Species-Specific Root Distribution and Leaf Iso/Anisohydric Tendencies Shape Transpiration Patterns Across Heterogeneous Karst Habitats.

The driving forces of transpiration are not only atmospheric evaporation but also root zone water supply and stomatal regulation among species. However, the biophysiological drivers of transpiration remain incompletely understood in heterogeneous karst habitats. This study investigated the commonly coexisting tree species Mallotus philippensis and Celtis biondii in two typical karst habitats: rock-dominated (RD) habitat and control soil-dominated (SD) habitat. Over 2 years, soil moisture, transpiration, root distribution, and leaf water potential were measured. The results showed that soil moisture in the RD habitat was significantly lower than in the SD habitat. Transpiration patterns also differed between habitats, with species-specific distinctions driven by biophysiological traits. M. philippensis showed small hydroscape areas and its root system mainly distributed in the soil zone in both habitats. The isohydric behaviour and lower root density in the RD habitat drove M. philippensis to reduce transpiration in response to soil water deficiency. Conversely, C. biondii had large hydroscape areas and roots capable of penetrating bedrock. It transpired higher relying on ample accessible water through anisohydric behaviour and having a more robust root system both in soil and bedrock zones in the RD habitat. Our study highlights the critical role of root water accessibility and leaf iso/anisohydric tendencies in driving transpiration.

Disentangling sources and transformation mechanisms of nitrogen, sulfate, and carbon in water of a Karst Critical Zone.

In the Karst Critical Zone (KCZ), mining and urbanization activities produce multiple pollutants, posing a threat to the vital groundwater and surface water resources essential for drinking and irrigation. Despite their importance, the interactions between these pollutants in the intricate hydrology and land use of the KCZ remain poorly understood. In this study, we unraveled the transformation mechanisms and sources of nitrogen, sulfate, and carbon using multiple isotopes and the MixSIAR model, following hydrology and surface analyses conducted in spatial modelling with ArcGIS. Our results revealed frequent exchange between groundwater and surface water, as evidenced by the analysis of δD-H2O and δ18O-H2O. Nitrification predominantly occurred in surface water, although denitrification also made a minor contribution. Inorganic nitrogen in both groundwater and surface water primarily originated from soil nitrogen (48 % and 49 %, respectively). Sewage and manure were secondary sources of inorganic nitrogen in surface water, accounting for 41 % in urban and 38 % in mining areas. Notably, inorganic sulfur oxidation displayed significant spatial disparities between urban and mining areas, rendering groundwater more susceptible to sulfur pollution compared to surface water. The frequent interchange between groundwater and surface water posed a higher pollution risk to groundwater. Furthermore, the primary sources of CO2 and HCO3- in both groundwater and surface water were water­carbonate reactions and soil respiration. Sulfide oxidation was found to enhance carbonate dissolution, leading to increased CO2 release from carbonate dissolution in the KCZ. These findings enhance our understanding of the transformation mechanisms and interactions of nitrogen, sulfur, and carbon in groundwater and surface water. This knowledge is invaluable for accurately controlling and treating water pollution in the KCZ.

[Research progress on structure and hydrological processes in the karst critical zone of southwest China]. / è¥¿å—å–€æ–¯ç‰¹å ³é”®å¸¦ç»“æž„åŠå ¶æ°´æ–‡è¿‡ç¨‹ç ”ç©¶è¿›å±•.

The southwestern region of China is the largest exposed karst area in the world and serves as an important ecological security barrier for the upstream of Yangtze River and Pearl River. Different from the critical zone of non-karst areas, the epikarst, formed by an interwoven network of denudation pores, is the core area of karst critical zone. Water is the most active component that participates in internal material cycle and energy flow within the critical zone. We reviewed relevant research conducted in the southwestern region from three aspects: the characte-rization of critical zone structure, the hydrological processes of soil-epikarst system, and their model simulations. We further proposed potential research hotpots. The main approach involved multi-scale and multi-method integrated observations, as well as interdisciplinary collaboration. Precisely characterizing the eco-hydrological processes of the vegetation-soil-epikarst coupling system was a new trend in the future research. This review would provide scientific reference for further studies on hydrological processes in critical zones and regional hydrological water resource management in karst areas.

Seasonal variations in water uptake and transpiration for plants in a karst critical zone in China.

Despite substantial drought conditions in the karst critical zone (KCZ), the KCZ landscapes are often covered with forest woody plants. However, it is not well understood how these plants balance water supply and demand to survive in such a water-limited environment. This study investigated the water uptake and transpiration relationships of four coexisting woody species in a subtropical karst forest ecosystem using measurements of microclimate, soil moisture, stable isotopes (δ18O, δ2H, and δ13C), intrinsic water-use efficiency (WUEi), sap flow, and rooting depth. The focus was on identifying differences within- and between-species across soil- and rock-dominated habitats (SDH and RDH) during the rainy growing season (September 2017) and dry season (February 2018). Species across both habitats tended to have higher transpiration with lower WUEi during the rainy season and lower transpiration with higher WUEi during the dry season. Compared to those in the SDH, species in the RDH showed lower transpiration with higher WUEi in both seasons. The dominant water sources were soil water and rainwater for supporting rainy-season transpiration in the SDH and RDH, respectively, and groundwater was the main water source for supporting dry-season transpiration in both habitats. A clear ecohydrological niche differentiation was also revealed among species. Across both habitats, shallower-rooted species with higher soil-water uptake, compared to deeper-rooted species with higher groundwater uptake, showed higher transpiration and lower WUEi during the rainy season and vice versa during the dry season. This study provides integrated insights into how forest woody plants in the KCZ regulate transpiration and WUEi in response to drought stress through interactions with seasonal water sources in the environment.

Alterations of ecosystem nitrogen status following agricultural land abandonment in the Karst Critical Zone Observatory (KCZO), Southwest China.

Background: Secondary succession after agricultural land abandonment generally affects nitrogen (N) cycle processes and ecosystem N status. However, changes in soil N availability and NO3 - loss potential following secondary succession are not well understood in karst ecosystems. Methods: In the Karst Critical Zone Observatory (KCZO) of Southwest China, croplands, shrub-grass lands, and secondary forest lands were selected to represent the three stages of secondary succession after agricultural land abandonment by using a space-for-time substitution approach. The contents and 15N natural abundance (δ 15N) of leaves, soils, and different-sized aggregates at the three stages of secondary succession were analyzed. The δ 15N compositions of soil organic nitrogen (SON) in aggregates and soil to plant 15N enrichment factor (EF = δ 15Nleaf -δ 15Nsoil), combined with soil inorganic N contents and δ 15N compositions were used to indicate the alterations of soil N availability and NO3 -loss potential following secondary succession. Results: Leaf N content and SON content significantly increased following secondary succession, indicating N accumulation in the soil and plant. The δ 15N values of SON also significantly decreased, mainly affected by plant δ 15N composition and N mineralization. SON content in macro-aggregates and soil NH4 + content significantly increased while δ 15N values of NH4 + decreased, implying increases in SON stabilization and improved soil N availability following secondary succession. Leaf δ 15N values, the EF values, and the (NO3 --N)/(NH4 +-N) ratio gradually decreased, indicating reduced NO3 - loss following secondary succession. Conclusions: Soil N availability improves and NO3 - leaching loss reduces following secondary succession after agricultural land abandonment in the KCZO.

[Speciation Characteristics and Risk Assessment of Soil Heavy Metals from Puding Karst Critical Zone, Guizhou Province].

The Community Bureau of Reference (BCR) sequential extraction method was used to analyze the distribution and chemical fractions of soil heavy metals (i.e., Cd, Cr, Zn, Fe, Ni, and Mn) under different land uses from the Puding karst critical zone, and the bioavailability and potential ecological risk of these heavy metals were evaluated. The results showed that the Fe, Zn, Cr, and Ni mainly were mainly concentrated in residual fractions and not likely to be absorbed by organisms, whereas the available fractions of Cd and Mn were in higher concentration, which retained strong potential migration and bioavailability. The richer organic matter in Puding soil might have promoted the transformation of oxidizable speciation of Fe, Ni, Cr, and Cd. The oxidizable forms of Fe, Mn, and Ni tended to accumulate in macro-aggregate soil. The risk assessment of RAC and RSP showed that the Puding karst soil was at slight ecological risk, and most heavy metals (except Cd) were at low risk to the ecological environment. Among the five land use types, the environmental risk of Cd in cropland and abandoned farmland was higher, which was mainly related to the input of Cd caused by fertilization, spraying pesticides, and other agricultural activities.

Karst rocky desertification diverged the soil residing and the active ectomycorrhizal fungal communities thereby fostering distinctive extramatrical mycelia.

Ectomycorrhizal fungi (EMF) are mutualists that play crucial roles in liberation, nutrient acquisition, transfer of growth-limiting resources and provision of water to host plants in terrestrial ecosystems, particularly in stressed prone climates. In this study, a field-based experiment was performed in Yunnan, China to assess the effect of karst rocky desertification (KRD) and natural forests (non-KRD) sites on the richness and composition of EMF communities. Inert sand-filled mesh bags were employed to characterize the active EMF and quantify the production of extramatrical mycelium (EMM). Results indicated that, EMF exhibited a significant differentiation among KRD and non-KRD sites, richness and diversity were higher across KRD areas, whereas the evenness showed the opposite trend. Ascomycota and Zygomycota were greater across KRD sites, however, Basidiomycota showed no difference across both study sites. The relative abundance of Clavaria, Butyriboletus, Odontia, Phyloporus, Helvella, Russula and Tomentella were higher across the KRD sites, whereas, Clavulinopsis, Endogone, Amanita, Inocybe and Clavulina were higher across the non-KRD sites. It's worth noting that, saprophytic (SAP) fungal community was found to be more abundant in the soil than the mesh bags at both sites particularly at KRD sites, which likely provide more free space and less competition for the EMF to thrive well in the mesh bags. In similar pattern, ergosterol concentration in mesh bags was observed relatively higher at KRD sites than the non-KRD sites. The Entoloma, Amanita, and Sebacina were found to be substantially higher in mesh bags than soil across both sites. Delicatula, Helvella and Tomentella on the other hand, showed higher relative abundance in mesh bags than soil over KRD sites, however they did not differ across non-KRD sites. Taken together, the presented results highlight relationship between the EMF community and the complex KRD environment, which is very important for the restoration of disturbed karst landscapes.

Effects of meteorological factors and groundwater depths on plant sap flow velocities in karst critical zone.

Determining water supply intensity of fracture/conduits is one of the difficulties involved in the research of plant transpiration water consumption in the Karst Critical Zone (KCZ). Our aims were to evaluate the effect of groundwater depth on plant sap flow velocities in KCZ. Thus, four sampled plots with different groundwater depth (GD) in boreholes KCZ7 (4 to 10 m GD), KCZ5 (2 to 9 m GD), KCZ1 (0 to 8 m GD) and KCZ3 (2 to 5 m GD), were selected, and the plant stem sap flow velocity in each plot were also monitored continuously and automatically using heat ratio techniques. The daily sap flow flux of Toona sinensis varied between 0.35 kg d-1 in KCZ3 and 1.50 kg d-1 in KCZ1. Photosynthetically active radiation (PAR), vapor pressure deficit (VPD), and gust velocity (ZWS) were the primary meteorological factors that determined the sap flow velocity of T. sinensis, which contributed to a regression equation, while the influence of GD on sap flow was complex. Most of the sap flow velocity had no obvious significant correlation with the GD; however, the sap flow velocity in four different GD showed significant differences (P < 0.05). Unit sap flow velocity changes induced by unit GD changes (Kv) in KCZ7 and KCZ1 samples was faster than that of other samples. In brief, the sap flow velocity was mainly affected by the PAR and VPD in KCZ7, KCZ5 and KCZ1 because of the sufficient epikarst water, while the sap flow velocity in KCZ3 was mainly affected by the rock water content. The karst aquifer medium and GD was the main factors causing the difference sap flow velocity in the four sample plots. This finding indicated that KCZ aquifer medium structure may have an important influence on plant water utilization.

[Soil-epikarst structures and their hydrological characteristics on dolomite slopes in karst region of southwest China]. / 西南喀斯特白云岩坡地土壤-表层岩溶带结构及水文特征.

Epikarst is the core area of karst critical zone, with important hydrologic regulation and storage function. However, the effects of karst development degree on hydrologic characteristics of epikasrt is still unclear. We used geophysical exploration and hydrogeological techniques, combined with the dynamic monitoring of moisture and water levels, to quantify the karst development degrees and their hydrologic characteristics on slope lands. We analyzed the responses of soil-epikarst systems to rainfall. Results showed that geophysical exploration technology could be well applied to the detection of surface-subsurface structures in the karst areas. The average thickness of soil and surface karst zone on the slope was less than 0.63 m and 2.60 m, respectively. The slopes of strong-karstification characterized by high apparent resistivity, well-developed joint fractures, and strong permeability (0.73 m·d-1). Such a result indicated that epikarst could regulate precipitation. The responses of soil moisture had a larger rainfall threshold (>20.50 mm·d-1) and the water level was determined by rainfall amount. In contrast, the slope with weak-karstification had low apparent resistivity and weak permeability (0.07 m·d-1). Moisture and water level were sensitive to rainfall. Karst channels were developed locally at 240-300 cm with a permeability coefficient of up to 432 mm·d-1. Obvious preferential flow was observed in extreme rainfall events on this slope, which could induce flood disaster in the adjacent depression. Our results would provide scientific basis for further research on water resources regulation, management, and eco-hydrology in karst areas of southwest China.

Contribution of soil microbial necromass to SOC stocks during vegetation recovery in a subtropical karst ecosystem.

Carbon sequestration is a key soil function, and an increase in soil organic carbon (SOC) is an indicator of ecosystem recovery because it underpins other ecosystem services by acting as a substrate for the soil microbial community. The soil microbial community constitutes the active pool of SOC, and its necromass (microbial residue carbon, MRC) contributes strongly to the stable SOC pool. Therefore, we propose that the potential for restoration of degraded karst ecosystems lies in the abundance of soil microbial community and the persistence of its necromass, and may be measured by changes in its contribution to the active and stable SOC pools during recovery. We investigated changes in SOC stocks using an established space-for-time chronosequence along a perturbation gradient in the subtropical karst ecosystem: sloping cropland < abandoned cropland < shrubland < secondary forest < primary forest. Microbial biomarkers were extracted from soil profiles from surface to bedrock and used to measure the contributions of the soil microbial community composition (using phospholipid fatty acids, PLFAs) and MRC (using amino sugars) to SOC stocks at each recovery stage. The results showed that the SOC stocks ranged from 10.53 to 31.77 kg m-2 and increased with recovery stage, with total MRC accounting for 17-28% of SOC. Increasing PLFAs and MRC abundances were positively correlated with improved soil structure (decreased bulk density) and organic carbon, nitrogen and phosphorus nutrient. Bacterial MRC contributes more to SOC stocks than fungal residue carbon during vegetation recovery. The PLFA analysis indicated that Gram positive bacteria were the largest microbial group and were relatively more abundant in deeper soils, and biomarkers for saprophytic and ectomycorrhizal fungi were more abundant in soils under woody vegetation. In conclusion, this study suggests that the soil microbial community in karst soils have the potential to adapt to changing soil conditions and contribute substantially to building SOC stocks after abandonment of agriculture in degraded karst landscapes.

Rainfall and conduit drainage combine to accelerate nitrate loss from a karst agroecosystem: Insights from stable isotope tracing and high-frequency nitrate sensing.

Understanding where nitrate is mobilized from and under what conditions is required to reduce nitrate loss and protect water quality. Low frequency sampling may inadequately capture hydrological and biogeochemical processes that will influence nitrate behavior. We used high-frequency isotope sampling and in-situ nitrate sensing to explore nitrate export and transformation in a karst critical zone. Nitrate was mobilised during light rainfall, and transferred from soil layers to the karst matrix, where some nitrate was retained and denitrified. Nitrate isotopic composition changed rapidly during the rising limb of events and slowly during the falling limb. The main nitrate source was synthetic fertiliser (up to 80% during event flow), transported by conduit flow following high rainfall events, and this contribution increased significantly as discharge increased. Soil organic nitrogen contribution remained constant indicating at baseflow this is the primary source. Isotope source appointment of nitrate export revealed that synthetic fertilizer accounted for more than half of the total nitrate export, which is double that of the secondary source (soil organic nitrogen), providing valuable information to inform catchment management to reduce nitrate losses and fluvial loading. Careful land management and fertilizer use are necessary to avoid nitrate pollution in the karst agroecosystem, for example by timing fertilizer applications to allow for plant uptake of nitrate before rainfall can flush it from the soils into the karst and ultimately into catchment drainage.

Coupled hydrological and biogeochemical modelling of nitrogen transport in the karst critical zone.

Transport of nitrogen (N) in karst areas is more complex than in non-karst areas due to marked heterogeneity of hydrodynamic behaviour in the karst critical zone. Here, we present a novel, distributed, coupled hydrological-biogeochemical model that can simulate water and nitrogen transport in the critical zone of karst catchments. This new model was calibrated using integrated hydrometric, water stable isotope, and nitrogen-N concentration data at the outflow of Houzhai catchment in Guizhou province of Southwest China. Hydrological dynamics appears to control N load from the study catchment. Combining flow discharge and water stable isotopes significantly constrained model parameterisation and mitigate the equifinality effects of parameters on the simulated results. Karst geomorphology and land use have functional effects on spatiotemporal variations of hydrological processes and nitrogen transport. In the study catchment, agricultural fertilizer was the largest input source of N, accounting for 86% of the total. Plant uptake consumed about 45% of inputs, primarily in the low-lying valley bottom areas and the plain covered by relatively thick soils. Thus, a large amount of N released from soil reservoirs to the epikarst (via fractures or sinkholes) is then exported to the underground channel in the limestone area to the south. This N draining into groundwater could lead to extensive, potentially long-term contamination of the karst system. Therefore, improving the efficiency of fertilization and agricultural management in valleys/depressions is an urgent need to reduce N losses and contamination risk.

[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.