Environmental Earth Sciences

, Volume 68, Issue 1, pp 151–158

Carbon cycle in the epikarst systems and its ecological effects in South China

Authors

  • Zhongcheng Jiang
    • Institute of Karst GeologyChinese Academy of Geological Sciences
    • Illinois State Water Survey, Prairie Research InstituteUniversity of Illinois at Urbana-Champaign
  • Xiaoqun Qin
    • Institute of Karst GeologyChinese Academy of Geological Sciences
Original Article

DOI: 10.1007/s12665-012-1724-x

Cite this article as:
Jiang, Z., Lian, Y. & Qin, X. Environ Earth Sci (2013) 68: 151. doi:10.1007/s12665-012-1724-x

Abstract

The carbon cycle in a global sense is the biogeochemical process by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the earth. For epikarst systems, it is the exchange of carbon among the atmosphere, water, and carbonate rocks. Southern China is located in the subtropical zone; its warm and humid weather creates favorable conditions for the dynamic physical, chemical, and ecological processes of the carbon cycle. This paper presents the mechanisms and characteristics of the carbon cycle in the epikarst systems in south China. The CO2 concentration in soils has clear seasonal variations, and its peak correlates well with the warm and rainy months. Stable carbon isotope analysis shows that a majority of the carbon in this cycle is from soils. The flow rate and flow velocity in an epikarst system and the composition of carbonate rocks control the carbon fluxes. It was estimated that the karst areas in south China contribute to about half of the total carbon sink by the carbonate system in China. By enhancing the movement of elements and dissolution of more chemical components, the active carbon cycle in the epikarst system helps to expand plant species. It also creates favorable environments for the calciphilic plants and biomass accumulation in the region. The findings from this study should help in better understanding of the carbon cycle in karst systems in south China, an essential component for the best management practices in combating rock desertification and in the ongoing study of the total carbon sink by the karst flow systems in China.

Keywords

EpikarstCarbon cycleEcologyCarbon sink

Introduction

There are about 22 million square kilometers (km2) of carbonate rock areas on Earth; a quarter of these are in China. The total carbonate rock area in southern China is about one million km2. Carbonate rocks store an estimated 6.1 × 1016 tons of carbon. They are not only the largest carbon pool (Houghton and Woodwell 1989) but also actively participate in the global carbon cycle through karstification (Yuan 1993, 1997). As global warming has become a worldwide concern in recent years, researchers have tried various ways to estimate the amount of atmospheric carbon exchanges with karst systems. The estimated annual absorption of atmospheric CO2 by the carbonate system was about 6.08 × 108 tons, which may be an important part to the “missing carbon sink” (Trudgill 1985; Amundson and Davidson 1990; Yuan 1993; Li 1996 Yang and Wei 1996; Xu and Jiang 1997). Although this volume may not be significant in terms of the global carbon cycle, it affects 22 million km2 of land areas on Earth. The carbon cycle in a karst system is the driving force for special karst environments. It can accelerate the physical, chemical, and biological processes in the epikarst systems and affect the ecosystems and the living environment in those regions.

Based on the chemical and ecological data collected from several typical karst regions in southern China (Fig. 1), this paper presents a systematic analysis of carbon cycles and their ecological impacts in the epikarst systems. The findings from this study are essential for an ongoing large-scale carbon sink study funded by the China Geological Survey in quantifying the amount of atmospheric CO2 taken by the karst system and for the ecosystem restoration and combat against rock desertification in southern China.
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Fig. 1

Distribution of carbonate rocks in south China and locations of four study sites

Mechanism of carbon cycle

The CO2–H2O–CaCO3 equilibrium

The carbon cycle in the epikarst system is about the exchange of atmospheric CO2 with soil, water, and carbonate rocks driven by the dynamic interaction of CO2 in the three-phase, air-water-carbonate rock system. The dissociation reactions for limestone and dolomite are, respectively:
$$ {\text{CaCO}}_{3} + {\text{CO}}_{2} + {\text{H}}_{2} {\text{O}} \Leftrightarrow {\text{Ca}}^{2 + } + 2{\text{HCO}}_{3}^{ - } $$
and
$$ {\text{CaMg}}({\text{CO}}_{3} )_{2} + 2{\text{CO}}_{ 2} + 2{\text{H}}_{2} {\text{O}} \Leftrightarrow {\text{Ca}}^{2 + } + {\text{Mg}}^{2 + } + 4{\text{HCO}}_{3}^{ - } $$
These chemical reactions show that dissolving one mole of CaCO3 or CaMg(CO3)2 needs one or two moles of CO2, respectively. The three-phase chemical reactions are dynamic and reversible. Saturation indices are often used to indicate the equilibrium of these dissociation reactions. The equilibrium status is controlled by the concentration of CO2 in the atmosphere and soil, solubility and pH of water, the temperature, the composition of carbonate rocks, etc. Carbon can be dissolved into water from the atmosphere, but it can also be released into the atmosphere in the form of CO2. High CO2 concentration into the system could result in the dissolution of carbonate rock and karstification. The release of CO2 from water can cause calcium carbonate deposition. Calcium deposits are often seen near the springs and groundwater seepages faces, where CO2 is released when groundwater transitions from a relatively closed environment into an open or semi-open environment. On the other hand, fresh precipitation with high CO2 concentration and low pH value into the groundwater system tends to dissolve carbonate rocks.

Factors affecting the carbon cycle

The carbon cycle in the epikarst in southern China is dynamic due to a combination of factors. Precipitation, evapotranspiration, atmospheric pressure, soil CO2 pressure, soil and vegetation cover, water temperature, and the hydrodynamic condition of the flow system are all contributors to this process.

Thick soil layers and good vegetation covers are favorable conditions for higher CO2 concentration and pressure in soils. The carbon cycle in soil layers is known to be an important factor for karstification but a rather complicated process because of the constraints of soil and moisture conditions. Studies by Trudgill (1985) and Amundson and Davidson (1990) showed that the soil CO2 concentration increases with depth and has seasonal variation. In the Nongla karst forest in Yunnan (Fig. 1), the peak concentration of soil CO2 appears usually between July and August, which is correlated with the high temperature of the season when microorganisms are very active in soil and soil moisture is high. Soil CO2 concentration drops to the lowest value for the year in fall and winter when the temperature is low and soils are dry. The distribution of soil CO2 concentration in the soil profile in this forest area is also unique. The soil CO2 concentration at 50 cm below land surface is much higher than it is at 20 cm below, which might be attributed to a combination of soil structure, CO2 diffusion, and biological processes in the soil profile. The soil CO2 concentration then decreases or has an inverse gradient towards the soil–bedrock interface (Xu and He 1999). However, the reversed gradient distribution of CO2 concentration does not occur in areas where the soil permeability and moisture are low.

Stable carbon isotope analysis indicates that CO2 in the soil layer is the major source of CO2 concentration in the epikarst system. The \( \delta^{13} {\text{C}} \) values of karst water from the Landiantang spring, upper Nongla spring, and Temple spring were −15.8, −17.88, and −13.68 % (PDB standard), respectively, which were apparently lower than that of limestone (0.4 %) and the atmospheric CO2 (−7 %), but rather close to that of soil organic carbon (−21 %). Considering the existence of inorganic carbon in carbonate rocks, the contribution of soil organic carbon has to be relatively high to form the low \( \delta^{13} {\text{C}} \) karst water. Therefore, the carbon cycle in the epikarst enhances the chemical and biological processes in the system.

Hydrodynamic conditions, particularly flow rate and flow velocity, have a major impact on the carbon cycle. The carbon flux in the cycle depends not only on the chemical reactions but also on the flow rate. For a closed karst groundwater system, it can be estimated from total discharge and its corresponding HCO3 concentration as:
$$ C = 1/2 \times 12/61 \times [{\text{HCO}}_{3}^{ - } ] \times Q $$
where C is the carbon flux and Q is the total discharge from the karst system.

With abundant rainfall and sufficient water storage, the carbon fluxes in the karst water system in southern China are relatively high. Yang and Wei (1996) analyzed the 2000 data collected from 76 hydrogeological regional survey reports in the six provinces in southern China. Their study showed that the highest CO2 absorption indices correspond well with the high groundwater flow rate and velocity, particularly in the Three Gorges region.

High solubility of water and dissolution speed of carbonate rocks tend to have a higher capacity for carbon absorption (Weng 1995). Data (Table 1) for the three underground rivers in Puding and Guizhou show that the dolomite underground river system in Gaoyang had the highest HCO3 concentration, whereas the limestone system with coal-bearing layers in the Longtan underground river system has the lowest HCO3 concentration. The fact that higher HCO3 concentration in a dolomite-dominant karst water system than in the limestone-dominant system was due to: (1) the solubility of MgCO3 is higher than CaCO3 under certain conditions; Song (1981) found that the solubility of MgCO3 can be two times or more higher than CaCO3 in pure water; (2) the existence of Mg can promote the dissolution of carbonate rock (Trudgill 1985); and (3) in an acidic or lower alkaline environment where pH < 7.8 dolomite dissolves faster than limestone (Shen et al. 1996). In a system where coal-bearing layers exist, sulfate is dissolved more easily than carbonate.
Table 1

Water chemistry data for three karst underground river systems in southern Puding of Guizhou, adopted from Yu et al. (1990)

Underground river system

Rock type

Chemical composition (mg/L)

HCO3

Ca2+

Mg2+

SO42−

Gaoyang

Dolomite

210.8–322.5

40.7–71.2

22.2–36.5

23.2–64.6

Wanzi

Limestone

146.9–157.3

47–54.5

4.1–7.4

23.3–30.9

Longtan

Coal-bearing

71.6–73.1

58.9–65.9

9.2–12.5

1.218–1.554

Characteristics of the carbon cycle

Active and rapid carbon cycle

The dissolution rate of rainwater on the bare surface of outcropped limestone at three different locations was monitored. The water chemistry data showed that the rainwater dissolves the bare rock surface rapidly. The HCO3 concentration in the rainwater increased from less than 20 mg/L to above 40 mg/L within 1 h (Fig. 2).
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Fig. 2

Increase of HCO3 concentration in rainwater on bare rock surface in three different locations

Good hydrodynamic conditions can accelerate the dissolution of carbon in soils. It was observed that the peak flow lag time for heavy rainstorms in most of the epikarst systems in south China is around 4 h. For example, the lag time in the Yaji experimental watershed in Guilin is a little over 4 and 3.5 h for the Landiantang spring system in Yunnan. The subsurface flow dissolves carbon along its path as it percolates in the soil and increases its HCO3 concentration. The HCO3 concentration in the outflows can be four times higher than that of water on bare rock surfaces. For example, the spring water from the Nongla karst system was as high as 287 mg/L compared to just 73.4 mg/L on the bare rock surface.

Sensitive to environmental change

In areas without rock desertification, the epikarst water system in south China is often covered with vegetation and some soils. Primarily because of different vegetation and soil covers, the carbon cycle and the karstification vary at different locations in southern China (Jiang 1996; Cao et al. 1999). It is easy to understand that the carbon cycle and karstification intensity are significantly different between the forest and bared mountain regions (Jiang 1999). The epikarst system is open to the atmosphere and the carbon exchange between the atmosphere and soils is active. It has been observed that the seasonal variation of karstification intensity corresponds to the seasonal variations of CO2 concentration in soils of epikarst systems, and the monthly variation of HCO3 in karst water correlates well with the change of soil CO2 even though with about 1 month delay.

On a regional scale, the carbon cycle and karstification intensity are more controlled by geological, climatic, hydrological, and ecological conditions. Shown in Table 2 is the comparison of precipitation, temperature, soil CO2, HCO3, and carbon absorption indices. In general, more carbon is absorbed in the hot and humid climate region and in the forest environment than in the dry and cold climate region and in the bared mountain environment. The data also show that the dolomite-dominant system can absorb more carbon than the limestone-dominant system due to the three reasons mentioned earlier.
Table 2

Comparison of precipitation, temperature, CO2 in Soil, HCO3 and carbon absorption in summer of 1999

Location

Nongla

Guilin

Maolan

Shuicheng

Average annual rainfall (mm)

1,650

1,937

1,320

1,363

Average summer temperature (°C)

28

29

23

19

Soil CO2 50 cm below surface (%)

2.6

2.1

4.4

5.0

HCO3 (mg/L)

415.41

269.62

258.03

198.25

Carbon absorption (mg/L)

81.72

53.04

50.76

39.00

Eco-environment

Sub-forest

Bare rock mountain

Forest

Bare rock mountain

Rock type

Dolomite

Limestone

Limestone

Limestone

Role of carbon cycle to atmospheric CO2

Karst flow systems have been recognized as a sink for atmospheric carbon by absorbing CO2 in the atmosphere and turning it into the liquid phase as HCO3 in the karst water system (Liu 2001). CO2 can also be released into the atmosphere when calcium carbonate precipitates out of water when physical and chemical conditions change. Calcium deposits are widely observed near waterfalls, springs, groundwater seepage faces, and inside caves in karst regions. After all, the CO2 released into the atmosphere by calcium deposition is in localized areas and the quantity is not significant on an annual base. Overall, the karst system in southern China absorbs more atmospheric CO2 than the CO2 it releases back into the atmosphere, which shows that the karst system is more of a carbon sink for the atmospheric CO2.

Table 3 compares the total amount of atmospheric carbon absorption by karst systems in the south and the whole of China. Results from four different estimation methods are quite different, ranging from 7.36 to 14 million tons per year (Yang and Wei 1996; Xu and Jiang 1997). The ratios of carbon absorption in southern China versus the whole country vary from 0.49 to 0.81. The average carbon sink from these four methods was estimated to be around 10 million tons per year (m t/a) in the karst area in southern China, which is about 64 % of the carbon sink for the karst areas in the whole of China. These data show that the karst system in south China plays an important role in atmospheric carbon absorption.
Table 3

Estimated atmospheric CO2 absorption by karst systems (in million tons per year)

Calculation method

I

II

III

VI

Average of the four

Karst areas in southern China

9.489

14.0

9.14

7.36

10.00

All karst areas in China

11.76

17.74

18.59

14.23

15.58

Ratio

0.81

0.79

0.49

0.52

0.64

I, Regional average based on dissolution tablet test results; II, Mass balance of runoff and karst water chemistry; III, Statistical method of CO2 absorption from each province; VI, GIS-based statistics from dissolution tablet test results

Impacts of carbon cycle to ecology

Effect on plant species

The active carbon cycle helps to expand plant species in the epikarst system by enhancing the movement of elements and dissolution of more chemical components. Water samples have been collected from the Landiantang and Upper Nongla forest karst flow systems (Table 4). The Landiantang forest karst system in Nongla, Yunnan has grown dense forest and the soil layers are about 2.5 m deep since it was restricted from logging in the 1960s. Quite contrarily, the upper Nongla forest has much less dense forest and the soil layer is only about 0.5-m thick because it was protected from logging only in 1985. The water sample data showed the Landiantang forest has a higher CO2 concentration, lower pH value, and more rich chemical elements in the soil than that of the upper Nongla forest (Table 4). The chemical elements in soils filled in the karst fractures had higher concentrations than in the A/B or B soil layers in the Nongla forest (Table 5). It indicates that the participation of karst water can increase the dissolution of nutritious chemical elements in soils.
Table 4

Chemical elements of karst springs in the Landiantang and upper Nongla forests

 

Soil CO2 (%)

pH

Concentration of chemical elements (mg/L)

Ca

Mg

Na

K

Al

PO4

Fe

Mn

Pb

Zn

Cu

Ni

Co

Landiantang

2.5

6.8

78.8

39.0

0.43

0.03

0.14

0.018

0.018

0.003

0.032

0.008

0

0.013

0.032

Upper Nongla

1.8

7.4

68.7

34.5

0.3

0.06

0.02

0.004

0.008

0.003

0.001

0.008

0

0.014

0.006

Table 5

Effective chemical elements in different soil layers in the Nongla forest

Soil layer

Effective chemical elements (mg/kg)

Ca

Mg

Fe

Na

K

Mn

Cu

Zn

A/B horizon

2038.9

838.48

15.21

9.38

28.5

17.51

0.23

0.19

B horizon

1651.6

741.33

14.45

9.55

29.25

12.87

0.31

0.22

Soil in fractures

4209.6

705.65

9.35

10.4

32.25

31.62

0.85

1.11

Plant species in the karst areas in south China are abundant and unique. Up until 1993, 195 families, 1,213 genera, and 4,287 species of vascular plants were found in Guangxi, Guizhou, and Yunnan (Xu 1993). Among them, many new genera, new species, and endemic species belong only to karst areas. Ten genera and over 100 new species were found for the gesneriaceae family alone (Xu 1993). In the mere one-million-hectare forest area in Maolan, Guizhou, more than 40 new species, 8 genera, 5 minority endemic species, and 14 majority endemic species were found (Zhou 1987). The species to genera ratio is about 3:5, which is significantly lower than the national average of 8:5 (Xu 1993). Eco-geological study shows that these diversified plant species are credited to the specific geological environment in the areas and the active carbon cycle in providing rich calcium in soils and water for the calciphilic plants to grow in the areas. Chemical analysis showed that the elements among rock, soil, and plants had a good correlation (Jiang 1997).

Effect on biomass accumulation

The calcium-rich water and soil environments are also favorable for biomass accumulation. A large amount of Ca2+ in the solution can combine with the humic acids in the soil to form stable humic calcium that promotes organic accumulations. It is quite common that the organic concentration in karst soil is often higher than in the red, yellow, and brick red soils in the non-karst areas. For example, organic materials in soils in Guilin, Nongla, Nonggang, and Maolan are usually above 10 % and could be as high as 15–34 %, whereas the organic materials in the red and brick red soils are often below 6 % (ISSAS 1978). A study by Zhou (1987) showed that about 85 % of the humic acids in the soils in the Maolan karst forest area are combined with calcium; however, almost none of the yellow and red soils are combined with calcium.

A study by the Integrated Survey Team from the Nonggang Natural Protection Group (1988) showed that biomass accumulation in the Nonggang karst forest was around 18.9–23.2 kg/ha (dry mass), much higher than in the brick red soil (9.4 kg/ha) in the Xishuangbanna rainforest and the brick red soil (12.3 kg/ha) in a forest in Hainan.

Some negative ecological effects

On the positive side, the carbon cycle can create environments with high HCO3, Ca2+, and Mg2+ concentrations in soil and water that help with plant growth and biomass accumulation. On the other hand, high HCO3 concentration could also have a negative impact on ecology. In areas where Mn, Cu, Zn, and Fe contents are not rich, high HCO3 concentration could form compounds with these elements and limit the supply of these alkaline elements available for plant growth. Because of that, the effective concentrations of Mn. Cu, Zn, and Fe are drastically lower than the total concentration as shown in Table 6. Listed in Table 7 are the total and effective Cu concentrations in soils from different areas in Guangxi. The effectiveness is the ratio of effective concentration to the total concentration. It can be seen that the effective Cu concentration was less than 8 % of the total Cu concentration in the Pinxiang forest area and as low as about 2% in Nongla. The deficiency of alkaline elements in those areas often results in brown tree leaves.
Table 6

Total and effective element contents in the calcareous soil in the Nongla forest

Elements

Total conc. (mg/kg)

Effective conc. (mg/kg)

Effectiveness (%)

Mn

1,368

12.87

0.94

Cu

23.11

0.31

1.34

Zn

415.8

0.22

0.05

Fe

132,600

14.45

0.01

Table 7

Comparison of the total and effective copper contents in different soils from Guangxi

Soil type

Crimson soil

Red soil

Yellow soil

Pinxiang

Nongla

Brown karst soil

Black karst soil

Brown karst soil

Black karst soil

pH

5.0

4.9

5.1

6.1

6.5

7.4

7.0

Total Concentration (mg/kg)

22.1

6.2

7.4

34.0

16.0

23.11

40.52

Effective concentration (mg/kg)

1.7

0.39

0.8

2.7

1.0

0.31

0.85

Effectiveness (%)

7.7

6.3

10.8

7.9

6.3

1.3

2.1

Summary and conclusion

The carbon cycle for climate change on the global scale has been discussed extensively in recent years. This paper focuses only on its cycle in the epikarst systems in south China and its impact on soil and water systems and the ecology in those areas. This study showed that carbon cycles in the epikarst systems in south China are more dynamic due to humid and warm monsoon conditions. The CO2 concentration in soils has clear seasonal variations and its peak correlates well with the warm and rainy months in southern China. The CO2 distribution in the soil profiles is a unique “v” shape, meaning it increases to a certain depth and then decreases below that depth. The CO2 concentration at the soil and carbonate bedrock interface is usually low. Stable carbon isotope analysis showed that the majority of the carbon in this cycle is from soils.

Hydrologic conditions such as flow rate and flow velocity in an epikarst system and the composition of carbonate rocks control the carbon fluxes. This study showed that a system with a high subsurface flow rate and velocity has a higher carbon flux and carbon absorption rate. The dolomite-dominant system has higher carbon flux than the limestone system, whereas the carbonate rock system with coal-bearing layers has the least carbon-carrying capacity.

The carbon cycle in south China karst areas is dynamic. The three-phase chemical reaction can reach equilibrium in a matter of 1 h after a rain event. Because of the high karstification in the region, most of the karst flow systems are small and the lag time between the peak discharge and peak rainfall is about 4–6 h. Even with such a short lag time, the bicarbonate concentration in the outflows from these systems can become more than four times higher than that of its recharge water, because of the dissolution of high concentration carbon dioxide in the soil along its path. This study proved that the carbon cycle is more active in the warm and humid areas, the forest areas, and the dolomite system than in the dry and cold areas, the bare rock areas, and the limestone systems. The karst flow system plays an important role in absorbing and acting as a sink of the atmospheric CO2. It was estimated that the karst areas in south China contribute to about half of the total carbon sink by the carbonate system in China; however, whether it can account for the mysterious “missing carbon sink” remains to be studied.

This study has also presented evidence of the impact of the carbon cycle on the rich and diversified plant species in those areas. The carbon cycle and karstification created a favorable environment for the calciphilic plants and biomass accumulation. On the other hand, the carbon cycle might produce a higher bicarbonate concentration in soil and water to form compounds with Mn, Cu, Zn, and Fe to make them less available for plant growth. Some types of tree diseases such as the “lack of green” disease for tree leaves are due to lack of those elements.

The karst areas in south China are typically low in soil and vegetation cover. Most of the areas are ecologically fragile and not productive. Rock desertification has become a serious problem in those regions (Jiang and Yuan 1998). In recent years, the Chinese government has been investing a tremendous amount of financial resources to combat the poverty in those areas and to improve the living conditions for local residents. The Chinese government has also sponsored a nationwide study on the carbon budget analysis. Karst system as a carbon sink is one of those focuses. We believe the findings from this study can provide some basic understanding on how the carbon cycle works in the epikarst systems for the best management practices in the region for ecosystem restoration in combating rock desertification and will help the ongoing study of the total carbon sink by the karst flow systems in China.

Acknowledgments

This research was jointly sponsored by the China Geology Survey (Grant No. 1212011087121) and the Ministry of Science and Technology of China (Grant No. 2010BAE00739). We would also like to thank Dr. Shiyi He, Mr. Jianguo Pei, and Dr. Cheng Zhang of the Institute of Karst Geology for their help in fieldwork.

Copyright information

© Springer-Verlag 2012