Community Structure Of Soil Microorganisms In Lichen Crusts And Its Function On Carbon Sequestration In The Mu Us Desert

Lichen crusts are an important component of the soil system in arid and semi-arid regions worldwide,and are significant carbon input of ecosystem carbon.Previous studies mainly focused on net carbon exchange of lichen crusts and their impacts on soil organic matter,neglecting the population of carbon-fixing microorganism and carbon allocation in active and stable organic carbon pool.To address these knowledge gaps,we used metagenomic approach to examine microbial community structure and carbon-fixing microorganism population,and applied continuously 13CO2 labeling experiment to detect the rate of microbial carbon fixation and the allocation of carbon fixation in the active and stable organic matter in the Mu Us Desert.This study would provide an insight into a fully understanding of carbon processes and amount of carbon fixation allocate into different turnover efficiency organic carbon pool.The results would contribute to accurate judgment on their effects on stability of organic carbon.In addition,this study would be of merits of estimiation of carbon sink or source of lichen crust.The main results and conclusions are as follows:(1)The microorganism of lichen crusts in the Mu Us Desert mainly composed of bacteria(relative abundance accounts for 89.93%),followed by archaea(1.07%),fungi(0.14%),viruses(0.02%)and unclassified(8.83%).In the level of phulum,lichen crusts composed of Proteobacteria(relative abundance accounts for 24.69%),Actinobacteria(21.72%),Cyanobacteria(14.35%),Acidobacteria(9.26%),Bacteroidetes(3.59%),Gemmatimonadetes(2.45%),Chloroflexi(1.90%),Planctomycetes(1.64%),Firmicutes(0.85%),and Thaumarchaeota(0.65%).The Calvin cycle(Calvin cycle),anaerobic acetyl-CoA pathway(WL cycle),reducing tricarboxylic acid cycle pathway(rTCA cycle),3-hydroxypropionic acid pathway(3-HP cycle),3-hydroxypropionate/4-hydroxybutyrate pathway(3HP/4-HB cycle)and dicarboxylic acid/4-hydroxybutyrate cycle(DC/4-HB cycle)pathway for carbon fixation were detected.(2)After incubation of 100 days,the soil carbon sequestration rate of photoautotrophic microorganisms in the lichen crust layer and their beneath soil was 0.266 g/m2·d and 0.048 g/m2·d,respectively,which were significantly higher than those of autotrophic microorganisms(0.057 g/m2·d and 0.007 g/m2·d,respectively).The active carbon component that was assililated by microorganisms accounted for about 22%of the microbial assimilated organic carbon.(3)The relative abundance of six carbon fixation pathway genes was negatively correlated with the carbon fixation of microbes.In addition,different microbial populations containing carbon sequestration genes had different effects on carbon sequestration of lichen crusts.The results show that there were abundant carbon-fixing microbial populations in lichen crusts,which can fix carbon dioxide from the atmosphere via six autotrophic microbial carbon-fixing pathways and converted CO2 into the more stable components in the organic carbon pool,forming a more stable carbon sink,which is conducive to the growth and development of lichen crusts,improving soil fertility in desert areas,and promoting vegetation restoration in desert areas.This study helped to enhance the understanding of lichen crust carbon processes of lichen crusts,to understand the deposit of fixed carbon in organic carbon pools at different turnover times,to judge its impact on the stability of surface soil organic carbon,and to provide scientific basis for accurately evaluating its carbon sink potential.