Effects Of Increasing Temperature And CO₂ Concentration On The Root Physiology And Rhizosphere Soil Microbial Communities Of Castor

The ecological effect of global climate change is one of the most complex challenges facing mankind in the 21st century.The changes of environmental factors characterized by the increase of temperature and CO₂concentration had significant effects on plant root growth and development and rhizosphere soil microbial community.Root architecture（RSA）refers to the spatial structure and distribution of roots in soil,and the ability to capture,move and fix resources（mainly water and nutrients）.The physiological characteristics of root system are particularly important for plants to adapt to the change of environmental factors.Plant rhizosphere microbial community also plays an important role in regulating plant growth and stress resistance.Castor（Ricinus communis）is one of the top ten oil crops in the world.Because of its high oil content and strong stress resistance,castor has attracted much attention in the fields of soil remediation（heavy metal,saline-alkali soil,etc.）and landscaping（ornamental castor）in recent years.However,it is not clear about the root growth,physiology and the response mechanism of rhizosphere soil microbial community to warming and CO₂.Based on this,this study used open-top climate chamber（OTC）to simulate the increase of temperature and CO₂concentration（CK:a CO₂+a T;e T:a CO₂+e T;e C:e CO₂+a T;e CT:e CO₂+e T;（a CO₂:400 ppm;e CO₂:550 ppm;a T:ambient temperature;e T:ambient temperature+2℃））to explore the response mechanism of castor root physiology and rhizosphere soil bacterial and fungal communities,which is of great significance to reveal the ecological effects of global change and the feedback mechanism of plant roots and important rhizosphere microorganisms responding to environmental changes.The main results are as follows:（1）Warming,increasing CO₂and their interaction all increased the total root length,surface area,total volume and bifurcation number.Among them,the number of root bifurcations increased most obviously.In addition,the root index of CO₂treatment was the highest,which was significantly higher than that of warming treatment and CO₂concentration and temperature increasing at the same time,indicating that the increase of CO₂concentration had the most obvious effect on the growth and development of castor roots,especially to promote more bifurcated growth of castor roots.In addition,warming,increasing CO₂and their interaction all increased the root biomass of castor,which was the highest under the treatment of increasing CO₂,which was 63.91%higher than that of the control group.The specific root length of castor showed an increasing trend after three treatments,and the specific root length was the highest under the warming treatment,which indicated that the construction cost per unit root length was the lowest under the warming treatment.（2）Under the action of increasing temperature and CO₂,the contents of reducing sugar,sucrose,soluble sugar,starch and non-structural carbohydrates in castor roots changed.Among them,warming significantly increased the contents of reducing sugar,sucrose,soluble sugar,starch and non-structural carbohydrates（P<0.05）.This may be due to the increase of temperature,which increases the photosynthesis and respiration ability of plants,and increases the energy conversion efficiency,which leads to the storage of more energy materials in roots,which indirectly leads to the increase of carbohydrates in roots,while the activities of antioxidant enzymes（APX,SOD,POD and CAT）do not change significantly.（3）In terms of bacterial community,the dominant phylum（relative abundance>5%）of each treatment was not affected by warming and CO₂treatment,which were Proteobacteria,Actinobacteria,Acidobacteria,Chloroflexi and Bacteroidetes.The relative abundance of bacteria in CO₂treatment was significantly different from that in other treatments.The effect of the interaction of warming and CO₂on the relative abundance of bacterial genera was greater than that of single treatment,which made the relative abundance of 10 genera changed significantly.Among them,the change trend of relative abundance of 5 genera was consistent with that of temperature increase under the treatment of simultaneous warming and increasing CO₂.This shows that under the treatment of simultaneous warming and increasing CO₂,the effect of warming on the relative abundance of bacteria is dominant.（4）In terms of fungal community,the dominant phylum（relative abundance>5%）of each treatment was not affected by warming and CO₂treatment,which were Ascomycota,Mortierellomycota and Basidiomycota.Compared with the control group,the relative abundance of Cercospora decreased under the warming treatment,while that of Tausonia,Pseudomonas and Pseudeurotium increased under the warming treatment,and the relative abundance of all dominant fungal genera increased under the increasing CO₂treatment.Under the interaction between the two,the change of the relative abundance of fungal genera was the same as that of the warming treatment.This shows that under the interaction of simultaneous warming and increasing CO₂,the effect of warming on the relative abundance of fungi is dominant.The effect of temperature and CO₂concentration on relative abundance and community structure of fungi in castor rhizosphere is due to the fact that fungi account for a large proportion of microorganisms decomposing litter,and their number is positively correlated with the amount of available organic matter,but the specific mechanism needs to be further studied.The results showed that warming,CO₂and their interaction had different effects on the relative abundance,diversity and community structure of castor root physiology and rhizosphere soil microbial community.Increasing temperature and CO₂can promote the elongation and bifurcation of castor root,increase its ability to absorb nutrients,and increase its root biomass.Under the interaction of increasing temperature and increasing CO₂,warming plays a dominant role in the composition of bacterial community and fungal community in castor rhizosphere soil.In contrast,at the gate level,bacterial community composition was significantly affected by increased CO₂,while fungal community composition was significantly affected by warming.