Soil organic nitrogen variation shaped by diverse agroecosystems in a typical karst area: evidence from isotopic geochemistry

Background

Soil organic nitrogen (SON) levels can respond effectively to crop metabolism and are directly related to soil productivity. However, simultaneous comparisons of SON dynamics using isotopic tracing in diverse agroecosystems are lacking, especially in karst areas with fragile ecology.

Methods

To better understand the response of SON dynamics to environmental changes under the coupling of natural and anthropogenic disturbances, SON contents and their stable N isotope (δ15NSON) compositions were determined in abandoned cropland (AC, n = 16), grazing shrubland (GS, n = 11), and secondary forest land (SF, n = 20) from a typical karst area in southwest China.

Results

The SON contents in the SF (mean: 0.09%) and AC (mean: 0.10%) profiles were obviously lower than those in the GS profile (mean: 0.31%). The δ15NSON values ranged from 4.35‰-7.59‰, 3.79‰-7.23‰, and 1.87‰-7.08‰ for the SF, AC, and GS profiles, respectively. Decomposition of organic matter controlled the SON variations in the secondary forest land by the covered vegetation, and that in the grazing shrubland by goat excreta. δ15NSON ranges were controlled by the covered vegetation, and the δ15NSON fractionations during SON transformation were influenced by microorganisms in all surface soil.

Conclusions

The excreta of goats that contained 15N-enriched SON induced a heavier δ15NSON composition in the grazed shrubland. Long-term cultivation consumes SON, whereas moderate grazing increases SON content to reduce the risk of soil degradation. This study suggests that optimized crop-livestock production may benefit the sustainable development of agroecosystems in karst regions.

High-frequency dynamics of CO2 emission flux and its influencing factors in a subtropical karst groundwater-fed reservoir, south China

Revealing the magnitude, dynamics, and influencing factors of CO2 emissions across the water-air interface in karst water with high frequency is crucial for accurately assessing the carbon budget in a karst environment. Due to the limitations of observation methods, the current research is still very insufficient. To solve the above problems and clarify the main influencing factors of CO2 emission in karst water, this study selected Dalongdong (DLD) Reservoir, located in the typical karst peak and valley area in southwest China, to carry out a multi-parameter high-frequency monitoring study from January to December 2021, and used the thin boundary model method to estimate the CO2 flux across the water-air interface (CF). The average annual flux of DLD reservoir is 84.48 mmol·(m2·h)-1, which represents a CO2 source overall. However, during the stratification period in August, there is a transient carbon sink due to negative CO2 emission. The alteration of thermal stratification in water is crucial in regulating the seasonal variation of CF. Meanwhile, the diurnal variation is significantly influenced by changes in hydrochemical parameters during the thermal stratification stage. Compared to low wind speeds (<3 m/s), high wind speeds (≥3 m/s) have a greater impact on the CO2 flux. Furthermore, high-frequency continuous data revealed that the reservoir triggered a CO2 pulse emission during the turnover process, primarily at night, leading to unusually high CO2 flux values. It is of great significance to monitor and reveal the process, flux, and control factors of CO2 flux in land water at a high-frequency strategy. They will help improve the accuracy of regional or watershed carbon budgets and clarify the role of global land water in the global carbon budget.

Geochemical behaviors of uranium and thorium during weathering and pedogenesis of carbonate rock: constraint from their speciation

During weathering and pedogenesis of carbonate rock with poor-uranium (U) and thorium (Th), U and Th present the characteristics of strong leaching (especially U) and significant residual enrichment, the cause of which is still unclear. In this paper, a weathering profile developed by dolomite in karst area of Guizhou province in southwest China was selected, which showed zonation characteristics of bedrock (Y), powdery rock (Yf), and soil layer (T1 to T12) from the bottom to up. Through the determination of the occurrence speciation of U and Th in Y and weathering profile, combined with mineralogical, geochemical characteristics, and element mass balance calculation, the constraints of U and Th speciation on the geochemical behavior of U and Th during the weathering of carbonate rock were revealed. The results proved that U and Th in Y preferentially existed in acid insoluble phase, for example, the contents of U and Th in Y were 0.90 mg·kg-1 and 0.28 mg·kg-1, respectively, while those in acid insoluble matter were 2.34 mg·kg-1 and 2.57 mg·kg-1, respectively, but because the mass percentage of acid insoluble matter was extremely low (0.95%), the mass percentages of U and Th in the acid soluble phase in the whole rock were absolutely superior (96% of U and 86% Th). The U and Th in the acid soluble phase of Y were mainly adsorbed on the crystal surface of carbonate minerals or existed in the cement, and the U and Th in the carbonate lattice only accounted for a small proportion. From Y to Yf with the initial dissolution, U and Th released from the surface of carbonate minerals and cements were in carbonate-rich alkaline environment, and these portions of U and Th were leached out, resulting in strong loss of U and Th in the Yf (the loss rates are 83% of U and 65% of Th, respectively). From the Yf to the overlying soil layer T1, the carbonate components were completely dissolved, and the U and Th released from the carbonate lattice showed different behaviors, where U was completely leached and Th tended to stay in the weathered residue. Thus, in the soil layer T1 formed by Y or Yf , the residual U was the inheritance of the U in the acid insoluble phase of Y; For Th, it not only inherited the Th of acid insoluble phase of Y, but also superimposed the Th from carbonate lattice in Y. On the other hand, during the evolution process from Y to Yf and to soil layer T1, with the dissolution of carbonate, the acid insoluble phase also showed a significant tendency of chemical weathering. However, the U and Th in the Y acid insoluble phase were not leached with the decomposition of the acid insoluble phase but were redistributed among the residual phases. For the geochemical behaviors of U and Th in the evolution of soil profile (T1~T12), they were subjected to the occurrence speciation of U and Th in T1 and the change of U and Th occurrence speciation with the upward direction of soil profile. The U and Th released from the carrier minerals were mainly redistributed among the residual solid phases, which weakened the intensity of their further loss. This study deepens the understanding of the geochemical behavior of radionuclides in karst environment and provides reference for the treatment of radioactive pollution in karst areas.

Plant growth-promoting rhizobacteria are important contributors to rice yield in karst soils

The difficulty of releasing nutrients from soils in karst areas limits the yield of local crops and leads to poverty. In this study, two strains of plant growth-promoting rhizobacteria (PGPR) were isolated from the rhizosphere soil of typical plants in karst areas, which were both identified as Bacillus sp. and named GS1 and N1. And two isolates were used to construct a composite PGPR named MC1. These three strains of PGPR were used for soil inoculation in the pot experiment and field trial and their capacity to promote rice development was assessed. The results showed that MC1 inoculation exhibited notable rice growth-promoting ability in pot experiments, and, respectively, had an increment of 16.96, 18.74, and 11.50% in shoot biomass, total biomass, and rice height compared with control. This is largely attributed to PGPR's capacity to secrete phytohormones and soil enzymes, particularly urease (UE) in GS1, whose secreted UE content was significantly higher by 12.18% compared to the control. When applied to the field, MC1 inoculation not only increased rice yield by 8.52% and the available nutrient content in rice rhizosphere soil, such as available phosphorus (AP) and exchangeable magnesium (EMg); but also improved the abundance of beneficial rhizobacteria and the diversity of microbial communities in rice rhizosphere soil. Results in this study revealed that inoculated PGPR played a major role in promoting rice growth and development, and a new strategy for facilitating the growth of rice crops in agriculture was elucidated.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03593-0.

Health risk assessment of heavy metal exposure through vegetable consumption around a phosphorus chemical plant in the Kaiyang karst area, southwestern China

The phosphorus chemical industry is an important source of heavy metals in farmland. Vegetables grown on contaminated soil potentially impose adverse effects on human health. In this study, the pollution status and health risks of heavy metals in vegetables around a phosphorus chemical plant in Kaiyang County, Guizhou Province, southwestern China, were assessed, and the low-accumulation vegetables were screened by bioaccumulation factor (BAF) and cluster analysis. Results showed the average concentrations of Hg, As, Cd, Pb, Cr, Mn, Co, and Zn in vegetables were 0.015, 0.728 0.382, 0.227, 0.850, 27.227, 0.525, and 6.438 mg/kg, respectively. The single-factor pollution index showed that Cd was moderately polluted, and Cr, Hg, As, and Pb were slightly polluted. The Nemerow pollution index showed that the overall heavy metal pollution was classified as moderately polluted. The accumulation of heavy metals in different vegetables varied greatly, and chard, crown daisy, chayote, pumpkin, eggplant, white radish, sweet potato, carrot, and potato were selected as the low-accumulator vegetables. The consumption of all vegetables except chayote poses both carcinogenic and noncarcinogenic risks; among them, the consumption of sweet potato leaves has the highest health risks. The local population needs to adjust plantation structure and change dietary habits, and government should strengthen the management of phosphorus chemical plant pollution.

Mercury migration to surface water from remediated mine waste and impacts of rainfall in a karst area - Evidence from Hg isotopes

Mine waste (MW) in historical mercury (Hg) mining areas continuously emits Hg into local environment, including aquatic ecosystems. Tracing Hg migration process from MW and determining its relative contribution to Hg pollution is critical for understanding the environmental impact of MW remediation. In this study, we combined data of Hg concentration, speciation, and isotope to address this issue in the Wanshan Hg mining area in southwest China. We found that rainfall can elevate Hg concentrations in river water and control the partitioning and transport of Hg in karst fissure zones through changing the hydrological conditions. A consistently large offset of δ²⁰²Hg (1.24‰) was observed between dissolved Hg (DHg) and particulate Hg (PHg) in surface water during the low-flow period (LFP), which may have been related to the relatively stable hydrologic conditions and unique geological background (karst fissure zones) of the karst region (KR). Results from the ternary Hg isotopic mixing model showed that, despite an order of magnitude reduction in Hg concentration and flux in river water after remediation, the remediated MW is still a significant source of Hg pollution to local aquatic ecosystems, accounting for 49.3 ± 11.9% and 37.8 ± 11.8% of river DHg in high flow period (HFP) and LFP, respectively. This study provides new insights into Hg migration and transportation in aquatic ecosystem and pollution source apportionment in Hg polluted area, which can be used for making polices for future remediation actions.

Research on the interaction of "tourism development-land use-landscape pattern" since the 1990s in a small karst basin, China

The rapid rise of tourism in the karst regions has promoted the development of the local economy by relying on the unique landforms and landscapes. However, tourism development is often accompanied by land use changes and has an impact on the ecological environment. Exploring the coupling relationship between "tourism development-land use-landscape pattern" is very important for ecologically fragile karst areas. Taking the Yulong River Basin as an example, this research applied 3S technology, spatial analysis based on POIs, and regression analysis to the following: (1) identifying the process and effects of land use change, (2) determining the spatial pattern of tourism land and its correlation with land use change, (3) determining the characteristics and impacts of landscape pattern evolution. As the results suggested: (1) The significant expansion of construction land occupies a large amount of farmland, there is a balanced relationship between farmland and forest land for mutual conversion. (2) The aggregation of tourist land is affected by the trend of tourist behavior and the distribution of scenic spots. There is a significant moderate positive correlation between tourism land and construction land. (3) With the land use change, landscape heterogeneity has improved, but landscape fragmentation is serious and landscape connectivity is reduced. This research provides new evidences for the effect of the rapid development of tourism on land use change and ecological environment and as a reference to future orderly and moderate land development and ecological sustainability in karst regions.

Tracing Fe cycle isotopically in soils based on different land uses: Insight from a typical karst catchment, Southwest China

Iron (Fe) isotopes can effectively unveil the Fe cycle mechanisms under redox and biological conditions during the weathering and pedogenic processes. Fe contents and Fe isotope compositions (defined as δ⁵⁶Fe) in the soil profiles under secondary forest land, abandoned cropland and shrubland were investigated in a typical karst area in Southwest China. The results showed that the Fe content ranged from 23.92 to 38.56 g/kg, 21.92 to 33.02 g/kg and 12.98 to 27.93 g/kg, and the δ⁵⁶Fe levels varied from -0.48 ‰ to 0.21 ‰, -0.24 ‰ to 0.11 ‰ and - 0.11 ‰ to 0.16 ‰ from the secondary forest land, abandoned cropland and shrubland, respectively. The correlation analysis results showed that Fe transportation and isotopic fractionation were regulated by the redox processes through soil pH and soil organic matter (SOM) in the abandoned cropland and shrubland. Heavier Fe isotope may be accumulated in the deeper soil of secondary forest land due to Fe-oxide precipitation. The Fe isotope fractionations were greatly altered by soil organic carbon (SOC) in surface soils due to negative surface charges. Soil pH also plays a key role in enriching lighter Fe in a medium-acidic environment (shrubland) by ligand-controlled dissolution and reductive dissolution. Long-term cultivation in abandoned cropland and grazing in shrubland reshaped the Fe cycle in soil profiles by changing soil pH and SOC contents. However, the similar values of δ⁵⁶Fe in different land use soils indicated that the agricultural activities have no significant impact on the Fe transformation in karst soil ecosystems. The land utilization is reasonable in the Yinjiang County. This study provided effective data and insightful analysis to understand the Fe cycle processes in the karst soils under varied land uses.

Characteristics and influencing factors of soil preferential flow in typical stands of Karst area in southwest China

The study of preferential flow phenomena in Karst areas and the identification of the main factors influencing preferential flow are of great importance for the recovery of local vegetation. The distribution of the dyeing solution in the vertical and horizontal directions was examined by field staining tracer test and image processing technique. We analyzed the total dyeing area ratio, matrix flow depth, preferential flow ratio, and length index as pre-ferential flow characteristic parameters, and 14 factors affecting preferential flow using grey correlation analysis. The results showed that there were two main types of preferential flow, funnel-shaped and dendritic, with lateral water movement occurring in the soil of typical Karst stands. The mean value of the dyeing area ratio of the understory in Karst areas was 19.4%, and that of the matrix flow depth, preferential flow ratio, and length index was 4.96 cm, 62.9%, and 385.5%, respectively. Among the 14 environmental factors influencing preferential flow, the initial soil moisture content had the strongest influence on the dyeing area ratio, the available potassium content had the most significant influence on the matrix flow depth, and available phosphorous content had the most significant influence on both the preferential flow ratio and the length index. The high degree of development and spatial variability of preferential flow under typical forest stands in Karst areas was strongly influenced by physical properties such as initial soil water content, while soil nutrient also exerts important influence on preferential flow.

Quantitative assessment of the relative contributions of climate change and human activities to NPP changes in the Southwest Karst area of China

Net primary production (NPP) is an essential component of the terrestrial carbon cycle and an essential factor of ecological processes. In global change research, it was the core content to study the driving forces of NPP change. In this paper, we focused on the Southwest Karst area of China and analyzed the response mechanisms of NPP to topography, land-use types, climatic change, and human activities. Our results showed that (1) changes in elevation and slope lead to significant differences in the spatial distribution of NPP. With the increase of elevation and slope, NPP first increased and then decreased, their critical values were 2000 m and 15°, respectively. (2) NPP varied significantly among different land-use types. The average NPP of the forest was the highest, and the average NPP of cultivated land increased fastest. (3) Temperature and precipitation had the most substantial influence on NPP, both of them promoted the increase of NPP, and the effect of temperature was more obvious in the Southwest Karst area. (4) Ecological engineering significantly promoted the change of NPP, while animal husbandry significantly inhibited the change of NPP. (5) There were significant spatial differences in the driving effects and corresponding contributions of climatic change and human activities; both of them promoted the increase of NPP in the Southwest Karst area of China. Under climatic change and human activities, NPP increased by 1.24 gC·m^-2·year^-1 and 2.29 gC·m^-2·year^-1, respectively. The contributions rates of climatic change and human activities separately accounted for 35% and 65%. The contribution of human activities on NPP was much higher than that of climatic change in the Southwest Karst area, and the results suggested that we should focus on the role of human activities on NPP change.

Spatial distribution and ecological risk assessment of heavy metals in karst soils from the Yinjiang County, Southwest China

BACKGROUND

Soil heavy metals (HMs) under different land-use types have diverse effects, which may trigger the ecological risk. To explore the potential sources of HMs in karst soils, the spatial distribution and geochemical behavior of HMs based on different land-use types are employed in this study.

METHODS

Soil samples (n = 47) were collected in three suites of karst soil profiles from the secondary forest, abandoned cropland and shrubland in Yinjiang, Southwest China. The concentrations of Ni, Mn, Cr, Pb, Cd and Mo were determined to give a comprehensive understanding of the possible sources of these HMs and evaluate the potential ecological risk in Yinjiang County.

RESULTS

The mean concentrations of HMs in all profiles followed the same order: Mn > Cr > Ni > Pb > Mo > Cd. Meanwhile, the concentrations of most HMs roughly increased with the depth. Additionally, the concentrations of HMs were mostly correlated with soil pH and SOC, rather than with clay and silt proportions. By contrast, with the enrichment factors (EF), geo-accumulation (Igeo) and potential ecological risk index (PERI) of HMs in soil under different land-use types, the results indicated that these HMs exhibited non-pollution (Igeo < 0) and no ecological risk (PERI < 30) to human health in soils of Yinjiang County.

CONCLUSIONS

The distribution of HMs is dominated by weathering in the karst area, and the effects of agricultural inputs on the enrichment of soil HMs in Yinjiang County are limited. This further state that the arrangement of the local agricultural structure is reasonable.

Ecological Security Pattern Construction in Karst Area Based on Ant Algorithm

Constructing the ecological security pattern is imperative to stabilize ecosystem services and sustainable development coordination of the social economy and ecology. This paper focuses on the Karst region in southeastern Yunnan, which is ecologically fragile. This paper selects the main types of ecosystem services and identifies the ecological source using hot spot analysis for Guangnan County. An inclusive consideration of the regional ecologic conditions and the rocky desertification formation mechanism was made. The resistance factor index system was developed to generate the basic resistance surface modified by the ecological sensitivity index. The Ant algorithm and Kernel density analysis were used to determine ecological corridor range and ecological restoration points that constructed the ecological security pattern of Guangnan County. The results demonstrated that, firstly, there were twenty-three sources in Guangnan County, with a total area of 1292.77 km², accounting for 16.74% of the total. The forests were the chief ecological sources distributed in the non-Karst area, where Bamei Town, Yangliujing Township and Nasa Town had the highest distribution. Secondly, the revised resistance value is similar to “Zhe (Zhetu Township)-Lian (Liancheng Town)-Yang (Yangliujing Township)-Ban (Bambang Township)”. The values were lower in the north and higher in the south, which is consistent with the regional distribution of Karst. Thirdly, the constructed ecological security pattern of the “Source-Corridor-Ecological restoration point” paradigm had twenty-three ecological corridors. The chief ecological and potential corridor areas were 804.95 km² and 621.2 km², respectively. There are thirty-eight ecological restoration points mainly distributed in the principal ecological corridors and play a vital role in maintaining the corridor connectivity between sources. The results provide guidance and theoretical basis for the ecological security patterns construction in Karst areas, regional ecologic security protection and sustainable development promotion.

Household livelihood choices under the different eco-environment in the karst area: A case study of Anshun City, southwest of China

Farmer household livelihood choice has been critical in ecosystem and natural resource sustainability in developing countries. This paper investigates the choices of farmer household livelihood in a Karst environment in China by using a multinomial logistic regression model to analyze 676 farmer household questionnaire responses from 6 counties in Anshun City, Guizhou Province. The results show that factors such as land quality, regional accessibility, government policies, and farmer households' characteristics have significant effect on farmer household livelihood choices. Even though farmers are unwilling to live in areas of rocky desertification and soil erosion, the proportion of those who live in those areas is high, because alternatives are limited by environment conditions and farmer households' situations. Good regional traffic accessibility facilitates farmer households' part-time non-agricultural employment. Meanwhile, the Rocky Desertification Control Project increases migrant laborers. The study suggests that government policies diversify the choices of farmer household livelihoods in different eco-environment regions.

Sources and migration of heavy metals in a karst water system under the threats of an abandoned Pb-Zn mine, Southwest China

Water quality is of great concern under the threats of an abandoned Pb-Zn mine within a vulnerable karst area. This study focuses on tracing the sources, migration and relevant geochemical controls of heavy metals along with surface- and groundwater flow, and assessing their hazards posed to the environment by indexes of heavy metal pollution (HPI) and ecological risks (ERI). We analyzed the concentrations of lead (Pb), zinc (Zn), cadmium (Cd) and copper (Cu) in filtered and digested surface water (SW) and groundwater (GW) samples. Whilst sulfate concentrations, δ³⁴S_SO4 isotope, and other physical-chemical parameters were measured in filtered SW and GW. Clearly elevated concentrations of Zn and Cd have been observed in the upstream of the Sidi River (max. 1540.3/1676.7, and 19.9/19.4 μg/L, for filtered/digested contents, respectively), and that of digested Pb and Cu (max. 17.8 and 114.6 μg/L, respectively) in the downstream karst groundwater. Zn and Cd are likely introduced by the Sidi River which is found mostly in the dissolved phase. While Pb and Cu are sourced from the remobilization of upper lying polluted soil which mostly existed in the particulate phase. Zn and Cd concentrations in the karst groundwater peak at the losing section while Pb and Cu in the river peaks at the gaining section as a result of metals transport with SW-GW exchange. Dilution significantly decreases total metals concentration in the non-karst area. Due to the buffer effect caused by carbonates dissolution and bacterial sulfate reduction, a steady neutral pH can reduce the concentrations of the dissolved metals in the karst area. The ERI and HPI assessments show an acceptable level for surface- and groundwater. A long-term observation on the contents of undissolved metals needs to be conducted in karst areas which are threatened by metal(s) mines.

Worldwide cadmium accumulation in soybean grains and feasibility of food production on contaminated calcareous soils

Elevated toxins in soybeans extensively threaten Asian residents and over one billion vegetarians worldwide. An integrated dataset of toxic trace metal(loid)s especially cadmium (Cd) analysis in soybean grain samples (n = 5217) from 12 countries/regions of origin was compiled for risk analysis. Worldwide grain Cd averaged 0.093 mg kg-1, but mean values varied 16-fold between regions, with South China (0.32 mg kg-1) > Argentina (0.15 mg kg-1) = German (0.13 mg kg-1) > Japan (0.11 mg kg-1) > the United States (0.064 mg kg-1) > Central-North China (0.020-0.60 mg kg-1) ≥ Iran (0.042 mg kg-1) = Brazil (0.023 mg kg-1) = South Korea (0.020 mg kg-1). Regression analysis suggested widespread contamination and acidic soil features significantly contributed the elevated food Cd contamination worldwide. Arsenic (As) and lead (Pb) are also of concern because excessive levels were often observed in grains. Given that soil Cd bioavailability is generally low in alkaline pH ranges, the feasibility of producing safe food from contaminated land was investigated by greenhouse experiments with one low-Cd soybean cultivar grown on 20 contaminated calcareous soils. Equilibrium-based approaches i.e., 0.01 M CaCl2 and in-situ porewater extractions, and diffusion-based diffusive gradients in thin-films technique were used to determine the plant-available fractions of soil metal(loid)s to explain the bioaccumulation variation. The results suggested that soybean grains bioaccumulated mean 0.76 mg Cd kg-1, ranging from 0.16 to 2.1 mg kg-1, whereas As and Pb bioaccumulation was low. Cadmium accumulation was closely correlated with plant-available Cd fractions especially the 0.01 M CaCl2-extractable Cd, but negatively correlated with soil pH. Even in the alkaline pH range, a slight decrease of soil pH would increase grain Cd significantly. Study region and those arable lands that have similar soil conditions are not recommended for growing soybean unless novel remediation strategies are developed.

Contrasting bacterial communities and their assembly processes in karst soils under different land use

Structure and assembly processes of soil bacterial communities under different land use at karst areas remained poorly understood to date. To address this issue, soil samples from arable land and pristine forest over a karst cave, located in the acid rain impacted area, Hubei province, were collected and subjected to high-throughput sequencing and multivariate statistical analysis. Bacterial communities and functions remarkably distinguished between soils under different land use. Both edaphic properties (the content of SO₄^2-, C/N, pH, TN) and weathering processes, such as Si concentration, Mg/Al and Ca/Al, significantly impacted on soil bacterial community structures. Variable selections were predominant ecological processes, and pH and SO₄^2- concentration were of significance in community assembly. Random molecular ecological network analysis revealed a more stable and complex microbial network in the forest ecosystem, which can quickly response to environmental change. Forest soil bacteria were mainly phototrophs, involving in C and N cycles, whereas those in arable soils were mainly chemoheterotrophs, capable of degrading organic fertilizers due to anthropogenic activities as confirmed by the analysis of keystone taxa, indicators and functional prediction. These results reveal that land use constructed soil bacterial communities in different aspects such as the structure, potential functions, microbial interactions and correlations with environmental variables. To our knowledge, this is the first report on bacterial community assembly in karst soils under different land use which enhances our understanding about how land use impact on microbial interaction and community assembly processes.

Potential Ecological Risk Assessment of Heavy Metals in the Fe-Mn Nodules in the Karst Area of Guangxi, Southwest China

We investigated the concentration of heavy metals in the Fe-Mn nodules in soils derived from the carbonate rocks of typical karst areas in Guangxi, Southwest China. Compared with the soil background values in Guangxi, heavy metals are substantially enriched in the following order: Cd (268.13) > Pb (39.46) > Cr (11.80) > Zn (8.43) > Ni (6.16) > Cu (3.65). CaCl₂ extraction results indicate that heavy metals in the nodules are substantially stable, while the proportions of the metals released to the surrounding environment are extremely low (≤ 0.003%). Moreover, pH-static leaching experiments indicate that heavy metals can hardly be dissolved into the surrounding soil environment under natural conditions (pH 6-8). However, once the soil environment is acidified, heavy metals incorporated in the nodules will gradually release into the surrounding environment, thereby causing potential ecological risks.

[Spatial distribution of preferential flow paths in paddy field in the Karst Area of Guangxi.]

The spatial distribution of the preferential flow path in soil affects the occurrence and movement of preferential flow. In afield soil staining and tracing experiment, with the help of morphological image analysis technology and community ecological analysis method, we examined the spatial distribution characteristics of preferential flow path in paddy fields under straw mulching (CM) and non-mulching (CK) in karst areas of Guangxi. The results showed that, under the same external water supply conditions, with the increases of soil depth, soil horizontal dyeing changed from overall distribution to clump aggregation distribution in paddy field under straw mulching. In the non-mulching paddy field, the main dyeing was branch, with an average shape coefficient of staining patch of 21.69. The dyeing morphology in the non-mulching paddy field was 1.04 times of straw mulching paddy field. The preferential flow paths of straw mulching and non-mulching paddy fields showed both clustered distribution. In straw mulching paddy field, the Morisita index was 1.28 as the preferential flow paths with radius of influence less than 1.0 mm were the main ones (importance value 0.31). The importance value of preferential flow paths with radius less than 1.0 mm was 0.28 in non-straw mulched rice field. Straw mulching affected the preferential flow paths in paddy fields, which could help to improve water and fertilizer utilization of crops in the field.

Comprehensive evaluation on water resource carrying capacity in karst areas using cloud model with combination weighting method: a case study of Guiyang, southwest China

It is important to maintain the sustainable development of water resources. Objective assessment on water resource carrying capacity (WRCC) is beneficial to the formulation of scientific and reasonable water management practices. In view of the problem that evaluation indicators of WRCC cannot describe the fuzziness and randomness, a cloud model was introduced into regional WRCC assessment. This study selected a typical karst area (Guiyang) as the research object to study WRCC by using cloud model with combination weighting method. WRCC was assessed from the following five dimensions: water environment subsystem, social subsystem, economic subsystem, ecological subsystem, and humanities (water resource management and policy regulation) subsystem. In addition, evaluation results after normalizing all of indicators data were also calculated. And these two kinds of evaluation results were compared with that of technique of order preference similarity to the ideal solution (TOPSIS), finding that evaluation results of cloud model were consistent with that of TOPSIS method. The cloud model realizes the transformation from qualitative evaluation to quantitative evaluation, which overcome insufficiencies of traditional evaluation methods in considering fuzziness and randomness. Results showed that during the period of 2008-2017, the state of WRCC in Guiyang was improving year by year, increasing from the serious overload carrying capacity level in 2008 to the strong carrying capacity level in 2017 (serious overload-overload-critical-weak carrying capacity-strong carrying capacity). However, some certain evaluation indicators are still in danger situation, such as population natural growth rate and use of the fertilizer per unit cultivated area, which needs to be further enhanced and improved. Moreover, the contradiction among economic development, population growth, and water resources is becoming increasingly apparent. To ensure the effective utilization of water resources in Guiyang, reasonable policies and measures should be formulated and put into effect. Research results could provide certain reference for the sustainable development of regional water resources.

[Effects of biochar addition on soil moisture infiltration process of the yellow soil in Karst area]

To investigate the effects of biochar addition on soil moisture infiltration characteristics of sloping farmland in Karst area, we used soil column simulation to study the variation characteristics of cumulative infiltration volume, infiltration rate, and wetting peak process under the different biochar addition amount (0, 1% and 2%) and different particle sizes (<0.25, 0.25-1 and >1 mm), and simulated the infiltration process in yellow soil on slope farmland. The results showed that soil infiltration process after biochar addition was significantly inhibited under the condition of constant bulk density. The cumulative infiltration amount and infiltration rate under biochar addition were significantly lower than those without biochar addition. There was no significant difference in the cumulative infiltration amount and infiltration rate of the soil with 1% and 2% biochar addition. The cumulative infiltration amount of the soil with different particle sizes followed an order of <0.25, 0.25-1 and > 1 mm after biochar addition. When the addition amount was 1%, the cumulative infiltration amount of 300 min had decreased by 20.9%, 35.2% and 45.0% compared with CK. When the addition amount was 2%, the decrease rate was 21.5%, 37.5% and 44.2%, indicating that the inhibition effect of large particle size biochar on soil infiltration being stronger than that of small particle size biochar. The change trend of soil wetting peak process to biochar addition of different contents and different particle sizes was consistent with the change trend of cumulative infiltration volume. Horton model and Kostiakov model could be used to simulate soil moisture infiltration process. The Horton model had higher fitting accuracy, the largest R² (between 0.91 and 0.98), and the smallest RMSE (between 0.14 and 0.21). The initial infiltration rate obtained by Kostiakov model was closer to the measured result. Our results could provide scientific basis for the rational application of biochar and provide a useful reference for soil improvement and soil and water conservation in slope farmland of Karst area.

Arsenic oxidation and immobilization in acid mine drainage in karst areas

High concentrations of arsenic (As) occur in acid mine drainage (AMD), while the mechanisms governing its distribution along the flow of AMD are not fully understood. In this study, As species distribution was surveyed along the flow of an AMD in Jiaole coal mine in a typical kast area, in which length of creek is about 1100 m. AMD from the discharging source contained 1754.2 μg/L As (1570.0 μg/L in As (III)) and 644.1 mg/L Fe (all in Fe (II)) at pH 3.45. Both As and Fe concentrations decreased drastically to trace levels along the flow in the creek. As(III) oxidation to As(V) and Fe(II) oxidation to Fe(III) were discovered in a short distance from the discharging source. Lab experiments were performed to unveil the mechanisms governing As and Fe species distribution. Biological mechanism governed As(III) and Fe(II) oxidation in the AMD phase without contact with solid matrix, while different mechanisms governed the oxidation in the presence of solid matrix at different stages of AMD flow. At the beginning of AMD discharge, its contact with the soil matrix in rich of carbonate minerals in the karst area facilitated Fe(II) oxidation by O₂ due to pH rise, which generated reactive oxidants for As(III) oxidation and iron oxyhydroxides for As adsorption or co-precipitation. Along the AMD flow, bacteria in the underlying sediments profoundly accelerated the biological oxidation of As(III) and Fe(II) as well as the co-precipitation into the sediments. Findings of this study deepen the understanding of As transport and transformation along the AMD flow, particularly in karst areas.

Accumulation of organic carbon in a large canyon reservoir in Karstic area, Southwest China

Artificial reservoirs play an important role in intercepting organic carbon (OC), which affects the carbon cycle of inland waters. However, limited by the methods of field observations, studies on the potential carbon sink of sediment in reservoirs are not accurate enough. In this study, we investigated OC accumulation in the Wujiangdu Reservoir (WJDR), a canyon reservoir located in a karst area of Southwest China, by coupling seismic survey data with physicochemical data of sediment cores. The results showed that irregular underwater topography resulted in spatial heterogeneity of the sediment deposition. The mean OC accumulation rate in the WJDR was 579.2 g C m^-2 year^-1, and the OC accumulation flux of the WJDR was 1.4 × 10¹⁰ g C year^-1. δ¹³C values coupled with a mixing model indicated that the autochthonous OC ranged from 14.2 to 97.0% of the total OC in the sediment, with an average of 48.4%. The significantly positive relationship between autochthonous OC and TN indicated that the autochthonous OC generated in the reservoir was associated with nutrient input. The autochthonous OC accumulation flux was 6.6 × 10⁹ g C year^-1, which was equal to 2.3 times the carbon emission from the reservoir surface to the atmosphere as CO₂-C, indicating that the autochthonous OC produced within the reservoir is a large carbon sink.

Zinc isotope revealing zinc's sources and transport processes in karst region

The heavy metal pollution, mainly caused by mining-related activities over extended period of time, is imposing a severe threat to environments and human health. Environmental systems, including rivers and paddy soils, have been widely established as one of the key sinks of potentially harmful metals. Aiming to understand contamination sources and pathways of Zn in karst area, we studied the Zn concentration and isotope composition of river waters, sediments, mine tailings, paddy soils, dust and three soil profiles with different levels of Zn-pollution around a Zn-mine, southwestern China. The distinct Zn isotope compositions among tailing (-0.42 ± 0.02‰), dust (-0.24 ± 0.02‰), and geogenic soil (-0.16 ± 0.03‰) allowed for separation of anthropogenic-Zn from native Zn. In the plot of δ⁶⁶Zn value and 1/Zn, all samples can be explained by the mixing of three components: mining-input, agricultural input, and background. Evolution of these three components helps produce direct sources: dust and geogenic soil. Under this framework, the Zn pollution in paddy soil and sediment can be explained by mixing of mine-tailing, dust, and geogenic soil. Our study shows that the contamination of mine drainage is limited in the area due to the relatively high pH buffered by carbonate in karst area. While the dust contributes most of the anthropogenic Zn with an average value of 19.5%. The dominant pathway of anthropogenic Zn from dust to paddy soil or sediment is through the long-term wind dispersion of fine-grained material from the tailing and the physical transmission. Under the special hydrogeological conditions of karst, mining activities will increase the migration of heavy metals. The Fe-Al oxides control the migration of Zn in soil profile, but probably do not lead to significantly Zn isotopes fractionation. This further enhances the reliability of Zn isotopes as a "fingerprint" in karst area.

Effects of afforestation on soil CH4 and N2O fluxes in a nsubtropical karst landscape

Afforestation is of importance for terrestrial carbon sequestration as well as soil and water conservation in karst landscapes. However, few studies have evaluated the effects of afforestation on soil CH₄ and N₂O emissions in subtropical karst areas. Thus, a year-round field experiment was conducted to quantify the effects of afforestation on soil CH₄ and N₂O fluxes from a subtropical karst landscape in South China. In this study, soil CH₄ and N₂O fluxes were simultaneously monitored using static chamber-gas chromatography from three paired sites, including a cropland site (SC) and adjacent sites at two stages of afforestation, a shrubland (SD) and a woodland (AF). The results showed that annual soil CH₄ uptake for SC, SD, and AF sites were 1.53 ± 0.20 kg C ha^-1 yr^-1, 2.90 ± 0.20 kg C ha^-1 yr^-1, and 5.68 ± 0.18 kg C ha^-1 yr^-1, respectively. Afforestation (i.e., SD and AF sites) significantly increased soil CH₄ uptake compared with the adjacent cropland. Annual soil N₂O fluxes for SC, SD, and AF sites were 2.38 ± 0.17 kg N ha^-1 yr^-1, 0.94 ± 0.14 kg N ha^-1 yr^-1, and 0.47 ± 0.01 kg N ha^-1 yr^-1, respectively. Afforestation significantly decreased soil N₂O fluxes compared with the adjacent cropland. The effects of afforestation on soil CH₄ and N₂O fluxes in the present study were mainly attributed to changes in soil characteristics, such as temperature and moisture, as these were significantly correlated with soil CH₄ and N₂O fluxes across different experimental sites. The present study highlights that afforestation is an effective land use management practice to mitigate non-CO₂ greenhouse gas emissions from subtropical karst landscapes in South China.

Identification of the sources and influencing factors of potentially toxic elements accumulation in the soil from a typical karst region in Guangxi, Southwest China

Southwestern China contains the largest and most well-developed karst region in the world, and the potentially toxic elements (PTEs) content in the soils of the region is remarkably high. To explore the internal and external control factors and sources of soil PTEs enrichment in this area and to provide a basis for the treatment of PTE pollution, 113 soil samples were collected from Hengxian County, a karst region in Guangxi Province, southwestern China. The importance of eighteen influencing factors including parent material, weathering, physicochemical properties, topography and human activities were quantitatively analyzed by (partial) redundancy analysis. The sources of PTEs were identified using the Pb isotope ratio and absolute principal component score/multiple linear regression (APCS-MLR) model. The contents of all soil PTEs were higher than the corresponding background values of Guangxi soils. The contents in Cu, Zn, Cd, Hg and Pb were the highest in the soil from carbonate rock. The factor group of geological background and weathering explained 26.5% for the accumulation and distribution of soil PTEs and the influence of physicochemical properties was less than 2% but increased to 25.6% through interaction with weathering. Fe (47.1%), Al (42.1%), Mn (22%), chemical index of alteration (12.8%) and clay (11.9%) were the key factors affecting the soil PTEs, while the influence of human activities was weak. Pb isotope ratio and APCS-MLR classified 62.8-74% of soil PTEs as derived from natural sources, whereas 18.23% and 18.95% were derived from industrial activities and agricultural practice/traffic emissions, respectively. The Pb isotope ratio showed that the natural sources account for up to 90% of the Pb in the soil from carbonate rock, the highest contribution among the studied soils. The results of the study can provide background information on the soil PTEs contamination in the karst areas of China and other areas worldwide.

Effects of vegetation restoration on soil quality in degraded karst landscapes of southwest China

Vegetation restoration was implemented to control soil erosion in the karst regions of southwest China. It is essential to assess the soil function and quality scientifically during this process and to adopt suitable management practices for this area. However, few studies have been conducted to comprehensively evaluate the effect of vegetation restoration on soil quality in this severely eroded karst area. By taking 302 soil samples from 11 vegetation types, this study investigated the influence of different types of vegetation restoration on soil quality using an integrated soil quality index (SQI) and a generalized linear model (GLM). Vegetation types had significant effects on soil properties and thus on soil quality. SQI was developed by using TN, TP, TK, AP, and clay content; TN had highest weighting values (0.58), which indicated that it contributed the most to final SQI. The highest and lowest SQI values were observed for primary forest and cropland, respectively. Overall, vegetation restoration (e.g., natural restoration, artificial forests and artificial grassland) improved soil quality significantly. A GLM model explained 73.20% of the total variation in SQI, and vegetation type explained the largest proportion (46.39%) of the variation, which implies that the vegetation restoration practices can greatly enhance the soil quality in karst landscapes of southwest China. The results of this study may be used to improve implication of ecological restoration and management in degraded regions.

Response of vegetation to water balance conditions at different time scales across the karst area of southwestern China-A remote sensing approach

This work identifies the vegetation communities, landform types and seasons in which vegetation is most sensitive to water imbalance in the karst area of southwestern China. The normalized difference vegetation index (NDVI) and standardized precipitation and evapotranspiration index (SPEI) were used to evaluate the effects of water balance conditions on vegetation in different seasons and at different time scales. During the growing seasons from 1982 to 2013, the vegetation growth in 79% of the study area was statistically significantly sensitive to the water balance condition (p < 0.05). The vegetation in the spring and autumn responded more visibly to water imbalances. The SPEI over the last 6 months was statistically significantly correlated with the monthly maximum NDVI during the growing season over the larger areas compared with the SPEI over other time periods. Therefore, the vegetation was most likely sensitive to six months of water imbalance in this area. Among the selected vegetation types, the shrubland and sparse woodland were the most sensitive to water imbalances, whereas grasslands and forests were less sensitive. The maximum correlation coefficient between the NDVI and SPEI for each karst landform type was statistically significantly different (p < 0.01). The vegetation in the peak-cluster depressions was the most sensitive to water imbalances, whereas the vegetation in the middle and high karst mountains was the least sensitive to water imbalances. Overall, although the climate of the karst region of southwestern China is humid and subtropical, the vegetation is still vulnerable to water imbalances in particular regions and soils.

Predicting of ultrafiltration performances by advanced data analysis

In order to optimize drinking water production operation, membrane users can use several analytical tools that help membrane fouling prediction and alleviate fouling by a proper feed water resource selection. However, during strong fouling event, membrane decision-makers still face short-term deadline to decide between different options (e.g. optimization of pretreatment or change in feed water quality). Hence, statistical approach might help to better select the most relevant analytical parameter related to fouling potential of a specific resource in order to speed-up decision taking. In this study, the physical and chemical properties and the filtration performances (at lab-scale) of five ground water resources, selected as potential resources of a large drinking production site of Paris (France), was evaluated through one year. Principal component analysis emphasizes the strong link between waters' organic matrix and fouling propensity. Cluster analysis of filtration performances allowed classifying the water samples into three groups exhibiting strong, low and intermediate fouling. Finally, multiple linear regressions performed on all collected data indicated that strong fouling events were related to a combined increase of carbon content and protein like-substances while intermediate fouling might only be anticipated by an increase of fluorescence signal associated to protein like-substances. This study demonstrates that advanced data analysis might be a powerful tool to better manage water resources selection used for drinking water production and to forecast filtration performances in a context of water quality degradation.

Lentzea guizhouensis sp. nov., a novel lithophilous actinobacterium isolated from limestone from the Karst area, Guizhou, China

A novel filamentous actinobacterium, designated strain DHS C013(T), was isolated from limestone collected in Guizhou Province, South-west China. Morphological and chemotaxonomic characteristics of the strain support its assignment to the genus Lentzea. Phylogenetic analyses showed that strain DHS C013(T) is closely related to Lentzea jiangxiensis FXJ1.034(T) (98.7 % 16S rRNA gene similarity) and Lentzea flaviverrucosa 4.0578(T) (98.0 % 16S rRNA gene similarity), but it can be distinguished from these strains based on low levels of DNA:DNA relatedness (~44 and ~37 %, respectively). Physiological and biochemical tests also allowed phenotypic differentiation of the novel strain from these closely related species. On the basis of the evidence presented here, strain DHS C013(T) is concluded to represent a novel species of the genus Lentzea, for which the name Lentzea guizhouensis sp. nov. is proposed. The type strain is DHS C013(T) (=KCTC 29677(T) = CGMCC 4.7203(T)).