Effects Of Elevated CO₂ Concentration And Cadmium Addition On Nitrogen Transformation In Soil And Alfalfa Leaves

The increase of atmospheric CO₂ concentration and heavy metal pollution will not only have a significant impact on the nitrogen transformation and nitrogen cycle process in soil and plants,but also have a significant change on the absorption of nitrogen and normal growth and development of plants.CO₂ is a necessary factor for plants to carry out photosynthesis.With the change of atmospheric environment,the increase of CO₂ concentration has become an inevitable trend,so the change of CO₂ concentration will certainly have an impact on plants and agriculture.The rapid development of industry has also brought a certain negative impact on the ecological environment.Heavy metals in the soil will have a toxic effect on plants,which will gradually accumulate and eventually inhibit the growth of plants.In this study,a CO₂spraying system was set in an open-top air chamber with 500 ppm and 700 ppm for artificial control of CO₂ concentration,using leachate and C3 plant alfalfa as research objects.At the same time,the content of two kinds of heavy metals cadmium was set under natural conditions to compare the soil artificially added with the soil without added cadmium.By conducting field positioning experiments,the effects of the increase of CO₂ concentration,Cd content and their synergistic effects on the contents of ammonium nitrogen,nitrate nitrogen and dissolved total nitrogen in soil and on the contents of ammonium nitrogen and nitrate nitrogen in alfalfa leaves were studied,respectively.and by using the method of bacterial denitrification in the soil and alfalfa leaf nitrogen isotopes,aims to explore the CO₂ and cadmium adding nitrogen in the soil and plant leaves under the conditions of transformation and change characteristics and mechanism of nitrogen cycle process,in order to high concentrations of CO₂ and heavy metal contamination of terrestrial ecosystems provide new insight into the nitrogen cycle process,is also seeking solutions under the condition of the soil and plant protection measures to provide a scientific basis.The main conclusions of this study are as follows:1.In the research experiment of chemical denitrification method,by testing the blank background value in the experiment,it can be found that Na Cl is the main reason for the fluctuation of the blank value.We found that burning Na Cl at a temperature of 450°C is sufficient to remove nitrogen impurities,and Na Cl should be used immediately after burning and should not be kept for more than 24 hours.When 0.3 g cadmium powder and 600 nmol Na N₃ are added,the reduction efficiency can reach 90%,it is sufficient to completely reduce NO₃-.The measurement of actual standard samples shows that the method has good accuracy and applicability.But since we are in the process of using chemical denitrification experiment of sample error would happen occasionally,and bacterial denitrification method requires little chemical reagent in the process of experiment were determined,avoids most of the experiment error caused by the reagent,and have good laboratory experimental conditions of denitrifying bacteria method.Finally,the bacterial denitrification method was selected as the method for determining the nitrogen isotope value in the subsequent field experiments.2.Compared with the atmospheric environment,the total inorganic nitrogen content and nitrogen isotope value in the soil decreased gradually with the increase of CO₂ concentration.The nitrogen andδ¹⁵N values in alfalfa leaves increased in different ranges.It shows that under the influence of different concentrations of CO₂ vane exists in the different levels of nitrogen transformation,these changes cause all sorts of nitrogen conversion and microbial activity in the process of fractional function has a close relationship.With the increase of CO₂concentration,the content of total inorganic nitrogen in the soil decreased,and the nitrogen that could be transported with soil erosion decreased,thus improving the fixation of nitrogen in the soil.At the same time,the change of nitrogen isotope in soil indicated that denitrification was weakened more than nitrification in soil.With the increase of CO₂ concentration,the nitrogen level in alfalfa leaves gradually increases,indicating that the increase of CO₂ concentration can promote the absorption of nitrogen by alfalfa and improve the utilization efficiency of nitrogen by alfalfa.Stable nitrogen isotopes can be used as another effective way to understand the nitrogen cycle in soil and plant systems under the background of increasing CO₂ concentration.3.Compared with the soil without cadmium addition,the concentration of ammonium nitrogen decreased with the increase of cadmium content,while the concentration of nitrate nitrogen increased,and the dissolved total nitrogen had no significant change.With the increase of cadmium content,the concentration of ammonium nitrogen in alfalfa leaves first decreased and then increased,while the concentration of nitrate nitrogen gradually decreased.In both soil and plant leaves,the nitrogen isotope values were negatively correlated with the cadmium supplemental content.The addition of cadmium enhanced the activity of nitrate reductase in the soil,which promoted the occurrence of nitrification.At the same time,heavy metals can cause the increase of nitrogen content in plant mycorrhiza,which leads to the increase of isotope fractionation and the decrease of nitrogen isotope value in soil and alfalfa leaves.