Soil Micro-food Web Evolution Characteris Tic And Mechanism Following Karst Ecosyst Em Restoration By Forestation In Guangxi Region

As one of "Chinese first three ecological disasters",the "rocky desertification"represents a serious eco-environmental problem in Karst regions of southwestern China.It can lead to natural disasters such as flood,mudslide and landslide,and thus is a great threat to ecological and geological safety.Forestation has been extensively adopted as an effective option to restore the degraded ecosystem.The restoration of ecosystems generally represents a complex and systematic process.During the restoration process,aboveground vegetation and belowground soil ecosystems can greatly interact and often show a co-sueccssion.Soil micro-food web(microorganism-protozoa-nematode),as an important component of ecosystems,plays important roles in soil organic matter decomposition and nutrient cycling,and can be a nice bioindicator for the restoration and succession of ecosystems.Investigating soil micro-food web dynamics during the Karst ecological restoration will be helpful to better understand the restoration process and mechanisms of degraded Karst ecosystems,and will also provide scientific basis for assessing the restoration status and remeding the degraded Karst ecosystems.In a representative Karst region of Huanjing county,Guangxi Province,using the "space instead of time" method,we selected rocky desertification sites and Toona sinensis foresting sites(with ages of 2,4,8 and 16 years)to conduct this study.Soil samples were collected from 0-10 cm depth range using the S shape sampling method.We investigated dynamics of size,activity,diversity,struncture,physical-ecology and stoichiometry of soil microbial community,of composition,size and diversity of soil protoza community,and of composition,diversity,structure and metabolic footprint of soil nematode community during the Karst ecological restoration,and clarified the changing mechanisms and main driving factors of soil micro-food web by examining the relationship between food web and vegetation,resource input and soil physicochemical properties based on redundancy analysis(RA),canonical corresponding analysis-variation partitioning analysis(CCA-VPA)and structure equation model(SEM).Microbial biomass carbon(MBC)and biomass nitrogen(MBN)were measured using the fumigation method;basal respiration(BR)was determined by the room incubation-alkali absorption method;enzyme activites involved in C and N transformation were measured using the fluorescence-microplate method;microbial community structure was determined by the phospholipid fatty acid(PLFA)method.After the Karst forestation,MBC,MBN,biomass(PLFAs)of bacteria,saprophytic fungi,and arbuscular mycorrhizal fungi(AMF),BR,?-1,4-glucosidase(BG),?-1,4-N-acetylglucosaminidase(NAG)and leucine aminopeptidase(LAP)activities tended to increase,group numbers,enveness index(J')and Shannon index(H')of microbial PLFAs increased while dominance index(?)decreased.These results,combining with the correlation analysis,indicate that soil microbial community size,activity and diversity increased due to the increased substrate resource(root,organica matter,etc.)and the imrproved environment(soil moisture,porosity,etc.)after the Karst ecological restoration.CCA-VPA indicates that the substrate resource was the relatively main driver of the microbial community.The value of microbial quotient(MBC%)was below 2.0 and metabolic quotient(qCO2)value was above 2.0 in the control site,reflecting the stressed environment induced by the rocky desertification.After the Karst ecological restoration,microbial quotient and specific enzyme activities(based on soil organic matter)tended to increase,while metabolic quotient tended to decline,reflecting the improved Karst environment.Consistent with root C:N,and soil C:N,microbial C:N ratio and enzyme activity ratio(BG/(NAG+LAP))both showed an increasing trend after the Karst ecological restoration.This suggests the adaptive response of microorganisms to resource stoichiometry.After the reforestation on Karst,the reduced quality of root resource(increased C:N ratio and lignin content and decreased N content)which relatively can favor to fungal utilization resulted in the elevation of fungi to bacteria ratio(F:B).This demonstrates that the micro-food web structure altered towards relative fungal dominance in decomposition pathway.Soil protoza couting and identification were measured by the "Most Proble Number" method and microccope observation,and then the protoza biomass was estimated.After the Karst forestation,the abundance and biomass of soil flagellate,sarcodina,ciliate and total protoza all showed an increasing trend,and the protoza community group numbers,richness index(SR),and Shnnon index(H')increased while dominance index(?)reduced.These indicate that soil protoza community size and diversity increase during the Karst ecologailc restoration.Correlation analysis suggests that the food resource such as microorganisms and environments such as soil moisture were driving forces of the soil protoza.Nematodes were extracted using the shallow plate method,and then were counted by dissecting microscope and were identified by optical microscope.Body length and maximum diameter of nematodes were also measured.A series of ecological inices of richness index(SR),maturity index(MI),structure index(SI),fungction foortprint(Ff)and nematode channel ratio(NCR)were calculated.After the reforestation on Karst,abundance,biomass and carbon footprints of all trophic groups of nematodes increased,enrichment footprint(Fe),structure footprint(FS)and fungction footprint(Ff)also increased,and nematode group numbers,richness index(SR),evenness index(J')and Shannon index(H')were elevated while dominance index(k,)declined.Maturity indices(MI,MI2-5)and structure index(SI)at later period were higher than early stage.These results indicate that nematode community size and metabolic fungction increased and community were more structured and complicated after the Karst forestation.This was attributed to the increased food resournce(root,microorganisms and protoza)and improved environment(soil moisture and porosity,etc.).CCA-VPA indicates that,in gerenal,the main driver was the food resource for r-stratigies in low trophic levels,and the environmental condition for K-stratigies in high trophic levels.According to structure equation model(SEM),bottom-up effect was a critical driving force of soil micro-food web.After the reforestation on Karst,fungi to bacteria ratio(F:B)tended to elevate,and in turn biomass carbon ratio of fungivores to bacterivores(FFC:BFC)also tended to increase,and nematode channel ratio(NCR)showed a decreasing trend.This also demonstrates that the micro-food web structure altered towards relative fungal dominance in decomposition pathway.