The Ecological Mechanisms Underlying Vegetation Patches’ Expansion During Natural Restoration Of Soda-Saline Grasslands Over The Songnen Plains

Grassland is the largest ecosystem on the earth,which plays a crucial role in carbon sequestration,global biogeochemical cycles,and biodiversity conservation.However,ongoing global climate change and intensive human activities have led to degradation and fragmentation of over half of the grasslands worldwide,characterized by fragmented patches.Natural restoration is a grassland restoration method that minimizes human disturbances and is widely used for degraded grassland restoration due to its simple management and low cost.Vegetation patches are widely distributed in Songnen saline-alkali degraded grassland and are essential for maintaining grassland plant species and community succession.Vegetation patches in saline-alkali grasslands have various forms and usually reflect different successional stages of the grassland.At the same time,there are significant differences in vegetation,soil,and soil microbial diversity among patches.In recent years,the issues related to grassland patches have received extensive attention from the academic community,and systematic research has been conducted.However,there is still a lack of in-depth understanding of the key ecological processes and the vegetation（biomass,litter,roots,etc.）-soil（physical/chemical/biological properties,major soil processes）-microbe chain and related mechanisms during the expansion of vegetation patches in the ecological restoration of soda-saline grassland.This study conducted by comprehensively applying methods such as field typical plot setting and positioning observations,theoretical analysis,laboratory tests,and high-throughput sequencing technology.In Songnen saline-alkali grassland,using the space-for-time substitution method and referring to the typical plant community composition and soil saline-alkali characteristics of the regional vegetation succession series,typical plots and their vegetation patches were selected as the research objects.The possible effects of patch location,distinguishing patch inside（PI）,patch ecotone（PE）,and patch outside（PO）on plant community characteristics and soil properties were clarified,and the changes in plant community characteristics and soil properties during the expansion process of vegetation patches were analyzed in depth.Plant community characteristics,soil properties,and soil microbial community composition and diversity among vegetation patches,distinguishing saline-alkali patch（SP）,forb patch（FP）,and Leymus chinensis patch（LP）were systematically analyzed to clarify the key aboveground-belowground ecological processes during the restoration of grassland patches.Based on data integration,the plant-soil-microbe chain and related mechanisms in the natural restoration process of Songnen saline-alkali grassland patches were analyzed and empirically demonstrated.The main results are as follows:（1）Vegetation patch characteristics in grassland natural restorationPlant community of saline-alkali grassland was characterized by discontinuous patchy forms,showing obvious patchiness.There were significant differences among PI,PE,and PO in terms of plant community dynamics and soil conditions.Specifically,plant height and coverage of PI were significantly higher than those in PE and PO（P<0.05）,while plant species richness（PR）did not differ significantly between PI and PE（P>0.05）.At the same time,soil properties in vegetation patches of saline-alkali grassland also exhibited significant differences in both horizontal and vertical directions.Soil p H value and electrical conductivity（EC）of PI were significantly lower than those in PE and PE（P<0.05）,with a relative decrease of 14.65%and 77.38%in the topsoil（0–20cm）compared to PO,respectively,and the topsoil was lower than the subsoil（20–40cm）.Soil organic carbon（SOC）,total nitrogen（TN）,and total phosphorus（TP）content of PI were significantly higher than those in PE and PO（P<0.05）,with a relative increase of 59.03%,133.33%,and 25%in the topsoil compared to PO,respectively,and the topsoil was higher than the subsoil.Therefore,a distinct"fertile island effect"was formed within patches,where the heterogeneity of habitat resources leaded to the distribution of surface vegetation in a discontinuous patchy cluster form,providing internal conditions for patch expansion.（2）Temporal and spatial changes of vegetation patches during grassland restorationA study on different types of patches in saline-alkali grasslands during four consecutive growing seasons（2016-2019）revealed significant changes in vegetation characteristics and soil properties.Overall,the dominant species shifted from Artemisia scoparia and Kochia scoparia to forbs such as Artemisia scoparia and Kalimeris integrifolia,and eventually L.chinensis.The accompanying plant species changed from predominantly Asteraceae to Poaceae.There was a certain degree of change in the plant community type and the original habitats.Grassland restoration significantly impacted vegetation characteristics（P<0.05）.Plant height,above-ground biomass（AGB）,litter biomass,and below-ground biomass（BGB）increased with grassland restoration,while coverage and PR initially increased and then decreased.Grassland restoration also had a significant impact on soil salinity and soil ions（P<0.05）.Soil p H and EC showed a decreasing trend with grassland restoration,and soil p H and EC of the topsoil were lower than those of the subsoil.Grassland restoration reduced Na⁺content by 18.38%and HCO₃^-content by 75.02%in the subsoil of SP.Moreover,grassland restoration significantly altered soil nutrient content of vegetation patches.SOC,TN,and TP content increased with grassland restoration,and the topsoil had higher levels than the subsoil.Available nitrogen（AN）showed an overall increasing trend with grassland restoration,and the topsoil had higher levels than the subsoil.However,available phosphorus（AP）decreased with grassland restoration,and SP had lower levels in the topsoil compared to the subsoil.（3）Changes in soil microbial community based on vegetation patches during grassland restorationIn the natural restoration of saline-alkali grassland,there were significant differences in soil microbial community diversity and structure among different patches.Firstly,grassland restoration significantly altered the composition of soil bacterial and fungal communities.The dominant species of soil bacteria and fungi in Songnen grassland was similar,but there were significant differences in the relative abundance of soil microbes（P<0.05）.Specifically,the relative abundance of Acidobacteria increased,with an increase of 116.45%in the topsoil and339.03%in the subsoil,while Gemmatimonadetes,Bacteroidetes,and Firmicutes showed a decreasing trend during grassland restoration.For fungi,the relative abundance of Ascomycota increased and then decreased,with a decrease of 8.86%in the topsoil and 30.18%in the subsoil,while the relative abundance of Mortierellomycota showed an increasing trend.Secondly,grassland restoration significantly altered soil fungal community diversity and OTU richness,while it had no effect on bacterial community.Specifically,during grassland restoration process,there were no significant differences in OTU richness and Shannon-Wiener diversity index of soil bacteria in the two soil layers（P>0.05）.However,Shannon-Wiener diversity index of fungi increased by 15.02%and OTU richness increased by 62.20%in the topsoil.Finally,grassland restoration significantly altered the community structure of soil bacteria and fungi（P<0.05）.MRM model analysis suggested that soil sodic salinization was key factor influencing the structure of soil bacterial community,while soil fertility and plant community traits were key factors influencing the fungal community.（4）"Plant-soil-microbe"interactions in vegetation patch response chainIn the study exploring the"plant-soil-microbe"interactions in grasslands,there was no significant correlation between plant alpha diversity and microbial alpha diversity in both the topsoil and subsoil layers during the process of grassland restoration（P>0.05）.However,there was a significant correlation between plant beta diversity and bacterial beta diversity in the topsoil（P<0.05）.Grassland restoration significantly altered the stoichiometric characteristics and related relationships of soil carbon,nitrogen,phosphorus,and microbial carbon/nitrogen.An increasing trend in soil C:P with grassland restoration,while soil N:P showed no significant changes.Soil C:N only showed an increasing trend in the subsoil with grassland restoration,increasing by 73.73%.Correlation analysis found that soil C,N,and P were significantly positively correlated with vegetation characteristics（P<0.05）,while the dominant phylum of soil bacterial and fungal community had significant correlations with vegetation and soil indicators（P<0.05）;SEM analyses also showed that changes in soil microbial communities were influenced by vegetation characteristics and soil properties during grassland restoration.In summary,considering plant-soil-microbe as a completed system to reveal the interactions between plant communities,soil environment and soil microbial communities during grassland restoration.The expansion of vegetation patches during the natural restoration process of the Songnen soda-saline grassland was the result of strong feedback effects among plant community succession,soil condition improvement,and changes in soil microbial communities.It was formed a closed loop of mutual promotion and continuous development of plant-soil-microbe feedback.The results of this study contribute to a deeper understanding of the ecological processes and related mechanisms of vegetation patch expansion during the natural restoration of the soda-saline grassland in Songnen Plain,providing possible new theoretical and strategic perspectives for the management of saline-alkali grassland restoration.