Guilin Karst Experimental Site is located in subtropical monsoon region, which is a typical peak-cluster depression. The annual average temperature 18.8℃ and the annual average rainfall1915.2mm are unevenly distributed. The total rainfall from April to August accounts for 70.32% of the annual rainfall, and the rainfall from September to March accounts for 29.68% of the annual rainfall. The karst stratum that constitutes the peak cluster depression of the site is the Upper Devonian Rongxian Formation (D3r), and the main rock composition is light gray to white dense pure medium-thick mud-bright granular limestone. Quaternary strata in the site are mainly residual products, mainly taupe, brown and brown calcareous soil. The soil layer on the upper part of the mountain is thin, and the soil layer on the hillside is thick, which can reach1~1.5m. The soil layer thickness in the depression can reach 5 ~ 6m, and the soil coverage rate is about 30%. See Table 3- 1 1 for the soil texture of sloping land measured by simple hydrometer method.
Table 3- 1 1 Soil texture of different levels of slope in Guilin Karst Experimental Site (wB/%)
The site is covered with secondary shrub community, 2 ~ 2.5m high, spiny, leathery, lobular, calcium-loving and drought-tolerant. The main tree species are Cinnamomum camphora, Lin Mang, Vitex negundo, Pyracantha fortunei, bamboo leaves pepper, Lithospermum, Marble, Rock Maple, Cliff Palm, Croton, Nandianzhu, Phyllostachys pubescens, Myrica rubra, Gracilaria lemaneiformis, Lithospermum Guilin, Bud Hair and Phyllostachys pubescens. Coverage rate is 60% ~ 80%.
In order to reveal the relationship between soil microbial activities and the operation of karst ecosystem, in addition to dynamic monitoring of temperature and rainfall, soil microbial metabolism index and karst development intensity index were also selected for monitoring. Metabolic indexes of soil microorganisms are: CO2 concentration in soil profile, soil respiration, soil microbial biomass carbon and soil water-soluble organic carbon; The strength index of karst development is the dissolution rate of carbonate rocks under soil. Its monitoring method:
CO2 concentration in soil profile: A self-made soil CO2 collection device (He Shiyi, 1997) was used for monitoring. Gastec vacuum pump and autocracy CO2 test tube produced by Gestec company in Japan were used. The observation period is 1 time per month.
Soil respiration: soil respiration rate was measured by alkali absorption method (AL Page et al.,1991);
Soil microbial biomass carbon and soil water-soluble organic carbon: soil samples are taken at the depth of 0 ~ 20~50cm and 20~50cm every month and kept fresh in the refrigerator; Soil microbial biomass was extracted by chloroform fumigation culture (Vance E D et al., 1987). Soil water-soluble organic carbon total organic carbon analyzer (Lu Rukun,1999);
Dissolution rate of carbonate rocks: observe the dissolution rate of rocks with standard dissolution test pieces.
(1) Monthly Dynamics of Soil Microbial Biomass Carbon
Soil microorganism is an important part of terrestrial ecosystem. In the process of its life activities, it constantly absorbs carbon in the environment and releases different forms of carbon components to the outside world. Therefore, the regulation of carbon cycle and bio-available nutrients in soil environment by microorganisms is closely related to the primary productivity of terrestrial ecosystem (Jen Kissons et al.,1992; Zak D R et al., 1990). In previous studies, people paid attention to the effects of terrestrial ecosystem types (forests and grasslands), farmland cultivation measures (Salinas-Garcia J R et al., 1997) and management modes on soil microbial biomass. However, the ecological observation of soil microorganisms in karst dynamic system and the driving effect of their metabolites CO2 and DOC (dissolved organic carbon) on karst ecosystem have not been paid enough attention.
From the monthly variation of microbial biomass carbon (taking the soil microbial biomass carbon at the depth of 0-20 cm in Yakou as an example) (Figure 3-29), there is a significant negative correlation between soil microbial biomass carbon and air temperature dynamics, that is, the soil microbial biomass carbon is the lowest in hot summer (304mg/kg) and the highest in cold winter (1 16544). There seems to be no obvious correspondence between microbial biomass carbon and rainfall. A correct understanding of this phenomenon will deepen the understanding of the mechanism of bio-driven karst development in soil environment.
Figure 3-29 Relationship between soil microbial biomass carbon dynamics and air temperature and rainfall in 0-20 cm pass of Guilin Karst Experimental Site.
Ⅰ. Basic characteristics of soil microorganisms
Soil microorganisms are extremely active active components in the soil environment. Although its content is only 65,438+0% ~ 4% of soil organic carbon, soil microorganisms have extremely high reproductive ability and very short life cycle. Under good conditions, the life cycle of bacteria is only 20 ~ 30 min, and fungi can be updated in a few hours. Casualty microorganisms are the best source of nutrition for new microorganisms. It is also the most easily decomposed into the final product CO2 (Marumoto T, 1984), so it is reasonable to think that microbial casualties are one of the important sources of CO2 produced by soil respiration. Among the factors affecting the dynamic change of soil microbial biomass, temperature is the main factor (Grisi B, 1998), and the alternation of soil drying and wetting is an important driving force to accelerate the circulation speed of soil microbial biomass (McGill W B et al.,1986; Ross D J, 1987), because most soil microorganisms can't adapt to low soil moisture (Reid D S, 1980). Therefore, it can be considered that soil microorganisms are the "constantly updated power drivers" of material circulation in the soil environment. "Continuous renewal" means that the life cycle of soil microorganisms is extremely short, and old microorganisms are constantly replaced by new microorganisms; "Power-driven" means that microorganisms are the direct driving force for the decomposition of organic matter (including microorganisms themselves) in soil environment to produce metabolites, and the living microbial community is constantly changing.
Ⅱ. Understanding that soil microbial biomass carbon is the lowest in summer and the highest in winter.
Although the temperature is low and the rainfall is low in winter, low evaporation can keep the soil moisture high, and the frequency of soil drying and wetting alternation is also very small, which reduces the turnover rate of soil microbial biomass, prolongs the turnover period and accumulates soil microbial biomass. Secondly, in autumn and winter, the increase of litter and the input of fresh organic matter can stimulate the increase of microbial biomass. 14C labeling tracer technique revealed that the decomposition of plant residues in soil was first transferred to microorganisms (Van Gestel et al., 1993).
(2) The relationship between soil microbial biomass carbon and soil respiration and dissolved organic carbon.
Ⅰ. Monthly dynamics of soil respiration
Soil respiration refers to the intensity at which organisms and metabolic entities in the soil absorb oxygen and release CO2 during metabolism. It has two sources. Biological sources: soil microorganisms, roots of soil plants, and respiration of soil protozoa; Chemical oxidation of organic carbon in soil environment. Among these sources, the respiration of soil microorganisms is the main one (Zheng Hongyuan, 1996), so the intensity of soil respiration can reflect the decomposition of organic matter in soil and the status of available nutrients in soil. It has long been regarded as an indicator of the total activity of soil microorganisms and one of the indicators for evaluating soil fertility (Li, 2000). In the soil ecosystem in karst area, CO2 produced by soil respiration and its migration in soil are not only closely related to the global carbon cycle (Tenglai,1999; Wallace S B et al., 199 1), but in karst areas, it can produce high concentration of CO2 near the ground and promote karst development (Liu Zaihua, 2000; Cao Jianhua et al., 1999).
Fig. 3-30 Relationship between 20cmCO2 concentration, soil respiration and discharge rate, rainfall and air temperature under Guilin karst experimental channel.
The temporal dynamics of soil respiration emission can be divided into three different stages (Figure 3-30): ① From March to August before/kloc-0, with the increase of temperature and rainfall, the soil respiration intensity gradually increased from 1 03.56 mg CO2/m2 h on April 6 to 229.52 mg CO2/day on July1day. From another point of view, with the increase of temperature and rainfall, the biological activity of soil is stimulated, which leads to the increase of soil respiration. ② In August, although the temperature remained at a high level, the soil respiration intensity fluctuated, especially in September, when it was 1, 8 1 .94 mg CO2/m2 h, and then it dropped to1,34.35 mg CO2/m2 h on September 25th. 654381rose to158.61mgco2/m2 h on October 26th. This Big bounce phenomenon is related to the uneven distribution of rainfall: before September 25th, 1 month rainfall was only 3.9mm, while1month rainfall was 259.4 mm from September 26th to June 26th. ③165438+1month later, with the decrease of temperature, the rainfall was less. The highest value of soil respiration emission rate appeared in July 1 day, which was 229.52 mg CO2/m2 h, and the lowest value appeared on February 22nd, which was 41.05 mg CO2/m2 h ... From the above analysis results, soil respiration was positively correlated with monthly average temperature and monthly rainfall, and the relationship between soil respiration and temperature was more obvious (.
Figure 3-3 1 Relationship between Soil Respiration Rate and Meteorology in Guilin Karst Experimental Site
Ⅱ. Relationship between soil respiration and partial pressure of CO2 in soil profile
There is an obvious positive correlation between soil respiration emission and soil profile CO2 partial pressure, and the correlation coefficient is r=0.74 (Figure 3-32). It can be seen from the above that soil respiration is a comprehensive response index of soil microbial metabolic emission, vegetation root respiration emission, soil animal respiration and soil organic carbon oxidation to produce CO2 in a region. Therefore, soil respiration can be used to indicate the intensity of regional biological activities. Karst is the most active in the soil environment, which is closely related to the partial pressure of CO2 in the soil environment. This has built a bridge between the operation of karst ecosystem and biological activities.
Figure 3-32 Correlation analysis between soil respiration and CO2 concentration in soil profile (CC=74.97SR+4 190.8, r2=0.5476).
(3) Monthly dynamics of soil DOC
Water-soluble organic carbon in soil environment is not only the metabolite of microbial decomposition of organic matter, but also an important source of microbial growth energy (Kalbitz K et al., 2000). According to previous studies, above 10℃, microbial activity increased with the increase of temperature, and reached the highest level at 25 ~ 35℃ (Paul E A et al., 1989). In addition, microbial activity is closely related to soil moisture. With the increase of soil water potential, microbial activity decreases, and when soil water content is equivalent to field water capacity, microbial activity is the strongest. Generally speaking, fungi can adapt to lower temperature conditions better than bacteria, and fungi are mainly distributed in shallow soil because of their aerobic nature. At the same time, fungi are more tolerant to high soil water potential than bacteria (Salinas-Garcia J R et al., 1997). In the contribution to water-soluble organic carbon, bacteria mainly contribute volatile components and fungi mainly contribute non-volatile components (A D McLaren et al., 1984). As can be seen from Figure 3-33, the change of soil water-soluble organic carbon has three different stages:
1) From March to July, the change of soil water-soluble organic carbon kept a consistent upward trend with the rate of CO2 release by soil respiration.
2) From August to June165438+1October, the temperature remained at a high level, but the rainfall was low, and the soil was easy to dry. Most microorganisms could not tolerate soil drying, and the activity of soil microorganisms was greatly weakened, resulting in the decrease of soil respiration and soil water-soluble organic carbon. It should be noted here that from September 27th 10 to October 26th/kloc-0, the rainfall of 259 mm stimulated the proliferation of microorganisms, and the soil respiration rate rose, while the water-soluble organic carbon showed the lowest value in the whole year. The possible reason for this phenomenon is that the leaching effect of rainwater after long-term soil drying is stronger than the rate of microbial metabolism to produce water-soluble organic carbon.
Figure 3-33 Relationship between Soil Microbial Biomass Carbon, DOC and Soil Respiration
3) From June 1 1 to February of the following year, with the decrease of temperature, microbial activity gradually decreased, while soil respiration rate continued to decrease and water-soluble organic carbon increased.
There are many factors that affect the existence and migration of soil water-soluble organic carbon. Kalbitz K et al. (2000) summarized the factors affecting the formation, migration and evolution of DOC into 23 kinds in the summary paper "Dynamic Control of Dissolved Organic Matter in Soil: A Review". Include litter input, soil organic matter content, C ∶N ratio of soil organic matter, forest community type growing on it, role of fungal community in microbial community, contents of iron, aluminum oxide, hydroxide and clay minerals in soil, soil pH, base saturation, sulfate, phosphate, multivalent cation, temperature, alternation of dry and wet, hydrological conditions, alternation of freezing and thawing (snowmelt), etc. The book believes that for a relatively stable ecosystem, the main factors affecting the dynamics of soil water-soluble organic carbon are:
1) temperature: if only from the results, the relationship between soil water-soluble organic carbon and temperature has the opposite result. First of all, the DOC content is higher under cold conditions (Park H C et al., 2000; He Z L et al.1994); Second, the DOC content in winter is higher than that in summer (Ross D J et al.,1981; Dalva m et al.,1991; Tips, etc., 1999). If the leaching and dilution process of rainfall is considered, this opposite phenomenon is better understood. From the fact that the DOC content in the top soil is higher than that in the bottom soil, it can be considered that microbial activity is high, which is beneficial to the production of DOC. Therefore, the author thinks that the appearance of DOC in soil is directly proportional to temperature, unless the supply of organic energy is insufficient and affected by hydrological conditions.
2) Soil moisture: frequent dry-wet alternation is the driving force for the increase of DOC content in soil (Haynes R J et al.,1991; Lundquist E J et al., 1999). The reasons are as follows: Drought leads to the decrease of microbial activity and decomposition ability, because microorganisms can not tolerate soil drying, resulting in a large number of casualties of soil microorganisms, microbial metabolites can accumulate in the soil, and the microbial assimilation of DOC in the soil is also reduced, which are beneficial to the production of high-concentration DOC when the soil is wetted again. Therefore, the high content of DOC in soil can often be measured at the early stage of rainstorm (Easthouse K B et al., 1992). However, when the rainfall is too large, the physical leaching and dilution of water is stronger than the ability of microorganisms to produce DOC, which will lead to the decrease of DOC concentration.
3) Composition and change of microbial community: In soil microbial community, Guggenberger et al. (1994) and Mooller et al. (1999) think that fungal community plays a more important role in DOC production than other microbial communities. On the one hand, fungi usually do not completely degrade in the process of decomposing organic matter, that is, a large number of small molecular organic compounds are produced, which makes the soil more tolerant to low temperature and higher soil moisture than bacteria. This is also one of the reasons for the increase of DOC content in soil in winter.
(4) The internal relationship among soil microbial biomass carbon, soil respiration and soil organic carbon.
Soil microorganisms are considered to live in soil, and the biomass less than 5× 103μm3 is a part of living soil organic matter (Jenkinson D S et al., 198 1). Soil microorganisms are "constantly updated power drivers" that drive soil carbon migration. With the change of environment, soil microbial community is constantly changing from old to new, decomposing external substances and organisms, absorbing and assimilating inorganic nutrients, synthesizing its own substances, and releasing its metabolites to the outside world, giving soil fertility and productivity. That is to say, soil microorganisms exist the process of growth assimilation and extinction decomposition at the same time, which is the microbial biomass turnover that academic circles pay more and more attention to. Generally speaking, the turnover period is prolonged at low temperature, but it is the opposite at high temperature. It is estimated that the turnover cycle of soil microbial biomass carbon in northern forest is 0.82a, that in temperate forest is 0.6a, and that in tropical forest is 0.14a. . The temperate grassland is 0.64a and the tropical savanna is 0.34a(Smith J L et al., 199 1).
Meanwhile, the supply of organic energy is an important factor restricting the growth and activity of soil microbial biomass. Heterotrophic microorganisms are dominant microorganisms living in soil environment, and it takes a lot of energy to maintain their life activities. According to He (1997), the total microbial biomass (based on microbial biomass carbon) in land soil is 6× 10 15gC, and the average turnover period of microbial biomass carbon in land soil is 0.42a, so the annual turnover of organic carbon through microorganisms is1.43×/. This data exceeds the total amount of organic carbon entering the soil in the form of litter every year (3.7× 10 16gC). The fastest sources of energy obtained by soil microorganisms are dead and injured microorganisms, water-soluble organic carbon (DOC) and plant root exudates. This is also the reason why the microbial biomass in rhizosphere soil is obviously higher than that in non-rhizosphere soil (Eiland F et al., 1994).
CO2 released by soil respiration is the final product of most biochemical processes in soil. In the soil environment where plants grow, soil respiration emissions are mainly composed of three parts: ① root respiration of plants; ② Microbial respiration in rhizosphere soil; ③ Non-rhizosphere microbial respiration. According to the research of Kelting D L et al. (1998), their contribution rates to soil respiration are 32%, 20% and 48% respectively. CO2 produced by soil respiration can be used as an indicator of soil organic carbon mineralization and soil microbial activity. In view of the above monitoring results and analysis, there is a negative correlation between soil respiration emission and soil microbial biomass carbon (Figure 3-34). Compared with soil respiration, the factors of soil DOC generation, occurrence and migration are more and more complicated, but from the research results of Guilin Karst Experimental Site, the monthly dynamic characteristics of DOC can be divided into two periods: in the warm rain period from March to September, soil DOC and soil respiration have the same changing trend; From 10 to the low temperature and low rainfall in autumn and winter in February of the following year, the change trend of soil DOC and soil respiration is opposite. Therefore, from the statistical analysis of the whole year, the correlation between soil DOC and soil microbial biomass carbon is poor (Figure 3-35a). If the correlation analysis is carried out from two different time periods, the negative correlation is obvious (Figure 3-35b).
The relationship between CO2 and DOC, the main factors driving karst development in soil environment, and soil microbial biomass carbon has been verified by field measured data.
Figure 3-34 Relationship among Soil Respiration, Soil DOC and Soil Microbial Biomass Carbon
Fig. 3-35 Correlation analysis of soil DOC and soil microbial biomass carbon in warm rain period and low temperature and little rain period.
Effects of different stages of vegetation community evolution on water cycle in 3.2.4.2 karst surface zone (comparative field test of vegetation restoration areas in Nongla peak cluster depression, Guangxi)
Nongla is one of the successful examples of ecological construction in karst peak-cluster depression in Guangxi. Two different vegetation communities, 40 a and 20 a, were compared and the dynamic changes of their surface spring water were monitored. The indigo pond spring area covers the top community of trees, and the vegetation coverage rate reaches 95%. Water was cut off from 5,438+0 in April 2006 to 31d in May 2002; ; The Dongwangquan spring area is covered with shrub communities, and the vegetation coverage rate is 65%. Stop water twice, 42d, 124d (Figure 3-36). In addition, the Ca2+ concentration of Landiantang Spring is higher than that of Dongwangquan Spring (Figure 3-37 and Figure 3-38), indicating that closing hillsides to facilitate afforestation and restoring vegetation communities not only enhance the storage capacity of karst surface zone for water cycle, but also contribute to the increase of karst development intensity.
Simulation of Effects of Different Vegetation Coverage on Carbon Cycle of Karst Ecosystem in 3.2.4.3
In karst ecosystem, karst and pedogenesis are complementary surface geological processes. According to the relationship between soil and ecosystem development in karst environment, Pan Genxing et al. studied the characteristics of soil carbon pool distribution and carbon transfer in Yaji village experimental site in Guilin, and showed that the development of vegetation endowed the active carbon component in karst system, thus driving the operation of karst ecosystem and accelerating the dissolution of carbonate rocks and carbon cycle of the system (Pan Genxing et al., 1999). The research results of Kelting D L et al. (1998) show that root respiration and rhizosphere soil microbial respiration play a major role in CO2 emission from soil respiration when higher plants inhabit (Zhang Fusuo et al., 1995). With the habitat of higher plants and the evolution of ecosystem, the characteristics of soil carbon cycle in karst ecosystem have changed as follows:
① The input of litter makes the organic matter in the soil continuously replenished; ② Plant root activity increased the supply of soil active organic carbon; (3) Plant roots form rhizosphere environment, which stimulates rhizosphere microbial activities, thus forming microbial-led rhizosphere carbon cycle microenvironment.
Figure 3-36 Comparison of Dynamic Changes of Water Level between Nonglalan, Diantangquan and East Wang Quan
Figure 3-37 Dynamic Change Characteristics of Spring Concentration in East Wang Quan and Landiantang
Figure 3-38 Dynamic Change Characteristics of Calcium Concentration in East Wang Quan and Landiantang Springs
Studying soil carbon transfer under vegetation cover is an important direction to combine geological and biological processes and karst processes on the earth's surface with systematic carbon transfer under the guidance of earth system science theory (Yuan Daoxian, 1999). Therefore, we designed three simple growth box simulation test devices for the vegetation-soil-carbonate system (Plate Ⅰ-1).
The main body of the growth box simulation test device is a cylindrical soil column box with a diameter of 50cm and a height of 60cm, which is made of a 5mm thick polypropylene plastic plate, and the bottom of the box is provided with a water outlet with a diameter of 2cm for discharging "groundwater". 0.3 mm thick triangular steel is used as the underframe support.
The limestone tested is the limestone of Rongxian Formation (D3r) in Yingxian County, a suburb of Guilin, which is broken into particles with a particle size of 2 ~ 6 cm and spread to the thickness of 10cm at the bottom of the box. The soil is taken from Fuhe River, Yanshan Town, Guilin City, and developed in layer A soil above the Donggangling Formation (D2d). The soil is filled in three soil boxes in equal quantities, with a thickness of 45 cm.
Boxwood box (tree box): The woody plants tested are mature boxwood in our garden. Boxwood has rich and long roots, which are distributed in the whole soil column.
Ophiopogon japonicus soil box (straw box): the tested herb is Ophiopogon japonicus, and the root system of Ophiopogon japonicus is concentrated in the soil at a depth of 5 ~ 10 cm;
Earth box (earth box): There is no plant cover.
A self-made CO2 collection device (He Shiyi et al., 1997) was installed at three different depths (10cm, 20cm and 45cm) in the soil column. At the same time, collect the water discharged from the water outlet at the bottom of the box for chemical analysis.
Monitoring indicators and methods: Alkali absorption method is adopted for CO2 emission from soil respiration (Li, 2000); The concentration of CO2 in the soil profile is monitored by GestecCO2 pump and CO2 test tube produced by Gestec Company in Japan. The pH value of water body is calculated by Cole ParmerpH; Calcium ion concentration test box; Determination of alkalinity by Merck, Germany; A thermometer measures the temperature; Rainfall pipes monitor the daily rainfall.
After the experimental device is installed, it is placed in the garden of karst research institute to receive natural rainfall and light. The observation and comparative study lasted for 74d (April 3 to June 16). The temperature and rainfall during the experiment are shown in Figure 3-39.
Figure 3-39 Daily average temperature and rainfall during the test.
Hydrochemical characteristics of soil-carbonate system drainage under different vegetation cover (1)
In field observation, the dynamic change of karst spring water is a complex process influenced by many natural factors. In this simulation experiment (Figure 3-40), the dynamic changes of karst water in the three systems have similar fluctuation patterns, which accords with the dynamic relationship between temperature and rainfall. However, there are significant differences in the intensity of karst water discharge in different systems. The average concentrations of tree box, grass box and soil box were 4.64 mmol/L, 2.69 mmol/L and 2.6 1 mmol/L respectively. During the whole test period, the rainfall was 638. 1 mm, and karst water was discharged for 8 times, with the total water discharge of tree box 1 10.6 L, grass box 70.92 L and soil box 67.56 L.. Therefore, the total amount of karst water discharged is: tree box 522.95 mmol, grass box 206.04 mmol and soil box 196.23 mmol respectively.
Figure 3-40 Comparison of Concentration in Drainage of Tree Box, Grass Box and Earth Box
The pH value of karst drainage reflects the comprehensive result that DOC and CO2, metabolites of soil biological activities, dissolve in water to produce carbonic acid and soil eluviates carbonate rocks. As can be seen from Figure 3-4 1, regardless of the influence of climatic conditions, the pH value of the tree box is always lower than that of the straw box and the soil box system. During the simulation experiment, the average pH value of the tree box is 7.13; The average pH value of straw box is 7.36; The average pH value of the clay box is 7.39. That is, the pH value of the tree box is 0.23 units lower than that of the straw box and 0.26 units lower than that of the soil box.
Figure 3-4 1 Dynamics of pH value of karst drainage under tree box, grass box and soil box
Figure 3-42 shows the dynamic change of Ca2+ concentration in karst drainage in the test system. It can be seen that the average concentration of Ca2+ in the discharged water of different systems is11.36mg/L for tree box, 1 16.07 mg/L for grass box and1/kloc for soil box. It shows that different plant treatments have no obvious effect on Ca2+ concentration in karst drainage. However, the total water discharge is 26.2 L, 10.5 L and 10.8 L respectively, and the total calcium discharge calculated by different systems is: 304.03 mmol in the tree box; The straw box is188.25mmol; The soil box is 176 438+0 mmol. Ca2+ excretion under tree box is 1.784 times that of straw box and 1.827 times that of soil box. At the same time, the total discharge of tree box is higher than that of grass box 153.82% and higher than that of soil box 166.45%. The main source of bicarbonate is the product of biological activity and water-rock interaction, so it can be considered that with the increase of biological activity intensity, the increase of bicarbonate concentration will lead to a corresponding increase in calcium ion excretion. That is, with the strengthening of biological action, the generated metabolites cause the decrease of pH value of karst water and the increase of bicarbonate, which promotes the water-rock interaction and the release of different forms of calcium in soil, and finally leads to the increase of limestone dissolution.
Fig. 3-42 Comparison of Ca2+ concentration in groundwater of tree box, grass box and soil box.
(2) Soil respiration and CO2 concentration in soil profile
CO2 in soil environment is one of the important sources of karst water, and it is also the reason why karst water is erosive. As can be seen from Figure 3-43, there are significant differences in CO2 concentration under different plant treatments in the system, although they all show fluctuations with air temperature and rainfall. However, during the whole experiment, the CO2 concentration in the tree box soil was higher than that in the grass box and soil box. Taking the CO2 concentration at the depth of 20 cm under the soil as an example, the average CO2 concentration during the experiment was: tree box 14389× 10 -6, grass box 8222× 10 -6 and soil box 5800× 10 -6. Compared with bare soil, Ophiopogon japonicus increased the CO2 concentration in soil by 465,438 0.76%, while Boxwood increased by 65,438 0.04%. Judging from the rate of CO2 emission by soil respiration (Figure 3-44), the soil respiration rate of tree box is much higher than that of grass box and soil box. During the experiment, the average rate of CO2 released by respiration in tree box soil was 365,438+00.83 MGC/m2 h, that in grass box was 65,438+065,438+04.64 MGC/m2 h, and that in soil box was 65,438+065,438+02.72 MGC/m2 h. The carbon emissions of tree box, grass box and earth box can be calculated: 104.438 gC for tree box, 38.5 19 gC for grass box and 37.875 gC for earth box (Figure 3-45). The calculation formula is:
Karst Ecosystem in Southwest China Constrained by Geological Conditions
Where: w-the amount of carbon emitted by CO2 during the experiment; V- the average rate of soil respiration; S—— the opening area of grounding box; T- the duration of the experiment.
Figure 3-43 Dynamic comparison of CO2 concentration under 20cm soil in tree box, grass box and soil box.
Figure 3-44 Dynamic ratio of CO2 emission rate of tree box, grass box and soil box.
(3) Carbon stable isotope tracer technique is used to estimate carbonate dissolution.
Many achievements have been made in studying carbon migration in soil environment by using carbon stable isotope and Ineson Petal. , 1996; Kellek, etc. , 1998; BernouxM et al. , 1998; Hesieh Yuch-Ping， 1996； Koronczi et al. , 2000), and it is also explored to understand and master the structural characteristics and operating rules of karst dynamic system by using carbon stable isotopes (Liu Zaihua et al.,1997; Pan Genxin et al. , 1997)。 In this paper, CO2-C released by soil respiration and -C released by groundwater were collected, and their carbon stable isotopes were determined to estimate the contribution of biological action to soil dissolution. Table 3- 12 gives the stable carbon isotope values of CO2-C emitted by soil and -C emitted by groundwater on May 15 and May 18.
Figure 3-45 Comparison of CO2-C emission of tree box, grass box and soil box during the experiment.
Table 3- 12 CO2-C and stable isotope values of groundwater released by soil respiration in tree box, grass box and soil box
CO2 emitted by soil respiration mainly comes from soil microbial respiration and plant root respiration. As can be seen from Table 3- 12, the isotope value of CO2-C is relatively heavy, indicating that carbonate rocks are dissolved (the δ 13 C value of CaCO3 is 0.5‰(PDB)), and the δ 13C value of CO2-C produced by tree boxes is relatively light. If the δ 13 C value of CO2-C emitted by soil respiration is taken as the isotope value of biological source in groundwater, the biological contribution rate of different systems in groundwater can be obtained by the following formula:
Karst Ecosystem in Southwest China Constrained by Geological Conditions
The results are shown in Table 3- 13.
Table 3- 13 Estimation of Biogenic Source Proportion and Carbonate Dissolution in Groundwater
The results of carbon stable isotope tracing show that the ratio of medium source and biological source of groundwater discharge is higher than that of carbonate dissolved source. Compared with the earth box, the enhancement of the biological function of the tree box and the grass box did not bring a larger proportion of biological sources, but brought a larger number and dissolution of carbonate rocks.
The carbon isotope value of CO2-C emitted by soil respiration in May 18 is lighter than that in May 15, indicating that the biological function is enhanced. It is reasonable to infer that more biogenic CO2 is dissolved in water and acts on carbonate rocks. The carbon isotope value of groundwater in May 18 is heavier than that in May 15, which seems to mean that with the enhancement of biological action, the number of biological sources in groundwater increases, and the dissolution of carbonate rocks makes a greater contribution to groundwater. More data is needed to confirm this.
The results of this experiment show that due to the increase of plant roots:
1) increased the CO2 concentration in the soil profile. Compared with simple soil microbial action (soil box 5800× 10-6), tree box (14389× 10-6) increased 140.08%, and grass box (8220.08%).
2) The rate of CO2 released by soil respiration increased. Compared with the soil box (112.72 MGC/m2 h), the tree box (310.83 MGC/m2 h) increased by 175.75%, and the grass box (16544.
3) The concentration of groundwater increased, compared with the soil box (2.6 1 mmol/L), the tree box (4.64 mmol/L) increased by 77.78%, and the grass box (2.69mmol/L) increased by 3.07%;
4) With the increase of Ca2+ emission from groundwater, compared with the soil box (196.23 mmol, 176.0 1 mmol), the tree box (522.95mmol, 206.04mmol) increased by 166.45% and respectively.
5) Reduce the pH value of groundwater, the tree box (7. 13) is 0.26 unit lower than the soil box (7.39) and the grass box (7.36) is 0.03 unit lower;
6) Increase the dissolution of limestone under the soil. According to the stable isotope tracing results of CO2-C released by groundwater and soil respiration, the dissolved amount of carbonate rocks in tree box is 2 1.875g, that in straw box is 9.57 1g and that in soil box is 6.748g. ..
The experimental results show that plant growth promotes soil biological activity, and then affects the dissolution of limestone under soil and the removal of system carbon. Finally, its contribution is to accelerate the carbon cycle of soil-limestone system, amplify the effect of carbon source and carbon sink, and further support that the surface karst dynamic system is a karst control system driven by the external power of climate-water and based on the carbon transfer and cycle of soil-biological system.