Microbial Pathways Of Atmospheric Carbon Dioxide Fixation In Soils In The Mu Us Desert

Drylands are widespread in terrestrial ecosystems.Due to the harsh conditions and low productivity of these areas,it had been considered as a source of carbon dioxide emission?CO₂?.The fact that soils in deserts can sequestrate atmospheric carbon dioxide has been proved wordwide in recent years,which provides important clues for exploring the issues of missing sink;however,the assumed physical and chemical processes cannot convincingly unveil the mechanism involved.On the basis of the existing research,here,we propose the hypothesis of microbial pathways of atmospheric CO₂ fixation in soils;that is,there are some microbial taxa which can transform atmospheric CO₂ into organic carbon and inorganic carbon in the desert soil.To test this hypothesis,the present study carried out in the Mu Us Desert is to investigate the soil microbes transform atmospheric CO₂ into soil organic carbon and inorganic carbon by means of the methods,including the high-throughput sequencing technology,stable carbon isotope tracing,carbonaceous mineral generation experiment,X-ray diffraction,and scanning electron microscopy analysis,etc.The main results and conclusions are as follows.?1?There were a large number of microbes containing genes involved in the six autotrophic carbon fixation pathways in the soil of the Mu Us Desert,and the relative abundance for genes encoding for the reductive citrate cycle was the highest among the six pathways.The autotrophic microbes belong to Actinobacleria,Proteobacteria,Chloroflexi,etc.Based on the stable carbon isotope tracing experiments,soil microbes incorporated atmospheric carbon into organic components,and the incorporated carbon in soil organic carbon accounted for 3.85%of total atmospheric carbon absorption.Such biotransformation of carbon from the atmospheric to soil via autotrophic carbon fixation represents a microbial pathway for persistent atmospheric CO₂ absorption in desert soils.?2?There were not only autotrophic carbon-fixing microbes,but also heterotrophic carbon-fixing microrbes in the soil.The number of autotrophic carbon-fixing microbes was much higher than that of heterotrophic carbon-fixing microbes.Only a small fraction of the carbon-fixing microbes employed the reductive pentose phosphate cycle?mainly used by the photosynthetic pathway?,most of which used the non-photosynthetic pathway.These carbon-fixing microbes are capable of transforming CO₂ into soil organic carbon,and their ability is modified by changes in the community composition of carbon-fixing microbes,as autotrophs employed specific pathways and heterotrophs used anaplerotic reactions.?3?Six bacterial strains?Arthrobacter sp.,Rhodococcus sp.,?were isolated from the desert soil.These bacterial strains contained the ability of precipitating carbonate precipitation?vaterite and calcite?via as the nucleation sites and increasing pH of solution.Through ¹³CO₂ tracing,it was confirmed that the atmospheric CO₂ is indeed involved in the process of carbonate formation by desert soil bacteria.?4?Culture experiments,together with ¹³CO₂ labeling,revealed that desert soil microbes converted atmospheric ¹³CO₂ into soil inorganic carbon.The microbially-convered rates were correlated with the compositions and metabolic functions of soil bacteria and fungi.Meanwhile,it was found that desert soil microbial community was able to induce the formation of inorganic carbon?calcite,carbonaceous aluminosilicate?under the control confitions.These results confirm that desert soil microbes sequestrate atmospheric CO₂ as inorganic carbon.The above results show that desert soil microbes can incorporate atmospheric CO₂ into organic components,have the ability of sequestrating atmospheric CO₂ into inorganic carbon pools.This study provides explanations of the microbial mechanisms for understanding the process of atmospheric CO₂ absorption by desert soils.The pathways that soil microbes transfer atmospheric CO₂ into organic and inorganic carbon may play an important role in desert soil carbon formation and accumulation,and would be of importance on understanding the carbon sink function of desert ecosystems.However,how to determine the rate and fate of microbial organic carbon and inorganic carbon,and the microbial carbonaceous minerals formation used as a new and promising countermeasure against desertification by wind erosion still need to be further investigated.