Study On The Patterns Of Natural ¹³C And ¹⁵N Abundance In Plant-soil System And Soil Microbial Community Diversity In Secondary Grassland In Thethree Gorges Region

Grassland is an important component of the global terrestrial ecosystems,which plays an important role in the global ecosystem carbon(C)nitrogen(N)cycle and the maintenance of soil microbial composition and diversity.However,most fragmentation secondary grasslands have been neglected in previous investigation as well as their functions response to global changes.In this study,we conducted experiments in the typical secondary grasslands(600 km transect)of the Three Gorges Region and riparian zones of the Three Gorges Reservoir(including four different flooding zones i.e.,low <145 m,an inundation area without plants;middle,145–160 m,an area with longer flooding duration and revegetation;high,160–175 m,an area with shorter flooding duration and revegetation;and the control,>175 m,a no-flooding area with original plants).Natural abundance of ?13C and ?15N measurements and Illumina Miseq sequencing were used to quantify ecosystem C and N cycling and their feedbacks to climatic change,and to explore the underlying mechanisms of bacterial community structure,diversity in grassland of the Three Gorges Region and the water level fluctuation zone of the Three Gorges Reservoir,respectively.The main results are showed as follows:1)In the typical secondary grasslands of the Three Gorges Region,the ?13C values in plants and soils declined with increasing mean annual precipitation(MAP),which was attributed to the shift in the dominant plant functional group from C4 to C3 as MAP increased.Moreover,the 13 C isotopic difference between soil and plant was generally lower than zero in sites with the MAP < 1200 mm,indicating large replacement of C4 grasses for woody C3 plants in relatively drier regions.In contrast,the soil ?15N values increased significantly with increasing MAP,possibly because the increasingly high-quality substrate(with lower soil C: N ratios)and N availability could favor N fractionation via turnover and outputs as MAP increased.The ?15N value of plant was positively correlated with the ?15N value of soil,and its 15 N isotopic difference with soil showed overall increasing but the non-linear trend with increasing MAP.2)In the typical secondary grasslands of the Three Gorges Region,the soil bacteria richness and alpha diversity index showed a marginal positive relationship with plant richness and Shannon index.The bacterial community compositions of both the taxonomic and phylogenetic structures were best explained by the soil p H and plant diversity and soil nutrient availability,with the peak richness at p H 6.8.Although the plant,soil and geographic factors were correlated with bacteria community dissimilarity,soil p H and plant diversity,nitrogen availability was relatively independent dominant community assemblage by quantifying taxonomic and phylogenetic turnover.Mantel tests indicated the assembly of soil bacterial community was predominated by the determinant processes.Overall,our findings suggest plant functional traits and abiotic soil properties(e.g.,soil p H and inorganic nitrogen)collectively drive soil bacterial diversity patterns but are not limited by pure spatial distance in grasslands of the three Gorges Region.3)In riparian zones of the Three Gorges Reservoir,soil bacterial community diversity was generally lower in the downstream sites compared to that of other sites with a higher level in the top soil than in the deep soil.Flooding significantly enhanced the bacterial community diversity compared to the control(permanent dry zone).Soil bacterial assemblages underwent deterministic processes in the upstream sites to stochastic processes in the downstream sites,with stronger stochastic processes in the top soil than in the deep soil across all sites and elevations.As expected,the soil variables,including the p H,moisture,NH4+-N,organic carbon and nitrogen,were proven to be determinants of the bacterial community composition.When considering plant traits(plant biomass,richness,and plant carbon content)for revegetation induced by submergence,these played an important role in structuring the bacterial community indirectly through plant and soil interactions.A network analysis revealed that the bacterial community exhibited a shorter averaged path distance(GD)in the flooding zones compared with that of the control,with the shortest average degree(avg K)and the least stability occurring in longer periodic inundation zones.Taken together,our results suggest that soil properties and plant functional traits are critical controls on the bacterial community structure and assembly at regional scales,whereas the water submergence can be an important factor of the variations in the bacterial community composition in riparian zones.