Soil Microbial Community Diversity Under Different Artificial Vegetation Types In The Hydro-fluctuation Zone Of The Three Gorges Reservoir

After the completion of the Three Gorges Reservoir,a water fluctuation zone with a vertical drop about 30 m was formed.The flooding for half a year has led to the gradual degradation of the original vegetation in the reservoir bank,and has caused various environmental problems such as soil erosion,biodiversity loss and epidemic situation,which has damaged the local ecological environment and seriously threatened the safety of the running of the Three Gorges Project.Environmental issues such as scientific vegetation restoration and reconstruction,prevention and control of soil erosion have become the focus of the study of the fluctuation zone of the Three Gorges Reservoir Area.The soil in the water-fluctuation zone is the premise to ensure the ecological service and function of the reservoir area.Especially after the vegetation is planted,the input of root exudates and litters will cause a series of spatiotemporal variations of soil properties,such as physical,chemical properties and biological characteristics.Therefore,it is necessary to conduct a detailed analysis of the soil properties after the vegetation reconstruction in the Three Gorges reservoir area.This present study explored the dynamic changes of soil properties for specific vegetation types and the diversity of soil biomes,in order to reveal the temporal and spatial variation of soil properties under different artificial vegetation and different hydrological change,and the soil quality and habitat status after the reconstruction of the vegetation.This study was carried out in the Ruxi River Basin of Zhong County,Chongqing.Several different vegetation soils were selected,including bald cypress(BC),pond cypress(PC),Salix(SL),Salix and Bermuda grass mixed(S+B),and Bermuda grass(BG).The bare soil was selected(no vegetationRecovery)as control.The sampling altitude in the fluctuation zone is divided into three altitudes: 160-165 m,165-170 m,and 170-175 m.The area above the fluctuation zone(180-185 m)is used as a control.In-situ sampling was carried out in June and September of 2014-2017,and 0-20 cm of topsoil was collected.The physical properties,chemical properties and biological properties of the soil were measured.The main results were as follows:(1)The vegetation of the trees in the Three Gorges reservoir area has a good growth trend.The plant heights of the BC,PC and SL are increased by 173%,138% and 148% respectively.The DBH and capony are increasing as well.The coverage of the BG community increased slightly,and the total coverage reached over 90%.The coverage of the Hemarthria compressa was reduced and gradually replaced by the BG.(2)The results of physical and chemical analysis showed that: in June and September,the soil properties in the fluctuation zone showed the same trends.The physical properties of artificial vegetation soil,such as soil moisture,porosity and density,were significantly higher than those of unplanted soil.Chemical properties such as organic matters and total nitrogen content were also higher than those of bare soil.After planting,soil pH value was reduced,and the content of other properties such as soil temperature,available nitrogen and available phosphorus were lower than that of unplanted soil overall.The difference of vegetation types also leads to the difference of soil physical and chemical properties.At the elevation of 170-175 m and 165-170 m,the organic content of BC and PC was higher than that of SL soil.However,the TN content of SL soil was higher than that of d BC and PC(except the elevation of 165-170 m in September).The mixed soil of SL and BG is basically the same with those of other vegetation types;The soil of BG had higher TN and TP contents in June in 165-170 m,and the TP content in September also ranked first in artificial vegetation soils.The soils in 180-185 m which did not experienced flooding has a lower oxidation-reduction potential than the soils in the water-fall zone,and has higher pH,TN,and TP content.The artificial vegetation soil structure has a significant improvement compared with unplanted soil.(3)The results of the study on soil microbial community diversity using T-RFLP technology showed that: the species abundance of the unplanted soil was lower than 10 in different sampling times and elevations.The bacterial abundance of BC and PC fluctuated around 14.The bacterial abundance of SL,S+B,and BG were slightly lower than those of BC and PC.Other indices showed no significant differences under different vegetation types,and no significant differences were found during different sampling times(p>0.05).The bacterial S value of the unplanted above the fluctuation zone varied between 10 and 11,H was around 2.4,and D,E were around 0.9,which were all higher than the unplanted soil in the fluctuation zone.The bacterial community diversity of the WC soil was similar to that of the artificial vegetation soils in the fluctuation zone,which has a relatively small variation.The WC soil bacterial Shannon index is significantly improved with its mean value around 2.4,which was about 13% higher than those of the artificial vegetation soils in the fluctuation zone.Soil fungi and actinomycetes communities change in bare and artificial soils in a similar way to bacteria.(4)RDA analysis of the relationship between soil microbial community and soil environmental factors showed that: Under the artificial vegetation type,the samples at different altitudes were concentrated,but the points in different periods were relatively dispersed.Different artificial vegetation was affected by different soil chemical properties,and BC and PC are significantly affected by soil pH,TN,AN,and AP.In addition,C/N and OM had a significant effect on the microbiome under BC and PC soils,respectively.The soil microbial community under SL was the most sensitive to soil AN content.The microbial community in the soil mixed with SL and BG was significantly affected by a lot of chemical properties,including pH,OM,TN,AP and C/N.The BG soil was significantly affected by OM and TN,while TK,AN and AK also significantly altered the soil microbial community of BG.In the region above the fluctuation zone,WC was less limited by soil chemical properties,expect pH and C/N.For the physical properties,temperature and water content had the most significant impacts on the microbial community,and temperature mainly affected the soil microbial community under BC,PC,and SL(p<0.05 or p<0.01),while water had a significant effect on the soil microbial community under BC,SL,and the mixed soil(p<0.05).Besides,soil microorganism of PC was mainly affected by porosity,soil density,and soil bulk density(p<0.01).The soil microorganism of WC was significantly affected by Eh.(5)The diversity of soil bacterial community conducted by high-throughput analysis indicated that: In different artificial vegetation and unplanted soils,Proteobacteria and Acidobacteria had the highest abundance.The analysis NMDS of bacterial community indicated that,in the altitude 170-175 m,the bacterial community of BC and PC tend to gather together,while the bacterial community of SL not gather with them.The bacterial community in unplanted soil was independent.The results of heat map analysis showed that,in 170-175 m,the bacterial communities of BC and PC had higher similarity in the family level,while the SL soil was more similar to the unplanted soil.The bacterial relative abundance in different soils was also varied;in 165-170 m,the BG and unplanted soil were also distinguished with each other obviously according to the difference of family level abundance.According to the correlation analysis,at 170-175 m elevation and 165-170 elevation,the systematic development distance of bacterial community was significantly correlated with soil pH,organic matter content,AN,Eh and soil temperature.In 170-175 m,the soil moisture content was also closely related with bacterial community.(6)The diversity of soil fungal community conducted by high-throughput analysis indicated that: The relative abundance of Ascomycota and Basidiomycota dominated the fungal sequences,accounting for 77% and 11%,respectively.The NMDS results showed that,the samples of BC,PC and SL were relatively concentrated in 170-175 m,in particular,the soil samples of BC was located in the middle of the soil of PC and SL.While the unplanted samples were separated with them.The fungal community of the BG soil was completely different from that of the bare soil in 165-170 m,indicating that there was a great difference between the two communities.The results of heat map analysis showed that the fungal communities in the soil of BC and PC had higher similarity at the family level,while the similarity between the unplanted soil and the artificial vegetation soils was poor.The relative abundance of fungi varies in different soils.Correlation analysis results showed that soil fungal community were significantly correlated with the content of soil OM and Eh in 170-175 m,while at 165-170 m elevation,soil fungal were significantly correlated with the content of AP,C/N,OM,and Eh.(7)The results of the study on soil microbial biomass indicated that: The soil microbial biomass of artificial vegetation,including SMC,SMN,and SMP showed an overall trend of lower in June and higher in September,while the soil in unplanted soil showed no obvious rules.The soil SMC of BC had the highest value in 170-175 m and 165-170 m,while there was no significant difference of SMN and SMP when compared with PC and SL.In the elevation of 165-170 m,the SMC content of BG was higher than that of mixed soil,while SMN and SMP were similar with each other.The WC soil has higher SMP content than that of the soil in the riparian zone,but the SMC content was lower than the overall level of artificial vegetation,especially in September.In terms of the carbon,nitrogen and phosphorus ratios of soil microbial biomass,in 165-170 m and 170-175 m,the microbial biomass ratios of BC and PC were higher than that of SL and unplanted soils.At the elevation of 165-170 m,the microbial biomass ratio of BG had obvious advantages over unplanted soil.The microbial biomass ratio of the soil above the water fluctuation zone was different from that in the riparian zone.The overall difference of the microbial biomass ratio between the WC soil and the bare soil was not very obvious,but there were advantages and disadvantages in individual ratios.In addition,strong community level elemental homeostasis was found in artificial vegetation soils.(8)The analysis results of soil enzyme activity showed that: the variation trend of artificial vegetation soil enzyme activity was relatively consistent,showing that it was lower in June and higher in September,which were both much higher than that of unplanted soil.In 170-175 m,the phosphatase,invertase and urease activities in the soil of BC ranked the top in artificial vegetation,followed by PC and SL.The activity of catalase was relatively consistent in the soils of BC and PC,but higher than that of SL.In 165-170 m,the activity of acid phosphatase and catalase in BC was higher than those of other vegetation soils and unplanted soil,followed by PC and SL.In contrast,sucrose enzyme had the highest activity in PC soil,which was obviously superior to those of BC and SL soils.However,the urease activity in the soil of the three artificial vegetation showed no significant differences,and basically remained at the same level.The phosphatase activity of mixed planting soil was similar to those of other arboreal vegetation soil(BC and PC),with no obvious fluctuation,while the BG showed an upward trend although they did not fluctuate much.In of 160-165 m,the activity of catalase and urease in the BG soil was significantly higher than that in the bare soil.The phosphatase activity of WC soil above the fluctuation zone was similar to that of the vegetation soil.Catalase,sucrase,and urease were generally higher than the elevation of 165-170 m and 160-165 m in the clearance zone,but they were not significantly different from 170-175 m of the altitude tree soils,especially the soil urease activity,and even lower than that of the high-altitude tree soils.(9)The analysis results of soil quality index(SQI)show that: The soil quality index in the reservoir area was generally lower,which was lower than 0.5 in the elevation of 170-175 m.The soil quality index of artificial vegetation,from high to low,were PC community soils >SL community soils >BC community soils >S+B mixed community soils >SL community soils in 165-170 m,which was increased by 74%,70%,63%,52% and 48%,respectively,when compared with the unplanted soil.Similarly,in 160-165 m,the SQI value of BG soil also showed an increasing trend compared to the bare soil,and was 1.48 times than that of the unplanted soil.For the elevation of 180-185 m,there was no significant difference between the cypress soil and the artificial vegetation soil in riparian zone,while the bare soil was slightly higher than the bare soil area in the fluctuation zone.