Soil Organic Carbon Dynamic And Microbiological Characteristics In The Fluctuating Zone Of The Three Gorges Reservoir

A special riparian zones of was formed in the Three Gorges Reservoir since the completion of the Three Gorges Dam.Large hydroelectric project can directly affect vegetation types,soil properties and hydrology,and ultimately modify soil carbon(C)and nitrogen(N)dynamics and microbial activities.However,studies on soil carbon and nitrogen dynamics and microbial characteristics in the Three Gorges Reservoir area remain unclear.In this study,we carried out field investigations to collect soil and plant samples in 6 sites(Changshou Fuling,Zhongxian,Wanzhou,Badong and Zigui)with four different elevation zones(i.e.low <145m,inundation area without plant;middle,145-160 m,longer flooding duration with revegetation;high,160-175 m,shorter flooding duration with revegetation;and control,>175 m,no-flooding area with original plant)at two soil depths(0-10 cm and 10-30 cm)in riparian zones of the Three Gorges Reservoir,China.We investigated plant traits(plant biomass,plant C and N content and plant C: N),soil C contents in different fractions [(i.e.soil organic carbon(SOC)labile and recalcitrant carbon(LC and RC)]and their isotope,C-hydrolyzing enzyme activities and soil microbial activities to identify the effects of flooding on plant community,the dynamics of soil C and N,microbes and enzyme activity,as well as their interactions,The main conclusions are as follows:(1)Impacts of flooding on plant community and the ?13C and ?15N of plant-soil systemFlooding significantly changed the vegetation community structure(i.e.,C3 and C4 vegetation composition).Generally,anti-seasonal flooding induced significant decline in the C4 plant by mean 25% in flooding zone compared with unflooding zone in the upstream,but significantly increased the C4 plant by mean 59% in the downstream.We found that the ?13C values of soils were relatively stabled and closed to the ?13C values of C3 plants in all sites,indicating that original plant was primarily dominated by C3 plant in this region.The 13 C isotopic differences between soil and plant(??13C)was lower than zero in both flooding and unflooding in the upstream,but was only lower than zero in flooding zone in the downstream.The proportion of C3-derived C in SOC and RC pool was higher than C4-derived C in almost sites,while the proportion of C4-derived C in LC was increased from upstream to downstream.Additionally,the ?15N values of soil were more enriched in unflooding zone compared to flooding zone in all sites,which were significantly positively correlated with soil C and N pool content,as well as dependent on soil p H.Overall,our results revealed different patterns and controls of soil C and N dynamics in response to flooding along riparian zones at regional scale.(2)Impacts of flooding on soil C fraction and enzyme activityFlooding greatly affected the spatial variations in soil C pool and C-hydrolyzing enzyme.Among sites,the SOC,C: N ratio,RC,and recalcitrance indices for C(RIC)generally decreased from the upstream to the downstream,whereas the higher LC content was observed in the midstream sites.Among elevations,SOC content was higher in the middle elevation than other elevations.In contrast,RC and RIC declined from the high elevation to low elevation zone,whereas the LC showed opposite trend.Thiscould be related to the spatial variation of plant biomass and plant C: N ratio.Plant C:N was negative with the LC but positive with RC.Flooding also significantly impacted soil enzyme activity.The four C-hydrolyzing enzyme activities were higher in the midstream than those in other sites,whereas the specific enzyme activities(i.e.,activity per unit SOC)showed a little bit different trend with greater level in midstream and downstream than other sites.The activities of four soil enzymes were significantly positively correlated with LC,indicating that LC was the tightest factor in regulating C-hydrolyzing enzyme activities.All the C-hydrolyzing enzyme activities and the specific enzyme activities were lower in the low elevation zone compared to other elevation zones.This variation was possibly because of multiple drives such as changes in LC content and anaerobic environment induced by flooding.Thus,our results revealled that the LC was the tightest factor in regulating Chydrolyzing enzyme activities.(2)Impacts of flooding on microbial community structureFlooding significantly altered microbial biomass and community composition.At different sites,the bacterial microbial biomass was significantly higher in the midstream(ZX and WZ,2.70 ?g g-1 dry soil)than in the downstream(BD and ZG,1.51 ?g g-1 dry soil)and upstream(CS and FL,0.66?g g-1 dry soil).The fungal microbial biomass was significantly higher in the middle stream(ZX and WZ,0.74 ?g g-1 dry soil)than in the downstream(BD and ZG,0.49 ?g g-1 dry soil)and upstream(CS and FL,0.21 ?g g-1 dry soil).Total microbial biomass(Total PLFAs)was significantly higher in the middle reaches of WZ and ZX(4.38 ?g g-1 dry soil),and the lowest values appeared upstream(CS and FL,1.12 ?g g-1 dry soil).Compared to controls,the bacterial,fungal,and total microbial biomass were significantly reduced by 46.32%,55.41%,and 49.27%,respectively,in flooding zones.There was no significant difference at middle and high elevation compared to controls(p <0.05).But bacteria and total microbial biomass had a decline tendency at high elevation.In different soil layers,the microbial biomass of the top soil was significantly higher than that of the bottom soil.The Gram-positive bacteria / Gram-negative bacteria ratios decreased gradually from upstream to downstream at different sites,while the fungal / bacteria ratios had no significant difference at different sites except in Badong.At different elevations,flooding significantly increased the proportion of Gram-positive bacteria and decredaed the proportion of fungal.In general,microbial biomass and community structure response strongly to flooding.In summary,flooding significantly change plant functional group with increasing the percentage of C4 plant at the whole rstudy area.Original C3 plant contributed to soil C accumulation primarily through in situ plant input and existing dominant C4 plant to enhance new C input.The flooding changed litter input and affected the spatial distribution pattern of soil C,which reduced the LC content and increased the RC content.Additionally,the relationship between C-hydrolyzing enzyme and microbial activity with LC content indicated that flooding leads to the reduction of available carbon source for microorganisms and thus could be beneficial to long-trem soil C sequestration.