The Mechanism Of CO₂ Fixation By Soil Carbon-fixation Microorganisms In The Loess Plateau

CO₂ fixation is one of the most effective ways to mitigate climate change.In addition to the contribution of plant roots and their exudates and litter decomposition to organic carbon in the process of soil carbon cycle,the CO₂ fixation by microorganisms in the soil may also be an important process for the accelerated accumulation of soil organic carbon sequestration.However,the carbon sequestration by microorganisms and their contribution of to SOC remains unclear.Typical soils of three different vegetation regions(forest,forest steppe and steppe)on the Loess Plateau were selected.Traditional microbial isolation and culture,metagenomic sequencing technology,combined with ¹³CO₂ laboratory labeling were conducted in our study.The main purposes were:1)to analyze the characteristics of soil carbon fixation microbial communities,carbon fixation pathways;2)to quantification soil microbial carbon assimilation capacity and the contribution to SOC;3)to discuss the influencing factors of carbon fixation microbial community and carbon fixation potential.The main results and conclusions are as follows:1)The process of CO₂assimilation into SOC by carbon sequestration microorganisms was confirmed by the laboratory labeling test of ¹³CO₂.The carbon assimilation amount of soil carbon fixation microorganisms in steppe area(1.64 mg kg^-1)was lower than that in forest steppe area(3.14 mg kg^-1)and forest area(5.30 mg kg^-1).However,the contribution rate of ¹³C-C_SOC to SOC increased from 0.01%in the forest area to 0.03%in the steppe area.These results suggested that the assimilation of atmospheric CO₂ into organic carbon by microorganisms was an important carbon accumulation pathway in steppe soils with low precipitation and nutrients limited.2)The sequence number of soil microorganisms ranged from 64,219,379 to67,773,074 along forest,forest steppe to steppe.The number of soil microorganisms in steppe area was much higher than that in forest steppe area and forest area.The soil microorganisms were mainly composed of bacteria(above 96%)in the three vegetation areas.The predicted number of carbon fixation genes ranged from 354,341 to 703,257,and the soil carbon fixation microorganisms accounted for 0.55%-1.04%of the total microbial communities.3)The dominant CO₂ fixation microorganisms in the soil of the three vegetation areas belong to the following 7 phyla:Actinobacteria,Proteobacteria,Acidobacteria,Chloroflexi,Firmicutes,Thaumarchaeota and Gemmatimonadetes.There were significant differences in the structure of carbon fixation microbial community among different vegetation areas.Proteobacteria was the dominant microbial in the soil of forest area,accounting for 41.1%.Actinomycetes were mainly found in forest steppe and steppe soils,accounting for 46.7%and 57.0%,respectively.Dominant carbon fixation microbial belong to the following 8 genera:Solirubrobacter,Rubrobacter,Conexibacter,Sphingomonas,Pyrinomonas,Bradyhizobium,Streptomyces,and Nocardioides.4)The carbon fixation microbial in the soil of the three vegetation areas fixed CO₂through eight metabolic pathways.The fixation pathways were including r TCA cycle,DC/4-HB cycle,Calvin cycle,3-HP cycle,3-HP/4-HB cycle CAM cycle,C4-Dicarboxylic acid cycle,and WL pathway.The metabolic pathways of soil carbon fixation microbial were significantly different in three vegetation areas.The r TCA cycle pathway was the most abundant in the three vegetation areas,accounting for 23.8%,23.2%and 22.3%in the forest area,the forest steppe area and the steppe area,respectively.The abundance of Calvin cycle in steppe area(13.2%)was significantly higher than that in forest area and forest steppe area(18.2%and 8.3%,respectively).In addition,the pathway of r TCA cycle and 3-HP cycle play an important role in soil assimilation of atmospheric CO₂ in forest area.The pathway of DC/4-HB cycle and 3-HP/4-HB cycle play an important role in soil assimilation of atmospheric CO₂ in the forest steppe area.The Calvin Cycle and CAM pathways played a greater role in soil assimilation of atmospheric CO₂ than other metabolic pathways,especially the Calvin Cycle.5)The enzymes coding genes involved in metabolic pathways determined the efficiency of carbon fixation pathways,and the abundance of key enzyme genes varied significantly among soils of different vegetation areas,and was significantly correlated with the content of microbial carbon assimilation.The relative abundance of the key EC(1.2.7.1)and EC(1.2.7.3)involved in the r TCA cycle in forest soil were 1.21 and1.23 times that of forest steppe and steppe soil,respectively.The relative abundance of the key EC(4.1.1.39)in the Calvin Cycle pathway was 1.04%in steppe soil,which was2.17 and 2.79 times that of the forest steppe and forest soil,respectively.The expression of key enzyme genes of microorganisms regulates the process of carbon fixation pathway,and affected the ability of microorganisms to assimilate atmospheric CO₂under the external environment changed.6)The mean annual precipitation,latitude and soil nutrients were the environmental factors that affected the differences of soil carbon fixation microbial communities in different vegetation areas.The mean annual precipitation was the dominant environmental factor that affected the microbial metabolism of soil carbon fixation in different vegetation areas.The carbon assimilation capacity of soil microorganisms in different vegetation areas were not only affected by environmental factors,but also by the composition of the carbon fixation microbial community,metabolic pathways(especially the Calvin cycle,the reduced citric acid cycle and the3-hydroxypropionic acid double cycle)and the enzyme genes involved in the metabolic pathways.7)The 10 culturable strains of CO₂fixation bacteria in grassland soil of forest steppe area were as follows:Pseudomonas?putida,Bacillus?cereus,Streptomyces?phaeochromogenes,Streptomyces?virginiae,Sinorhizobium?meliloti,Agrobacterium?sp.?BD-32,Enterobacter?kobei,Brevundimonas?vesicularis,Rhizobium?sp,Acinetobacter?lwoffii.The strain Bacillus?cereus belonging to Firmicutes,the Streptomyces?phaeochromogenes and Streptomyces?virginiae strain belong to Actinobacteria,and the remaining strains belong to Proteobacteria.