Karst Region Of Southwestern China Research Articles

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

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

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

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

Soil organic carbon stock and its changes in a typical karst area from 1983 to 2015

Changes in soil organic carbon (SOC) stock have major impacts on global terrestrial carbon cycling. However, their responses to land use conversions are poorly characterized for karst areas with extremely fragile geology and intensive human disturbance. To investigate the effects of soil type and land use on SOC stock in a typical karst region of southwestern China, 0–15 cm topsoil samples were randomly collected in 2015. Furthermore, in the same locations as the sites in 1983, 0–100 cm stratified profile soil samples (0–10, 10–20, 20–30, 30–50, 50–70, and 70–100 cm) were collected to evaluate the changes in SOC stock as affected by land use conversions from 1983 to 2015. The current SOC stock in 0–15 cm differed significantly between Calcisols and Ferralsols, and was highest in the secondary forest, followed by shrubland, grassland, plantation forest, and cropland. Changes in the stratified SOC stock (both recalculated to 0–20, 20–40, 40–60, 60–80, and 80–100 cm intervals in 1983 and 2015) varied among different land use conversions. Average stratified SOC stock decreased after forest degradation and reclamation, except in 80–100 cm. After reforestation, it had decreases in 0–20 and 20–40 cm, whereas it increased in subsoil (40–60, 60–80, and 80–100 cm). However, compared with the cropland in which soil was exposed to continuous conventional tillage, average stratified SOC stock increased in all soil layers after reforestation. The increases in SOC stock after reforestation and re-cultivation (short-term reuse in abandoned croplands) indicated the positive role of agricultural abandonment in increasing terrestrial SOC stock.

Spatial heterogeneity of soil organic carbon in a karst region under different land use patterns

AbstractUsing the grid‐based sampling method, 2755 soil profiles consisting of 22,057 soil samples were sampled and analyzed to study the spatial distribution and storage of soil organic carbon (SOC) in the Houzhai River Basin in Guizhou Province, a typical karst region in southwestern China. The results suggest that the total levels of SOC stored in the 0–10, 0–20, 0–30, and 0–100 cm soil layers of the Houzhai River Basin are approximately 1.48 × 108, 2.65 × 108, 3.44 × 108, and 5.39 × 108 kg C, respectively. SOC content and soil bulk density levels were found to vary depending on the land use. In the 0–50 cm soil layer, forest SOC content levels (including arbor forests, arbor–shrub mixed forests, shrubland, and shrub grassland) are generally higher than those of uncultivated land, followed by grassland and cropland (including paddy fields, arid land, sloping cropland, and abandoned cropland), gardens, and orchard. It is believed that land use is a vital factor directly affecting carbon storage patterns in the study area. Soil thickness, rock outcrop, and slope gradients are the main factors in arrangement of land use by local governments and peasants. This study has established a feasible method for assessment of SOC stock in karst mountainous areas. The findings of this study are applicable to other karst mountainous regions globally.

Temporal changes in vegetation around a shale gas development area in a subtropical karst region in southwestern China

Over the past decade, various aspects of China's fragile karst environments, including net primary productivity (NPP), have been changed or threatened by shale gas development. This industry is still developing, so it is important to understand what drives environmental changes, particularly in NPP, when shale gas pads are constructed in sensitive areas. Few previous studies have addressed this issue, so we quantified how the NPP changed, and what drove the changes, when a large shale gas area was developed at the end of 2012 in a mountainous karst area in Sichuan Province. We calculated the trend in the normalized difference vegetation index (NDVI) from 2012 to 2017 and used the Carnegie-Ames-Stanford Approach (CASA) model to calculate the changes in NPP at different distances from the pads using remote sensing images for July 2012 and July 2017 and field survey data from July 2017. We then identified the factors that drove the changes with Geodetector. The results showed that the NDVI increased across 64.2% of the shale gas development area from 2012 to 2017 because of climate change, and only showed a significant decrease across 0.3% of the area, mainly because of the shale gas development. The NPP decreased by 110.1 t because of the shale gas development in July 2017, or by about 0.35% of the total NPP. Of this, 93.8 t were associated with the pad construction areas, and 16.3 t were associated with the area around the pads. The changes in NPP around the shale gas pads were mainly confined to within 150 m during the construction phase and 90 m once the construction was completed. The NPP at different distances from the pads during the construction period was related to the distance from the pad, slope, and land use. Once completed, the NPP mainly varied with distance, land use, and the distance from the pad to rural settlements. The NPP was most strongly influenced by the distance from the pad and the area of the pad. We suggest that, when planning the construction of shale gas pads, the pads should be sited on gently sloping areas, the number of wells on each pad should be optimized, land use type changes outside the pad should be limited, and the land beyond the pads should be reclaimed in a timely manner to allow the NPP to recover.

Moss-dominated biological soil crusts modulate soil nitrogen following vegetation restoration in a subtropical karst region

Vegetation restoration often promotes the development of biological soil crusts (biocrusts), which have a significant role in the terrestrial nitrogen (N) cycle. However, biocrusts are not always considered when assessing soil N responses to restoration efforts, especially in subtropical regions. We assessed the effect of various restoration strategies on moss-dominated biocrust properties and soil N variables (0–5 cm topsoil and 5–10 cm sub-topsoil) in the subtropical karst region of southwestern China. Four restoration strategies were included: restoration with plantation forest, forage grass, and a combination of forest and grass, and spontaneous regeneration to natural grassland. Cropland under maize-soybean rotation (CR) was used as reference. The biocrust under combination restoration strategy had significantly higher biomass, saturated water adsorption ratio, and carbon content than did the other strategies; and the soil generally had the highest total N, ammonium, and microbial biomass carbon and N content but the lowest nitrate content. Redundancy analysis, variation partitioning, and stepwise multiple linear regression all indicated that biocrust properties play an important role in affecting soil N variables, especially in the topsoil. Vegetation restoration strategies that use various plant functional group assemblages, such as forest and grass, are preferable in enhancing the development of biocrusts and thus improving soil N properties.

Effect of Soil Moisture and Temperature on the Soil Inorganic Carbon Release of Brown Limestone Soil in the Karst Region of Southwestern China

In order to understand the influence of environmental factors on the carbonate conversion of the Karst soil, typical brown limestone and red soil samples were collected from the Karst ecosystem, and a 100-day incubation experiment was conducted. The characteristics of inorganic carbon release from the soil under three temperature gradients (15, 25, and 35℃) and water contents (30%, 65%, and 100% WHC) were studied by adding 14C-CaCO3 for 100 d. The results showed that under the different soil moisture and temperature conditions, the maximum rate and the cumulative amount of inorganic carbon release from the soil over 100 days varied between 0.7-16.8 mg·(kg·d)-1and 5.9-29.4 mg·kg-1, respectively, in the brown limestone soil, and varied between 39.7-103.3 mg·(kg·d)-1 and 83.3-135.1 mg·kg-1, respectively in the red soil. Under drought conditions (30% WHC), the cumulative amount of inorganic carbon release was the highest for the two soils and increased with increasing temperature. At 65% WHC and 100% WHC, increasing temperature can still promote inorganic carbon release from the soil. The temperature sensitivity of the soil inorganic carbon release in the brown limestone soil is greater than that of the red soil, which is significantly affected by soil moisture. The soil pH and MBC content were remarkably increased after adding CaCO3, and the difference between the two soils was significant. The variance partition showed that temperature and soil moisture can explain 7.6% and 2.0% of the soil inorganic carbon release variability, respectively. In conclusion, warming and drought aggravate inorganic carbon release from brown limestone soil in the southwestern Karst region. Therefore, in the context of global warming and more frequent extreme precipitation events, the effects of soil moisture and temperature on inorganic carbon conversion in soil should be fully considered when studying the soil carbon cycle and its dynamic changes in southwestern Karst. This research can provide a scientific basis for further understanding the influence of climate change on the global carbon cycle.

Contributions of Carbonates to Carbon Dioxide Release From a Calcareous Soil in Response to Experimental Warming

Seasonal asymmetric warming is a distinguishing feature of climate change, which is not often considered when evaluating the effects of global change on terrestrial ecosystems. Responses of soil CO2 efflux from organic and inorganic sources to asymmetric and symmetric warming are not well understood. A 5-year warming experiment was performed to evaluate the effects of symmetric and asymmetric warming on soil organic carbon (SOC)–derived and carbonate-derived effluxes from a calcareous soil in the karst region of southwestern China. On average, the carbonate-derived efflux represented 31% of the total CO2 efflux in the six treatments. Warming of 2.0°C substantially stimulated soil CO2 efflux from both sources, except for the carbonate-derived efflux in summer-autumn. Soil CO2 efflux and SOC-derived efflux were appreciably reduced when ratios of warming in winter-spring to summer-autumn ratio of asymmetric warming (RAW) increased, while no changes (full year or in summer-autumn) were found for the carbonate-derived efflux. However, soil CO2 efflux from both sources were considerably enhanced in winter-spring with increasing RAW values. Temperature sensitivity temperature coefficient (Q10) of CO2 efflux from both sources decreased with increasing RAW values. The average Q10 of SOC-derived efflux was 1.3 times greater that of the carbonate-derived efflux. Symmetric warming potentially overestimates CO2 efflux from calcareous soils, compared with the projected asymmetric warming scenario, likely because of overestimation of SOC-derived effluxes in summer-autumn.

Effect of rice husk biochar addition on nutrient leaching and microbial properties of Calcaric Cambisols

The use of biochar to improve soil properties and decrease the leaching of nutrients has received attention, yet very little research has focused on its effects on Calcaric Cambisols. We evaluated the effect of rice (Oryza sativa L.) husk biochar at the highest pyrolysis temperatures of 450°C and 650°C (RH450 and RH650) on nutrient leaching, nutrient availability, and microbial properties of Calcaric Cambisols in the karst region of southwestern China. In a soil column leaching experiment, rice husk biochars produced under two temperatures were added at 1%, 2%, and 5% (weight/weight) to Calcaric Cambisols following application of fertilizer (ammonium nitrate [NH4NO3] and monopotassium phosphate [KH2PO4]) and incubated for 28 weeks. A control treatment (without rice husk biochar) was also included. After a 28-week incubation, both temperature rice husk biochars significantly reduced the cumulative amounts of ammonium (NH4+) in leachate from Calcaric Cambisols, while they increased cumulative amounts of leachate phosphate (PO43−) and potassium (K+). Addition of 2% and 5% RH650 significantly reduced cumulative amounts of leachate nitrate (NO3−), while RH450 amendment had no significant influence on NO3−-N leaching. Addition of 5% RH450 significantly increased soil available nitrogen (N) content simultaneously accompanied by a significant increase in microbial biomass N, suggesting net N mineralization was enhanced and soil microorganisms utilized a portion of mineral N for their own cell building. Both temperature rice husk biochar additions caused a significant increase in soil available phosphorus (P) and K. The highest P availability was observed in RH450-treated soil samples at 5% rate. Compared with RH450 treatment, higher K availability was found in RH650-treated soils. Both rice husk biochar additions significantly increased soil microbial biomass carbon (C), and RH450 addition also significantly increased soil microbial biomass P and soil fluorescein diacetate activity (FDA). This is largely due to the more labile C in RH450 (high oxygen [O]/C and hydrogen [H]/C ratios). The results suggest that high-temperature rice husk biochar amendment may be an effective strategy for reducing N leaching of Calcaric Cambisols, whereas low-temperature rice husk biochar has more beneficial effect on soil microbial biomass and microbial activity.

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.

Diversity of Fungal Communities in Heshang Cave of Central China Revealed by Mycobiome-Sequencing.

Deciphering of the mycobiome in pristine karst caves has been impeded by constraints of remote locations, inaccessibility to specimens and technical limitations, which greatly restricted in-depth understanding of mycobiomes in subterranean ecosystem. Here, mycobiomes of Heshang Cave in south-western karst region of China were investigated by Illumina HiSeq sequencing of fungal rRNA-ITS1 gene across different habitats. In total 793,502 ITS1 reads and 2,179 OTUs from 778 Mb reads after stringent quality control (Q30) and 453 genera, 72 orders and 19 classes within 6 phyla were detected. Ascomycota (42% OTUs) dominated across the five habitats. Shannon-Wiener index varied from 1.25 to 7.62 and community richness was highest in drip waters, followed by weathered rocks, bat guanos, sediments, and air samples. Mycobiomes displayed specificity to five habitats and more distinct OTUs were found in weathered rocks (12%) and drip waters (9%). In contrast, only 6.60% core OTUs were shared by five habitats. Notably, weathered rocks possessed more indicator groups and were revealed for the first time to be dominated by Sordariomycetes (43%). The community richness of air mycobiomes increased from cave entrance to the innermost part and dominated by the indicator groups of Penicillium mallochii (>30%) and P. herquei (>9%). Our work represents the largest attempt to date to a systematical investigation of oligotrophic solution-cave-associated mycobiomes in China. Our discovery of high diversity of mycobiomes in Heshang Cave also suggests that eukaryotic microorganisms may play a crucial role in subsurface environments.

Effects of nitrogen, phosphorus and potassium addition on the productivity of a karst grassland: Plant functional group and community perspectives

Rocky desertification is currently the most serious ecological and environmental problem in karst region of southwestern China. Its negative consequences for both natural ecosystems and the human inhabitants of the karst region have created a need for effective ecosystem restoration strategies, but success in these efforts has been limited. We hypothesized that scarcity of mineral nutrients could be a crucial factor in vegetation recovery, but relatively little information is available about the limiting roles of various mineral elements. We investigated responses of aboveground biomass and nutrient concentrations of a degraded karst grassland to nitrogen (N), Phosphorus (P) and Potassium (K) additions over a period of three years. Nutrient additions significantly increased aboveground biomass and nutrient concentrations for both the plant community and individual plant functional groups. Total aboveground biomass was significantly increased by N (by 35.6%), P (by 35.3%) and K (by 11.7%) fertilization over three years of nutrient additions. The interaction effects of year × N and year × P on total biomass were significant. Additions of N and P increased the biomass of grasses by 39.2% and 15.0%, respectively, and additions of P increased the biomass of forbs by 69.3%. The biomass of shrubs was significantly increased by P (by 111.3%), K (by 45.3%) and N (by 38.5%), and there were strong interaction effects of N, P and K on shrub biomass. P and K additions significantly increased the relative biomass of shrubs but decreased that of grasses, especially under the NPK treatment. Our results suggest that the productivity of degraded grassland in the karst region of China is co-limited by N, P and K, with N and P being the primary limiting factors. Among functional groups, grasses are mainly limited by N and P, forbs by P, and shrubs by all 3 elements, with P being the most limiting factor overall. Mineral fertilization stimulates plant growth and may be a useful tool in efforts to restore woody vegetation in degraded grasslands, thus counteracting the process of rock desertification in the karst region of southwestern China.

Environmental variables controlling soil organic carbon in top- and sub-soils in karst region of southwestern China

Detailed information about the variability of soil organic carbon (SOC) content in different horizons is vital for the effective management of soil fertility and can improve understanding of SOC accumulation and decomposition. However, few studies have explored the variations of SOC content of cultivated lands in different horizons at the regional scale, particularly in a karst region in southwestern China. In this work, 235 soil samples were collected from the A to C horizons at 80 sites over the study area. Classical statistics and classification and regression trees (CARTs) were applied to investigate the critical environmental variables (topography, climate and parent material) that control SOC content variability in various horizons. Results indicated that SOC content decreased and its variability increased with an increase in horizon depth. The SOC content in the three horizons was predicted well by the CART models. The mean absolute prediction error, root mean square error and coefficient of determination ranged from 2.38 to 3.85 g kg−1, 3.23 to 5.2 g kg−1 and 0.5837 to 0.856, respectively. The importance of the environmental variables in SOC content varied with the horizons. In the A horizon, topographical variables, such as terrain wetness index (TWI), terrain ruggedness index and slope, were the key factors affecting SOC content variability. In the B horizon, the topography still showed the primary influence by elevation and TWI, meanwhile the importance of the parent material strengthened. Climate variables had the greatest impact on SOC content in the C horizon. TWI and precipitation that directly influenced soil moisture were the primary factors controlling SOC content in the three horizons.

The application of geospatial techniques in monitoring karst vegetation recovery in southwest China

The karst region in southwestern China, one of the largest continuous karst areas in the world, is special for its high landscape heterogeneity, unique hydrology, high endemism among vegetation species and high intensity of human disturbance. The region had experienced severe degradation through karst rocky desertification (KRD) between the 1950s and 1990s. Starting in the late 1990s, various levels of the Chinese government conducted several ecological projects to recover degraded karst ecosystems. It was reported that the implementation of these projects had been successful in facilitating the recovery of karst vegetation in many areas. However, global climate changes may compromise the efficacy of recovery. Geospatial techniques had been employed to map and monitor karst ecosystem conditions during the recovery process. We examined the history and progress of the various geospatial techniques applied to monitor and evaluate karst vegetation conditions. In addition, we reviewed the techniques used to assess and monitor KRD, KRD influencing factors, vegetation community type, fractional vegetation cover, vegetation dynamics, vegetation productivity, ecosystem goods and services, vegetation biodiversity, ecosystem health and rural society changes. We also explored the potential to apply geospatial techniques for karst vegetation recovery in the future. It is projected that there will be more remotely sensed images for the vegetation dynamics monitoring at numerous scales. New techniques (e.g. image fusion and data assimilation) will be available to manage scale and heterogeneity issues in the karst landscape.

Assessing spatial distribution of soil erosion in a karst region in southwestern China: A case study in Jinfo Mountains

Soil erosion is serious with rocky desertification areas appearing in mountainous Karst regions in southwest China due to a conspicuous contradiction between man and the land resource. Land use and land cover play significant roles in regional soil erosion by water. This paper aimed to quantify regional soil erosion and to explore relationships between soil erosion and land use/land cover in order to locate high risk areas requiring soil conservation. Based on GIS, the Revised Universal Soil Loss Equation (RUSLE) was employed for erosion assessment for a typical Karst region, Jinfo Mountain region in southwest China, using local parameters. Spatial distribution of topsoil erosion was analyzed and relationships between soil erosion and land use/land cover changes (LULC) were statistically explored and discussed for regional erosion control. The overall values were under 25 t.ha.a, with the medium erosion areas accounting for 12.7% and the intense and very intense erosion areas totalled about 6%. The relations between soil erosion and LULC are complicated in this Karst mountainous region. Generally, the amount of ground cover, soil conservation measures, and cultivation disturbance have played critical roles in topsoil loss in the Jinfo mountain region. The reduced ground cover levels accompanying greater cultivation disturbance lead to higher erosion intensity in each landscape, and vice versa.

Aboveground carbon stock, allocation and sequestration potential during vegetation recovery in the karst region of southwestern China: A case study at a watershed scale

Karst landscape in southwestern China is one of the most typical landscapes developed on carbonate bedrock and has the largest area in the world. Carbon sequestration potentials during secondary karst vegetation recovery remain uncertain. Based on the vegetation map and 87 sampling plots at five stages of natural vegetation succession, this study estimated aboveground (AG) vegetation carbon stocks and dynamics at a watershed scale. AG carbon density of grasslands, shrublands, shrub forests, secondary forests and primary forests was 1.70, 4.15, 22.3, 70.3, 142.2Mgha−1, respectively. The ten most important species stored 71.6–96.1% of total AG carbon stock, indicating that carbon pool in karst vegetation was determined by a few dominant species. Main contributors to AG carbon stock shifted from individuals in small diameter classes in shrublands to individuals in large diameter classes in primary forests, indicating that carbon increases in the early vegetation succession resulted from high recruitment of woody plants, while carbon accumulations in the later forests were mainly due to tree growth. The long time required for secondary forests to recover carbon density to the level of primary forests could be explained by the slow speed of large evergreen trees reaching a high level of dominance during secondary succession on the harsh habitats. The total AG carbon stock of the studied watershed (7.50×103ha) was 85.9×103 Mg, of which paddy fields, dry lands, grasslands, shrublands, shrub forests and secondary forests accounted for 22.6%, 3.49%, 0.34%, 5.97%, 12.3% and 55.3%, respectively. The AG carbon stock in this watershed would increase by 92.5% in 50–100 years and by 4.40 times in 140–200 years if the degraded vegetation types could continue to develop into mature forests. Although carbon density of karst forests was significantly lower than that of the forests on non-karst habitats in the same latitudinal zone, the degraded karst vegetation showed a great carbon sequestration potential due to the large distribution area in southwestern China.

Mixing litter from deciduous and evergreen trees enhances decomposition in a subtropical karst forest in southwestern China

In the karst region of southwestern China, mixed evergreen and deciduous broad-leaved forest is typical vegetation, differing remarkably from the evergreen broad-leaved forest developed in non-karst regions in the same latitudinal zone. Litter input from deciduous trees influences the characteristics of the litter layer, which could affect decomposition dynamics. We evaluated the role of deciduous trees in regulating the decomposition of mixed leaf litter in a karst forest. Four species richness treatments (one, two, four and six species) were designed to evaluate the decomposition rates of litter mixtures for two years. The ratio of deciduous to evergreen species numbers was 1:1 in all mixtures. Litter mass loss was positively correlated with initial nitrogen concentrations and negatively correlated with carbon:nitrogen ratios, suggesting that mass loss was controlled by nitrogen concentration of the litter mixture. Litter mixing accelerated the mass loss by 5.0% and enhanced nutrient release by 4.8%–26.2% for different elements. Synergistic effects in the four-species mixture were usually strongest and increased over time. Although individual litter species within the mixtures showed idiosyncratic responses to litter species richness, mixing effects enhanced decomposition of evergreen litter species more than deciduous species. This study suggests that species composition was more important than species richness in driving non-additive effects on decomposition in this forest. Deciduous trees in karst ecosystems significantly contribute to nutrient cycling, through enhancing the decomposition of evergreen leaf litter.

Low nitrate retention capacity in calcareous soil under woodland in the karst region of southwestern China

A 15N-tracing study was conducted to investigate the gross N transformation rates involved in the production and consumption of NH4+ and NO3− in karst region soils (a calcareous and a red soil) from southwest China under woodland. The gross mineralization rate of organic N to NH4+ was much lower in calcareous soil (3.71 mg N kg−1 d−1) than in red soil (5.57 mg N kg−1 d−1), while the total gross nitrification rate was significantly higher in calcareous soil (5.80 mg N kg−1 d−1) than in red soil (0.70 mg N kg−1 d−1). Microbial NO3− immobilization accounted for 2.9% and 153% of the total nitrification in calcareous soil and red soils respectively. Thus the calcareous soils in the karst region have a low biological N retention capacity and high N losses, which are further aggravated by the high precipitation and fractured soil structure.

Community size, activity and C:N stoichiometry of soil microorganisms following reforestation in a Karst region

Reforestation has been widely adopted to restore soil fertility and ecosystem service in the rocky-desertified Karst region of southwestern China. As a key indicator of ecosystem restoration, soil microbial characteristics however remained poorly understood following reforestation on the degraded Karst soils. This study employed site comparisons along a chronosequence of Toona sinensis reforestation in soil microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), basal respiration (BR), microbial metabolic quotient (qCO2), microbial quotient (MBC:TOC), and microbial C:N stoichiometry. Results showed that MBC, MBN, BR as well as soil organic C and N contents, moisture and porosity increased with the reforestation age. The 0-yr control of bare soil showed the highest microbial metabolic quotient (qCO2, 2.48 mg g−1 h−1) and lowest microbial quotient (MBC:TOC, 2.29%), reflecting the disturbed and stressed habitats. After 12-yr reforestation, qCO2 decreased to 1.49 mg g−1 h−1 and MBC:TOC increased to 4.70%, due to the disturbance cessation and the weakened stress. The consistent increases in microbial C:N ratio and soil C:N ratio under the reforestation indicated microbial adaptive response to substrate resource stoichiometry. This study suggested that these microbial indices independently or in combination can be used as the early indicators of the restoration of Karst ecosystems.

Interannual Variations in Growing-Season NDVI and Its Correlation with Climate Variables in the Southwestern Karst Region of China

In this study, the updated Global Inventory Modeling and Mapping Studies (GIMMS) Normalized Difference Vegetation Index (NDVI) dataset for growing season (April to October), which can better reflect the vegetation vigor, was used to investigate the interannual variations in NDVI and its relationship with climatic factors, in order to preliminarily understand the climate impact on vegetation and provide theoretical basis for the response of ecosystem to climate change. Multivariate linear regression models, including the Ordinary Least Squares (OLS) and Geographically Weighted Regression (GWR), were adopted to analyze the correlation between NDVI and climatic factors (temperature and precipitation) together. Average growing-season NDVI significantly increased at a rate of 0.0015/year from 1982 to 2013, larger than several regions in China. On the whole, its relationship with temperature is positive and also stronger than precipitation, which indicated that temperature may be a limiting factor for the vegetation growth in the Karst region. Moreover, the correlation coefficients between grassland NDVI and climatic factors are the largest. Under the background of NDVI increasing trend from 1982 to 2013, the period of 2009–2012 was chosen to investigate the influencing factors of a sharp decline in NDVI. It can be found that the reduced temperature and solar radiation, caused by the increase in cloud cover and precipitation, may play important roles in the vegetation cover change. All in all, the systematic research on the interannual variations of growing-season NDVI and its relationship with climate revealed the heterogeneity and variability in the complicated climate change in the Karst ecosystem for the study area. It is the Karst characteristics that hinder obtaining more representative conclusions and tendencies in this region. Hence, more attention should be paid to promoting Karst research in the future.