Karst Critical Zone Research Articles

Land use interacts with changes in catchment hydrology to generate chronic nitrate pollution in karst waters and strong seasonality in excess nitrate export

Agricultural land in karst systems can pollute water courses, with polluted waters travelling quickly to and through the sub-surface. Understanding how rapidly nitrate moves within the highly-transmissive karst critical zone (from soils to aquifers) is limited by low resolution data. To understand nitrate behavior and its controls, we deployed sensor technology at five sites to generate autonomously high-resolution time series of discharge and NO3−–N, which is the major nitrogenous component, in a farmed karst catchment in Southwestern China. The [NO3−–N] time series exhibited rapid response to rainfall-induced increases in discharge and a large magnitude in [NO3−–N], from 0.72 to 16.3 mg/L across five sites. However, the magnitude of NO3−–N response at each site was varied during rainfall events (wet season) and dry season. The highest mean [NO3−–N] and normalized annual fluvial export occurred in a headwater catchment with a developed karst aquifer system. Seasonal variation in NO3−–N export occurred in response to source availability, most notable in catchments with valley agriculture: in the wet season up to 94% of nitrate was exported from the headwater catchments within two months, but at the larger catchment scale, over the 6 month wet season, only 61% of total export occurred. At the larger catchment scale, [NO3−–N] were lower due to buffering by the karstic aquifer network. From the time series we observe little decrease in [NO3−–N] as discharge decreases in the dry season, indicating the karst aquifers are chronically-polluted with nitrate through slow flow pathways.

Spatiotemporal variations of dissolved inorganic carbon and controlling factors in a small karstic catchment, Southwestern China

AbstractDissolved inorganic carbon (DIC) is the most important carbon component in karst aquatic system where fluid is highly transmissive, but has rarely been examined in the subtropical karst critical zone (K‐CZ). In this study, concentrations of dissolved solutes and isotopic compositions of DIC (δ13CDIC) at 11 sites of a 73.4 km2karstic catchment in Southwestern China were analysed monthly in order to uncover the spatiotemporal variations of both DIC and its dominant sources, and to identify relevant controlling factors. Both DIC concentrations and δ13CDICwere highly variable, ranging from 2.52 to 5.85 mmol l−1and from −15.7 to −4.5‰, respectively. DIC in underground water (UGW) was higher in concentration and more depleted in13C compared to surface water (SFS). DIC concentrations showed an inconsistent seasonal trend with other solutes, with higher values in the wet season at some sites. δ13CDICvalues were lower in the wet season than in the dry season. The results of mixing model IsoSource revealed spatiotemporal patterns of DIC sources. During the dry season, carbonate weathering was the primary contributor to DIC in UGW (excluding in the middle reaches). However, during the wet season, soil CO2was the dominant source of DIC in both UGW and SFS, and it was higher than in the dry season. Overall, there are significant spatiotemporal disparities and highly transmissive characteristics of both DIC and its sources in the K‐CZ, which are controlled by multiple factors. This study also highlights that rainfall may play a crucial role in accelerating carbon dynamics in the K‐CZ. High‐frequency sampling campaigns in high‐flow periods and deep analyses are needed in future work to elucidate the related processes and mechanisms. © 2019 John Wiley & Sons, Ltd.

Soil enzyme activity and stoichiometry along a gradient of vegetation restoration at the Karst Critical Zone Observatory in Southwest China

AbstractThe “Grain for Green Programme” was implemented in the 1990s as a solution to the extreme degradation of karst landscapes that cover one‐third of China, largely caused by decades of poorly managed intensive agriculture. The recovery of soil functions is key to the success of ecosystem regeneration of abandoned croplands where the carbon (C) and nutrient cycles have been severely perturbed by cultivation. However, an ecological ‘tipping point’ beyond which soil functions are unrecoverable in manageable timescales may have been passed in the fragile, subtropical karst ecosystem. The aim of this study was to use the activity of key enzymes for C, nitrogen (N), and phosphorus (P) acquisition in the soil as a proxy for the biological response to vegetation restoration after agricultural abandonment in a severely degraded karst catchment at the Karst Critical Zone Observatory in Guizhou Province. In 2016, a space‐for‐time approach was used to establish a chronosequence of vegetation recovery: sloping cropland < recently abandoned sloping cropland < shrubland < secondary (regenerated) forest < primary (natural) forest. Soils were sampled from the surface to the bedrock (up to 80‐cm depth) in each recovery phase. The activity of all enzymes in the top 0 to 30‐cm depth increased after abandonment and was positively correlated with soil nutrient and water contents. Nitrogen deficiencies were indicated by the reduced ratios of C‐ relative to N‐hydrolase activity and the increased ratios of N‐ relative to P‐hydrolase activity in the abandoned croplands and shrublands. Phosphorus deficiencies were indicated by the reduced ratios of N‐ relative to P‐hydrolase activity and C‐ relative to P‐hydrolase activity in the soils of the shrubland and secondary forest compared with the primary forest. Our results revealed that near‐to‐natural biological soil function was recoverable as vegetation naturally restored and suggested that the rate of recovery may be accelerated by managed nutrient amendments during the early stages after abandonment. This new information may help to inform the managed regeneration of degraded agricultural land in nutrient‐poor, subtropical environments.

Soil organic carbon sequestration in soil aggregates in the karst Critical Zone Observatory, Southwest China

Soil organic carbon (SOC) sequestration in aggregates under land use change have been widely concerned due to intimate impacts on the sink (or source) of atmospheric carbon dioxide (CO<sub>2</sub>). However, the quantitative relationship between soil aggregation and SOC sequestration under land uses change has been poorly studied. Distribution of aggregates, SOC contents in bulk soils and different size aggregates and their contributions to SOC sequestration were determined under different land uses in the Puding Karst Ecosystem Observation and Research Station, karst Critical Zone Observatory (CZO), Southwest China. Soil aggregation and SOC sequestration increased in the processes of farmland abandonment and recovery. SOC contents in micro-aggregates were larger than those in macro-aggregates in restored land soils, while the opposite results in farmland soils were obtained, probably due to the hindrance of the C-enriched SOC transport from macro-aggregate into micro-aggregate by the disturbance of agricultural activities. SOC contents in macro-aggregates exponentially increased with their proportions along successional land uses. Macro-aggregates accounted for over 80% on the SOC sequestration in restored land soils, while they accounted for 31–60% in farmland soils. These results indicated that macro-aggregates have a great potential for SOC sequestration in karst soils.

Structural Features and Function of the Karst Critical Zone

Structural Features and Function of the Karst Critical Zone

Characterization of karst structures using quasi-3D electrical resistivity tomography

Karst is characteristically complex, hydrogeologically, due to a high degree of heterogeneity, which is often typified by specific features, for example, cavities and sinkholes, embedded in a landscape with significant spatial variability of weathering. Characterization of such heterogeneity is challenging with conventional hydrogeological methods, however, geophysical tools offer the potential to gain insight into key features that control the hydrological function of a karst aquifer. Electrical resistivity tomography (ERT) is recognized as the most effective technique for mapping karstic features. This method is typically carried out along transects to reveal 2D models of resistivity variability. However, karstic systems are rarely 2D in nature. In this study, ERT is employed in valley and hillslope regions of a karst critical zone observatory (Chenqi watershed, Guizhou province, China), using a quasi-3D approach. The results from the extensive geophysical surveys show that there is a strong association between resistivity anomalies and known karstic features. They highlight the significance of a marlstone layer in channeling spring flow in the catchment and confining deeper groundwater flow, evidenced by, for example, localized artesian conditions in observation wells. Our results highlight the need to analyze and interpret geophysical data in a three-dimensional manner in such highly heterogeneous karstic environments, and the value of combining geological and hydrogeological data with geophysical models to help improve our understanding of the hydrological function of a karst system.

Spatial distribution and controlling factors of heavy metals in soils from Puding Karst Critical Zone Observatory, southwest China

Soil profile samples under different land-use types were collected at the Puding Karst Critical Zone Observatory to investigate the composition, distribution and controlling factors of heavy metals. The heavy metal contents of Cr, Mn, Fe, Ni, Cu, Zn, Cd and Pb were determined, and their relationships to soil properties were examined. The Mn, Zn, Cd and Pb contents were larger in the topsoil than the deeper layer in all land use. In secondary forest land, the Cr, Fe, Ni and Cu contents in the topsoil were lower than in the deeper layer; however, all eight heavy metal contents were enriched in the topsoil under cropland. The results showed soil organic carbon played a fundamental role in controlling of Cd, while it showed negative relationship with Cr, Fe, Ni and Cu. Soil pH was positively associated with Ni and Cu. The Cr, Fe, Ni and Cu contents were significantly correlated to proportion of micro-aggregates, while the Cd and Pb contents showed a positive correlation with proportion of macro-aggregates. According to principal component analysis, the Cr, Fe, Ni and Cu contents might be affected by soil organic matter; however, the Mn, Zn and Pb contents might be influenced by atmospheric heavy metal deposition, and the Cd content might be controlled by both organic matter and atmospheric deposition. The geoaccumulation index and enrichment factor were calculated to assess the pollution level of soils, and the results showed that most of the selected heavy metals in soils may not be present at sufficient levels for contamination, and Mn and Fe were not from a pollution source and might come from natural weathering processes. This research will help researchers make strategic decisions about food security in the choice of agricultural land.

Variations and Indications of δ13CSOC and δ15NSON in Soil Profiles in Karst Critical Zone Observatory (CZO), Southwest China

Soil carbon and nitrogen storage and stabilization are the key to solving the problems of mitigation of global warming and maintaining of crop productivity. In this study, the contents of soil organic carbon (SOC) and soil organic nitrogen (SON) and their stable isotope compositions (δ13CSOC and δ15NSON) in soil profiles were determined in two agricultural lands (including a farmland and an abandoned farmland) and four non-agricultural lands (including two shrub-grass lands and two shrub lands) in the karst critical zone observatory (CZO), Southwest China. The contents of SOC and SON were used for research on the effects of land use on SOC and SON storage, and the change of δ13CSOC and δ15NSON values in soil profiles were used to indicate SOC and SON stabilization. The results showed that agricultural activities reduced SOC and SON storage in the whole soil layers of farmland compared to non-agricultural lands, and farmland abandonment slightly increased SOC and SON storage. Crop rotation between peanut (C3) and corn (C4) affected the δ13CSOC in surface soils of agricultural lands (−21.6‰), which were intermediate between shrub lands (−22.7‰) and shrub-grass lands (−19.6‰). 15N-depleted SON in surface soils in farmland compared to those soil in other lands possibly associated with synthetic N fertilizer application. In soil layers below 30 cm depth the δ13CSOC deceased with depth, while the δ15NSON displayed irregular fluctuation. The change in δ13CSOC and δ15NSON through soil profiles in karst soils were more intensive than those in semiarid grassland soils indicating the less stabilization of SOC and SON in karst soils.

Rock crevices determine woody and herbaceous plant cover in the karst critical zone

The study of the critical zones (CZs) of the Earth link the composition and function of aboveground vegetation with the characteristics of the rock layers, providing a new way to study how the unique rock and soil conditions in karst regions affect the aboveground vegetation. Based on survey results of the rocks, soils and vegetation in the dolomite and limestone distribution areas in the karst area of central Guizhou, it was found that woody plant cover increases linearly with the number of cracks with a width of more than 1 mm, while the cover of herbaceous plants shows the opposite trend ( p 20 cm, which is suitable for the distribution of herbaceous plants with shallow roots. Due to the development of crevices in the limestone distribution area, the soil is deeply distributed through the crevices for the deep roots of trees, which leads to a diversified species composition and a complicated structure in the aboveground vegetation. Based on moderate resolution imaging spectroradiometer (MODIS) remote sensing data from 2001 to 2010, the normalized differentiated vegetation index (NDVI) and annual net primary productivity (NPP) results for each phase of a 16-day interval further indicate that the NDVI of the limestone distribution area is significantly higher than that in the dolomite distribution area, but the average annual NPP is the opposite. The results of this paper indicate that in karst CZs, the lithology determines the structure and distribution of the soil, which further determines the cover of woody and herbaceous plants in the aboveground vegetation. Although the amount of soil in the limestone area may be less than that in the dolomite area, the developed crevice structure is more suitable for the growth of trees with deep roots, and the vegetation activity is strong. At present, the treatment of rocky desertification in karst regions needs to fully consider the rock-soil-vegetation-air interactions in karst CZs and propose vegetation restoration measures suitable for different lithologies.

Response of soil water hydrochemistry and δ13CDIC to changes in spatio-temporal variations under different land covers in SW China karst catchment

Abstract Soil water plays a crucial role in biogeochemical processes within karst ecosystems. However, geochemical variations of soil waters under different land covers and the related karst critical zone processes are still unclear. In this study, five land covers, including grassland, dry land, shrub land, reforestation land, and bamboo land in the Qingmuguan karst area of Chongqing Municipality, Southwest (SW) China were investigated in order to better understand the spatio-temporal variations of soil water geochemistry and its controlling mechanisms. The hydrochemistry of soil water and stable carbon isotopic compositions of dissolved inorganic carbon (δ13CDIC) in soil water were analyzed by a semi-monthly sampling strategy. The results show that there is remarkable spatio-temporal variation in the hydrochemistry and δ13CDIC of soil waters under different land covers in the studied area. Soil waters collected from shrub, dry, and afforestation lands have higher total dissolved solids (TDS), Ca2+, and HCO3− concentrations and heavier δ13CDIC, which is probably associated with the stronger carbonate dissolution caused by higher soil CO2 and carbonate content in soils under these land covers. However, lower TDS, Ca2+, and HCO3− concentrations as well as δ13CDIC values but higher SO42− concentrations are found in soil waters collected from bamboo land and grassland. The reason is that higher gypsum dissolution or oxidation of sulfide minerals and less soil CO2 input occurs in soils under these two land covers. Under the shrub, dry, and afforestation lands, higher concentrations of Ca2+ and HCO3− in soil waters occur in rainy seasons than in dry seasons, which are probably linked to higher CO2 input due to stronger microbial activities and root respiration in the wet summer seasons. In addition, seasonal variations of NO3− concentrations in soil waters from the dry land are observed, and much higher NO3− concentration occurs in the rainy seasons than that in the dry seasons, which suggest that the agricultural fertilization may lead to high NO3− in soil water. On the vertical soil profile, except for the bamboo land, soil waters under different land covers commonly show an increasing trend of main ion concentrations with the increase of depth. This vertical variation of hydrochemistry and δ13CDIC values in soil waters is primarily controlled by the intensity of carbonate dissolution related to carbonate content in soils and soil CO2 production. The soil waters under different land covers have great variations in δ13CDIC values which ranged from −20.68‰ to −6.90‰. Also, the [HCO3−]/([Ca2+] + [Mg2+]), [NO3−]/[HCO3−], and [SO42−]/([Ca2+] + [Mg2+]) molar ratios in soil waters show a large amplitude of variation. This suggested that carbonic acids could not be a unique dissolving agent and sulfuric/nitric acids may play a role in the weathering of carbonate in the Qingmuguan karst area.

Characterizing the heterogeneity of karst critical zone and its hydrological function: An integrated approach

AbstractSpatial heterogeneity in the subsurface of karst environments is high, as evidenced by the multiphase porosity of carbonate rocks and complex landform features that result in marked variability of hydrological processes in space and time. This includes complex exchange of various flows (e.g., fast conduit flows and slow fracture flows) in different locations. Here, we integrate various “state‐of‐the‐art” methods to understand the structure and function of this poorly constrained critical zone environment. Geophysical, hydrometric, and tracer tools are used to characterize the hydrological functions of the cockpit karst critical zone in the small catchment of Chenqi, Guizhou Province, China. Geophysical surveys, using electrical resistivity tomography (ERT), inferred the spatial heterogeneity of permeability in the epikarst and underlying aquifer. Water tables in depression wells in valley bottom areas, as well as discharge from springs on steeper hillslopes and at the catchment outlet, showed different hydrodynamic responses to storm event rainwater recharge and hillslope flows. Tracer studies using water temperatures and stable water isotopes (δD and δ18O) could be used alongside insights into aquifer permeability from ERT surveys to explain site‐ and depth‐dependent variability in the groundwater response in terms of the degree to which “new” water from storm rainfall recharges and mixes with “old” pre‐event water in karst aquifers. This integrated approach reveals spatial structure in the karst critical zone and provides a conceptual framework of hydrological functions across spatial and temporal scales.

Nitrogen functional gene activity in soil profiles under progressive vegetative recovery after abandonment of agriculture at the Puding Karst Critical Zone Observatory, SW China

At the end of the 20th century, China launched the ‘Grain-for-Green’ Project (GGP) that recommended the abandonment of low-yielding sloping farmland (>15°) prone to soil degradation by erosion, to allow recovery through natural vegetative regeneration. The effect of this policy on soil nitrogen (N) cycling, as fertilization applications are also withdrawn after abandonment, is poorly understood. A space-for-time approach was applied to investigate the responses of nitrogen functional genes (NFGs) in soil profiles (surface to bedrock) associated with progressive vegetative recovery (sloping farmland > recently abandoned sloping farmland > secondary forest > primary forest) at the Puding Karst Critical Zone Observatory in Guizhou province, southwest China. Coincident soil chemical properties (dissolved organic carbon (DOC), nitrate (NO3--N), ammonium (NH4+-N), available inorganic phosphorus (AP), soil organic carbon (SOC), total nitrogen (TN) and total phosphorus (TP)) were also quantified. We found that the absolute abundance of NFGs significantly varied according to the phase of vegetation recovery, and that concentrations of AP and NO3--N were the best explanatory variables. The external N from fertilizer application promoted the absolute abundances of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) in sloping farmland. The relative abundances of chiA (associated with decomposition) increased with soil depth across all vegetation recovery phases. The relative abundances of chiA and nifH (associated with N fixation) accounted for the largest proportion (58–72%) of the measured NFGs, indicating that active N-acquisition increased along the vegetation recovery gradient. The ratios of (chiA + nifH)/(AOA + AOB) and the sums of (nirK + nirS) were larger in the forest soil than those of sloping farmland and abandoned sloping farmland, implying a greater capacity for N storage potential, though accompanied by increased gas N emission potential, in the karst forest ecosystems. Our results provide a new and comprehensive understanding of soil N cycling potentials at the microscale in degraded and recovering karst ecosystems.

Co-regulation of photosynthetic capacity by nitrogen, phosphorus and magnesium in a subtropical Karst forest in China

Leaf photosynthetic capacity is mainly constrained by nitrogen (N) and phosphorus (P). Little attention has been given to the photosynthetic capacity of mature forests with high calcium (Ca) and magnesium (Mg) in the Karst critical zone. We measured light-saturated net photosynthesis (Asat), photosynthetic capacity (maximum carboxylation rate [Vcmax], and maximum electron transport rate [Jmax]) as well as leaf nutrient contents (N, P, Ca, Mg, potassium [K], and sodium [Na]), leaf mass per area (LMA), and leaf thickness (LT) in 63 dominant plants in a mature subtropical forest in the Karst critical zone in southwestern China. Compared with global data, plants showed higher Asat for a given level of P. Vcmax and Jmax were mainly co-regulated by N, P, Mg, and LT. The ratios of Vcmax to N or P, and Jmax to N or P were significantly positively related to Mg. We speculate that the photosynthetic capacity of Karst plants can be modified by Mg because Mg can enhance photosynthetic N and P use efficiency.

喀斯特关键带植被时空变化及其驱动因素

喀斯特关键带植被时空变化及其驱动因素

Analysis of Temporal and Spatial Variations in Trace Element Migration in Karst Critical Zone:An Example of Jiguan Cave, Henan

To explore the temporal and spatial variations and the process of trace element migration in the karst critical zone, continuous fixed-point monitoring and sampling analysis was applied to measure each cave system component, which includes rainfall, soils, bedrock, drip waters, and their aerial sediment. Approximately 650 experimental data were obtained from October 2009 to May 2015 in a typical karst critical zone in north China-an interactive zone of the Jiguan Cave in the west Henan Province. The variations in and the migration rules of Ca, Mg, Ba, Sr, and δ13C as well as their ratios to different components were studied. The results show that:① Soil and bedrock are the main sources of drip water. The values of Mg, Ba, and Sr are consistent with the "soil-bedrock" two end-members model and their respective proportions are 43.6:56.4, 1.01:98.09, and 47.2:52.8. ② From the spatial perspective, element migration of each component in the cave system interactive zone is interrelated. Drip water inherits the signals of the soil and bedrock and the modern sediment can continue the element information of the drip water. Elements in the soil profile cause the leaching and deposition effect and the subsoil better inherits the information of trace elements in the bedrock. ③ From the temporal perspective, the migration of elements in the cave system interactive zone is complex. Because of the leaching effect, soil and drip water show obvious seasonal discrepancies. However, under the influence of karst water migration path, the prior calcite precipitation (PCP), and extreme arid and annual precipitation type, the seasonal discrepancy in the element concentrations in drip water is smaller than in soil. The PCP effect, selective leaching, and other factors change the continuity of elements released from sediment to drip water. ④ In order to systematically and comprehensively ascertain the trace element migration in the karst critical zone and energy transformation rules, the study of cave system should be based on the research of the cave's critical zone and should consider comprehensive information ranging from the atmospheric origins to secondary sediments.

Role of hydro-geochemical functions on karst critical zone hydrology for sustainability of water resources and ecology in Southwest China

Focusing on sustainability of water resources and ecology in the complex karst critical zone, we illustrated functions of the hydro-geochemical analysis on hydrology from the aspects of connection and interaction among hydrology–vegetation–soils/rock fractures along the karst subsurface profile. We reviewed isotopic and geochemical interpretations on tracing water sources for plant uptake, quantifying watershed outlet flow composition and residence times, and evaluating long-term evolution among climate–landscape–hydrology in the karst critical zone. In this paper, the application of the hydro-geochemical analysis on the above aspects in the karst areas of southwest China was summarized.

Catchment‐scale conceptual modelling of water and solute transport in the dual flow system of the karst critical zone

AbstractHydrological and hydrochemical processes in the critical zone of karst environments are controlled by the fracture‐conduit network. Modelling hydrological and hydrochemical dynamics in such heterogeneous hydrogeological settings remains a research challenge. In this study, water and solute transport in the dual flow system of the karst critical zone were investigated in a 73.5‐km2 catchment in southwest China. We developed a dual reservoir conceptual run‐off model combined with an autoregressive and moving average model with algorithms to assess dissolution rates in the “fast flow” and “slow flow” systems. This model was applied to 3 catchments with typical karst critical zone architectures, to show how flow exchange between fracture and conduit networks changes in relation to catchment storage dynamics. The flux of bidirectional water and solute exchange between the fissure and conduit system increases from the headwaters to the outfall due to the large area of the developed conduits and low hydraulic gradient in the lower catchment. Rainfall amounts have a significant influence on partitioning the relative proportions of flow and solutes derived from different sources reaching the underground outlet. The effect of rainfall on catchment function is modulated by the structure of the karst critical zone (e.g., epikarst and sinkholes). Thin epikarst and well‐developed sinkholes in the headwaters divert more surface water (younger water) into the underground channel network, leading to a higher fraction of rainfall recharge into the fast flow system and total outflow. Also, the contribution of carbonate weathering to mass export is also higher in the headwaters due to the infiltration of younger water with low solute concentrations through sinkholes.

Carbon sequestration potential and its eco-service function in the karst area, China

The karst critical zone is an essential component of the carbon (C) pool, constituting the global C cycle. It is referred to as one of the “residual land sink” that remains largely indeterminate. Karst area (2.2×107 km2) comprises 15% of the world’s land area, and karst area comprises 3.44×106 km2 of area in China. Due to the complexity of karst structure and its considerable heterogeneity, C sequestration rate estimations contain large inaccuracies, especially in relation to the different methods used in calculations. Therefore, we reevaluated rock weathering-related C sink estimations in China (approximately 4.74 Tg C yr–1), which we calibrated from previous studies. Additionally, we stipulated that more comprehensive research on rock-soil-biology-atmosphere continuum C migration is essential to better understand C conversion mechanisms based on uncertainty analyses of C sink estimations. Moreover, we stressed that a collective confirmation of chemical methods and simulated models through a combined research effort could at least partially eliminate such uncertainty. Furthermore, integrated C cycling research need a long-term observation of the carbon flux of multi-interfaces. The enhanced capacity of ecosystem C and soil C pools remains an effective way of increasing C sink. Karst ecosystem health and security is crucial to human social development, accordingly, it is critical that we understand thresholds or potential C sink capacities in karst critical zones now and in the future.

Ecosystem service delivery in Karst landscapes: anthropogenic perturbation and recovery

Covering extensive parts of China, Karst landscapes are exceptional because rapid and intensive land use change has caused severe ecosystem degradation within only the last 50 years. The twentieth century intensification in food production through agriculture has led to a rapid deterioration of soil quality, evidenced in reduced crop production and rapid loss of soil. In many areas, a tipping point appears to have been passed as basement rock is exposed and ‘rocky desertification’ dominates. Through the establishment of the “Soil processes and ecological services in the karst critical zone of SW China” (SPECTRA) Critical Zone Observatory (CZO) we will endevaour to understand the fundmental processes involved in soil production and erosion, and investigate the integrated geophysical-geochemical-ecological responses of the CZ to perturbations. The CZ spans a gradient from undisturbed natural vegetation through human perturbed landscapes. We seek to understand the importance of heterogeneity in surface and below-ground morphology and flow pathways in determining the spatial distribution of key stocks (soil, C, vegetation, etc.) and their control on ecosystem service delivery. We will assess the extent to which the highly heterogeneous critical zone resources can be restored to enable sustainable delivery of ecosystem services. This paper presents the CZO design and initial assessment of soil and soil organic carbon stocks and evidence for their stability based on caesium-137 (137Cs) data.