Effects Of High Concentration CO₂ In The Aeration Zone On The Soil-Plant System

The average concentration of CO₂ in the atmosphere before the Industry Revolution was about 280ppm,while now it has increased to 400ppm.With the rapid development of industry and the wide use of fossil fuel,more and more CO₂ were discharged into the atmosphere without control,which caused the aggravation of the greenhouse effect.The excessive emissions of greenhouse gas will threaten the human living environment and the social development.To reduce CO₂ discharge,national governments have taken a series of actions,and one of them,carbon capture and storage?CCS?technology,is an important method for carbon emission reduction.Essentially,CCS technology is to separate CO₂ from the site of generation and transform gaseous CO₂ into a supercritical state by changing pressure and temperature.Subsequently,the supercritical CO₂ is injected into the geologic structure through a drilling.Although there are many successful CSS projects in the world,people have always been worrying about the risk of CO₂ leakage.Therefore,the natural test site and the simulation experiment platform were used to simulate the natural CO₂ leakage,exploring probable effects of high CO₂ concentration?>2000ppm?on the living environments of human and ground surface vegetation.The field experiment platform was located at a natural CO₂ leak point in Sahe Town,Ping'an County,Qinghai Province.The formation of this leak point was due to the imperfect well digging technology at the begging of the 21th century.A portion of CO₂ leaks directly to the ground surface?atmosphere?along the wellbore wall,and the other diffuses into the soil.Through our visual observation,there were noticeable bad odors,dead birds and unhealthy plants damaged by CO₂ leakage including soybeans,rapeseed,highland barley and vegetables at the leak point.The experiment platform,simulating the process that CO₂ invades vadose zone,included the main simulation platform,greenhouse and threshold test system.All of them have four experimental zones,three with high,medium,low CO₂ concentration respectively,one is control.The main simulation platform includes the basement and surface CO₂ testing tower.In the basement,there were 9 FRP testing columns with a height of 2.8m,a diameter of 1m.Gas mixing cabinet was set up to prepare different concentrations of CO₂.The soil filled in the testing column was taken from CCS demonstration base?Ejin Horo Banner,Erdos?.Different vegetations?wheat,corn,etc.?were planted in the test column depending on the planting season.Different concentrations of CO₂?5×10⁴ppm,10×10⁴ppm,15×10⁴ppm?were input from the bottom of the testing column to observe the changing status of ground surface vegetations and the physical,chemical and biological characteristics of the soil.Greenhouse was used to eliminate the restriction of temperature.Different vegetations could plant there across seasons,broadening the test species.According to the design,the planting experiments were arranged in the greenhouse.The characteristics of plants,the test soil and the microbial status were tested after inputting different concentrations of CO₂?5×10⁴ppm,10×10⁴ppm,and 15×10⁴ppm?.In order to explore the tolerance and resistance of plants and soil microorganisms to ultra-high CO₂ concentration,different test barrels containing soil from CCS demonstration base were set up for threshold test.The target plants were planted in test barrels and different concentrations of CO₂?50×10⁴ ppm,70×10⁴ ppm,and 90×10⁴ ppm?were input into the soil to investigate the thresholds of CO₂ concentration that could kill different plants and the stress responses of plants and soil microorganisms.The test results showed that CO₂ intrusion in vadose zone would change the physicochemical properties of the soil,thus affecting the microecological environment,and resulting in a significant impact on the ground surface vegetations.?1?CO₂ intrusion in vadose zone could change the physicochemical properties of the soil to some extent:the on-site pH of soil was varied inversely with CO₂ concentration.When the concentration of CO₂ increased,the pH of soil decreased.The chemical composition of soil had been changed by the complex reactions among CO₂,water and soil,including dissolution,exchange,hydration,corrosion and so on.Moreover,CO₂ intrusion could cause the changes of mineralogical phase:with the increase of CO₂ concentration,the crystallization of soil increased and some compound contents were found to a higher level,especially for the CaCO₃,anorthite,illite and montmorillonite.In addition,the increase of CO₂ concentration could also lead to the improvement of soil temperature,thermal conductivity,thermal diffusion coefficient and thermal capacity.Overall,the greater concentration of CO₂,the higher content of carbon in the soil,which reflected the ability of soil to adsorb CO₂.?2?The development of microbial communities in soil was closely related to the ecological environment.Simulation experiments showed that high concentration of CO₂ had a greater impact on the species and amounts of protozoa,especially soil microorganism diversity.Some microbial growth were inhibited,while other microorganisms were stimulated which resulted in increased abundance.The dominant bacteria in soil were Proteobacteria,Bacteroides and Acidobacteria.Among them,Acidobacteria could be used as indicator microorganism to warning CO₂ leakage from geological storage,since it will multiply rapidly when the concentration of CO₂ in soil reaches the level of 40×10⁴ppm.?3?The effects of CO₂ leakage on ground surface vegetations include:The high concentration of CO₂ in soil had a positive impact on photosynthesis of pea,wheat and amaranth,and the low concentration of CO₂ caused the inhibition of photosynthesis in wheat and amaranth.The photosynthesis of ryegrass and corn was inhibited to different degrees;The protective enzyme system of dicotyledon pea and amaranth showed a small-scale variation.the CAT and SOD activity of some monocotyledons such as wheat,ryegrass,and corn were changed complexly.The activity of POD changed greatly and was sensitive to the oxygen free radicals caused by the CO₂ stress;The degree of membrane lipid peroxidation in all plants had increased and the progress of senescence accelerated under CO₂ stress;Comparison of the sensitivity of different kinds of plants and their key physiological and biochemical characteristics,soluble sugar and POD were changed significantly to the increase of CO₂concentration in soil.Pea?C3 plant?and corn?C4 plant?were more sensitive to the environment stress.?4?The effect of natural CO₂ leakage in the field on surrounding plants and environment:The amount of plants increased with the improvement of CO₂ concentration,but there was not any certain correlation between plant population distribution and CO₂ concentration.Most plants growth increased at a medium concentration?6490ppm?,while were inhibited at a higher concentration?>10×10⁴ppm?.Among these plants,Argentina anserina was more sensitive to the change of CO₂ concentration in soil.The important achievements are described below:?1?On-site pH of soil is sensitive to CO₂ leakage.Higher CO₂ concentration,lower pH of soil,and the temperature of soil also enhanced.?2?Planting Argentina anserina,peas and corn in CCS site is helpful for warning through observation of plant growth combined with testing physiology and biochemistry of plants.?3?At the CCS project sites,it is necessary to measure the background value of Acidobacteria in soil.After the implementation of the CCS project,soil samples should be collected at a certain frequency?for example,once a month?.If the relative abundance of Acidobacteria in soil shows an increasing trend,CO₂ leakage may happen.?4?Since the soil has a certain adsorption capacity for CO₂,CO₂ leakage must be paid more attention if the C content in the soil of the CCS project site is found to be higher than the background value.These conclusions have certain reference value and great significance for the risk assessment during the implementation of CCS project,the determination of leakage warning indicators and the enactment of relevant measures.