adjacent regions of other soils, despite the relatively higher soil organic matter (SOM) content in limestone soil. The underlying mechanism remains ambiguous. We speculated that the geochemical characteristics of limestone soils in the karst region plays an essential role, especially the high calcium content of limestone soil. To test this hypothesis, we collected limestone soil samples from a limestone soil profile in the southwestern China karst region and extracted humic acid (HA) from these limestone soil samples. We investigated the interaction of Ca2+ and three HA samples on a joint experimental platform, which consists of an automatic potentiometric titrator, a UV–visible spectrometer, and a Fluorescence spectrometer. HA solutions were titrated by Ca2+ and optical spectra of the HA solutions were monitored during the titration experiments. The results indicated that: (1) the interaction of Ca2+ and HA is a combined process of adsorption and complexation. Adsorption dominated the overall distribution behavior of Ca2+, which could be fit by Langmuir and Freundlich isotherm models. Complexation was distinguished only when the concentration of Ca2+ is low; (2) the changes of UV–visible spectroscopy and excitation–emission matrix fluorescence spectroscopy spectra of HA samples when they were binding with Ca2+ implied the apparent molecular size and structure of HA became larger and more complex; (3) the combination of Ca2+ and HA plays an important role in the SOM preservation of limestone soils but the stability of the Ca–HA association was relatively weak. The present study draws attention to maintaining the relatively higher Ca2+ concentration in limestone soils in ecologic restoration attempts in karst regions.

Influence of shale gas development on core forests in the subtropical karst region in southwestern China

Core forests are an important component of forest landscapes and wildlife habitat. Although the core forests were damaged during the development of shale gas sites, it remain unclear how much damage the shale gas development has caused to this ecologically vulnerable region. We analyzed high-resolution remote sensing images of a shale gas development area in 2012, 2014, and 2017 in the karst region in southwestern China. The results showed that the core forest area decreased by approximately 4.0% from 2012 to 2017. Of this decrease, approximately 32.3% was related to the shale gas development activities, while 67.7% was related to other human activities, i.e., agricultural lands and residential developments. Approximately 5.6% of the decrease in the core forest was for new pipelines, with 0.5 ha occurred in 2012–2014 and 248.6 ha occurred in 2014–2017. Of the shale gas development activities, the pipeline constructions were most detrimental to the core forest. The patchiness of the core forest increased by 8.2% from 2012 to 2017 by the expansions of dry fields, towns, and settlements. The core forest Effective Mesh Size (MESH) decreased by 86.3%, primarily caused by the shale gas development pipelines. In conclusion, human activities that were not directly related to shale gas development were the main driver of the core forest decreases. The pipelines caused most losses of the core forest among the shale gas activities and the impacts deteriorated as the shale gas development proceeds. Therefore, we propose that new shale gas pads should be placed adjacent to existing shale gas pipelines and new shale gas pipelines should be constructed in parallel with existing roads to reduce the damages on core forest.

Effects of geosorbent and solution properties on sorption and desorption of PAHs

Characteristics of phenanthrene and pyrene’s sorption and desorption on two local soils in solutions of simulated groundwater, simulated lung fluid, and simulated saliva were studied with batch equilibrium experiments to understand the fate of PAHs in the karst region of southwestern China and to assess the environmental exposure and the health risk of PAHs. The results showed that the sorption and desorption isotherms of phenanthrene and pyrene on two target soils in the three solution systems could be adequately described by the Freundlich model, while the fitted isotherm parameters for the simulated groundwater solution distinguished notably from those for the simulated body fluid solutions. For the sorption experiments, in the simulated groundwater, the n values were 0.722 and 0.672 for phenanthrene and were 0.724 and 0.663 for pyrene, respectively, on the yellow soil and the limestone soil; The log KF values were 3.118 and 3.323 for phenanthrene and were 3.648 and 3.846 for pyrene, respectively, on the yellow soil and the limestone soil. In the simulated body fluids, the n values for phenanthrene and pyrene ranged from 0.622 to 0.836 and from 0.590 to 0.865, respectively, and the log KF values of phenanthrene and pyrene ranged from 2.845 to 3.327 and from 3.344 to 3.779, respectively. For the desorption experiments, in the simulated groundwater, the n values were 0.662 and 0.744 for phenanthrene and were 0.702 and 0.647 for pyrene, respectively, on the yellow soil and the limestone soil. The log KF values were 3.666 and 3.686 for phenanthrene and were 4.128 and 4.225 for pyrene, respectively, on the yellow soil and the limestone soil. In the simulated body fluids, the n values for phenanthrene and pyrene ranged from 0.612 to 0.668 and from 0.631 to 0.819, respectively, and the log KF values of phenanthrene and pyrene ranged from 3.134 to 3.407 and from 3.533 to 3.839, respectively. The limestone soil had relatively higher log KF values but lower KOC values compared to those of the yellow soil, indicated that the nature of sorbent soils played the dominant role in sorption and desorption behaviors of PAHs. The experimental results showed a remarkable differences in sorption and desorption behaviors of PAHs in simulated body fluids and groundwater. The nonlinearities of measured isotherms and the measured sorption capacities of soils in simulated body fluids were significantly lower than corresponding those in the simulated groundwater, and HI values for simulated body fluids systems were significantly smaller than corresponding those for the simulated groundwater systems. The results underscore cautions in assessing environmental exposure and health risks of PAHs based on their sorption–desorption data in simulated groundwater as this is traditionally done.

[Characteristics and Source Apportionment of Ambient VOCs in Spring in Liuzhou].

The objective was to investigate the characteristics and sources of ambient volatile organic compounds (VOCs) in the karst region in southwestern China. We monitored atmospheric VOCs in Liuzhou by the GC955 VOCs Online Monitoring System and analyzed the pollution characteristics, ozone formation potential (OFP), aerosol formation potential (AFP), and the positive matrix factorization (PMF) model in March 2019. The results show that ① 50 kinds of VOC components were detected during the supervised period, with an average daily concentration of 25.52×10-9 mol·mol-1, which was composed of alkanes (56.08%), alkenes (19.63%), alkynes (14.25%), and aromatics (10.04%), respectively. ② The concentration of VOCs was lower during the day and higher at night, with the highest value at 23:00. The VOC concentration was low in daytime and high at night. The peak value of VOCs with regard to diurnal variation was correlated with the time of morning and the evening traffic peak and may be influenced by various factors. ③ The contribution of alkenes, aromatics, and alkanes to OFP was 44.30%, 33.03%, and 19.96%, respectively. This indicates that the control of aromatic and olefin should prioritize alkanes. In addition, Liuzhou city is in the VOC sensitive area of O3 generation, and the reduction of VOCs had a controlling effect on O3 generation. ④ The contribution of aromatic hydrocarbons to AFP was up to 95.27%. Therefore, the improvement and control of the processes in motor vehicle exhaust emissions, solvent use, and the automobile industry and the chemical industry could effectively suppress ozone and haze pollution. ⑤ The emission sources of VOCs in spring were mainly industrial emission sources (28.34%), motor vehicle sources (25.47%), combustion sources (24.37%), solvent sources (13.28%), and plant emission sources (8.54%), respectively. This indicates that the control of industrial emission sources, motor vehicle sources, and combustion sources is the main way to control VOC pollution in Liuzhou City. Meanwhile, the olefin and aromatic hydrocarbons emitted by these emission sources should be mainly considered.

Long-Term Cultivation of Fruit Plantations Decreases Mineralization and Nitrification Rates in Calcareous Soil in the Karst Region in Southwestern China

Determination of rates of mineralization of organic nitrogen (N) into ammonium-N (NH4+-N) and nitrification of NH4+-N into nitrate-N (NO3−-N) could be used to evaluate inorganic N supply capacity, which, in turn, could guide N fertilizer application practices in crop cultivation systems. However, little information is available on the change of mineralization and nitrification in soils under fruit cultivation systems converted from forestlands in karst regions. In a 15N-tracing study, inorganic N supply capacity in forest soils and three typical fruit crop soils under long-term cultivation was investigated, in addition to factors influencing the supply, in calcareous soils in the karst regions in southwestern China. Long-term fruit crop cultivation decreased soil organic carbon (SOC), total N, and calcium concentrations, cation exchange capacity (CEC), water holding capacity (WHC), pH, and sand content, significantly, but increased clay content. Compared to that of forests, long-term fruit crop cultivation significantly decreased mineralization and nitrification rates to 0.61–1.34 mg N kg−1 d−1 and 1.95–5.07 mg N kg−1 d−1, respectively, from 2.85–6.49 mg N kg−1 d−1 and 8.17–15.5 mg N kg−1 d−1, respectively, but greatly increased the mean residence times of NH4+-N and NO3−-N. The results indicate that long-term fruit crop cultivation could decrease soil inorganic N supply capacity and turnover in karst regions. Both mineralization and nitrification rates were significantly and positively correlated with SOC and total N concentrations, CEC, and WHC, but negatively correlated with clay content, suggesting that decreased soil organic matter and increased clay content were responsible for the decline in mineralization and nitrification rates in soils under long-term cultivation of fruit crops. The results of the present study highlight the importance of rational organic fertilizer application in accelerating soil inorganic N supply and turnover under long-term cultivation of fruit crops in karst regions.

Decreased inorganic N supply capacity and turnover in calcareous soil under degraded rubber plantation in the tropical karst region

Investigating soil inorganic nitrogen (N) supply and availability can guide soil improvement of rubber (Hevea brasiliensis) plantations in tropical regions, but the mechanisms controlling the inorganic N supply remain unknown. In this study, three natural forests and three degraded rubber plantations located in a tropical karst region of southwestern China were sampled to determine the gross N transformation rates using a 15N tracing method. The soils of the natural forests were characterized by a high inorganic N supply capacity and a high-level nitrate (NO3−) production potential, due to the high rates of organic N mineralization to ammonium (NH4+) (MNorg) and NH4+ oxidation to NO3− (ONH4) but relatively low rates of immobilization of NH4+ (INH4) and NO3− (INO3) to organic N and dissimilatory NO3− reduction to NH4+ (DNRA). In the soils of the degraded rubber plantations, the rates of MNorg, ONH4, INO3, and DNRA were lower but the rates of NH4+ adsorption on cation-exchange sites (ANH4) increased, resulting in reductions in the inorganic N supply capacity and N availability. In addition, NO3− turnover in the soils of the degraded rubber plantations decreased, accompanied by a high mean residence time of NO3− and low δ15N values. Soil total N, organic C, phosphorus, and potassium concentrations, water-holding capacity, cation-exchange capacity, and sand content were significantly lower in the soils of the degraded rubber plantations than in those of the natural forests, indicating a decline in soil quality in the former. The significant, positive relationships between these soil properties and the rates of MNorg, ONH4, INO3, and DNRA highlight the importance of the appropriate application of organic N fertilizers as well as phosphorus and potassium fertilizers to stimulate soil N cycling and thereby increase the inorganic N supply. A reduction of the N deficiency in soils used for rubber tree cultivation would alleviate the soil degradation that characterizes many rubber plantations in tropical karst regions.

Effects of agricultural activities coupled with karst structures on riverine biogeochemical cycles and environmental quality in the karst region

The karst region in southwestern China is a typical region with fragile ecological environments, and the coordination of regional agricultural development and environmental protection faces enormous challenges. Based on the soil and hydrological characteristics of karst in southwestern China, this study summarized water-related environmental issues caused by agricultural activities in this karst region. Agriculture in the karst hills is more likely to cause soil and nutrient losses due to the fast hydrological flow through special karst structures with high permeability. Thus, this review emphasized the impacts of agricultural development on the riverine biogeochemical cycles of elements based on previous studies. Meanwhile, the carbon cycle is also strongly impacted by agricultural activities in this karst region due to enhanced carbonate weathering by nitric acid from the nitrification of ammonium. This weathering mechanism represents a net source of atmospheric CO2 and might impact regional and global carbon cycling in the Anthropocene. Based on the results summarized in this study, we advocate that in the future, better management of agricultural land, improvement of fertilizer use efficiency, and boosting of nutrient recycling rate should be taken into account for reducing nutrient losses and water quality deterioration. Targeted management of local agricultural practices along with guidance from scientific research results is needed to be devoted to sustainable development of agriculture and economies while protecting water environment.