The Evolution Characteristics And Driving Mechanisms Of Soil Nematode Community In The Typical Vegetation Ecosystems Of Loess Plateau

Vegetation restoration is an important measure to improve of the fragile ecological environment.During vegetation restoration,there are close relationships between aboveground plant and belowground biological communities.Nematodes exist in various habitats and occupy key positions of different trophic levels of soil food webs,which participate soil organic matter decomposition and nutrient cycling.The community characteristics of soil nematodes can indicate soil food web structure and functioning,which can effectively reflect the process of ecosystem restoration.In recent years,the metabolic footprint analysis of nematodes provides an effective method for further exploring the energy flow of soil food web,which can help to reveal the changing rule and driving mechanisms of soil food web.The Loess Plateau is one of the most fragile ecological areas in the world,and it is also the key area of vegetation restoration in China.In the past several decades,scientists have carried out a series of studies to explore the effects of vegetation restoration on plant communities,soil physical and chemical properties and microbial communities in the Loess Plateau area.However,little attention has been paid on the evolution characteristics and driving mechanisms of soil nematode community in the process of vegetation restoration in this area.This study selected naturally restored grassland,secondary forest,artificial shrub forest and artificial grassland on the Loess Plateau as the research objects to investigate soil nematode communities individual number,community composition and structure,and metabolic footprint at different successional stages(or after different restoration years)and analyze their environmental influential factors.Finally,the evolution characteristics and driving mechanisms of soil nematode communities in different vegetation ecosystems were explored and compared to provide new indicators and scientific basis for evaluating the ecological and environmental effects of vegetation restoration on the Loess Plateau.The main results are as follows:(1)In naturally restored grassland,the abundance,community structure and metabolic footprint of soil nematodes greatly varied at different successional stages,which were mainly mainly related to the changes in aboveground biomass,plant communities diversity,living root biomass,soil organic carbon,total nitrogen and microbial biomass carbon.Form Artemisia scoparia community(AS)to Thymus mongolicus & Stipa grandis community(TS),soil nematode abundance gradually increased,with the percentage of plant-feeding nematodes decreasing to the bottom(30.82%)and that of omnivorous-predatory nematodes reaching the highest level(21.25%).Therefore,in Thymus mongolicus & Stipa grandis community(TS),the trophic structure of soil nematode was more equivalent,and the diversity index(?H'),maturity index(MI),Wasilewska index(MI),total nematode metabolic footprint(TNMF)and functional metabolic footprint(FMF)all reached the peak,which indicate that the diversity and energy metabolism of soil nematode community were greatly improved,therefore soil food web keeping mature and stable.At the stages of Artemisia gmelinii(SB)and Stipa bungeana(SB)communities,the TNMF and FMF decreased to the lowest,implying the significant decreases in the energy input and metabolic efficiency.(2)During the process of secondary forest succession,the response patterns of soil nematode communities were distinctly different among grassland stage,shrub forest stage and arbor forest stage.The fine root biomass was the main factor influencing soil nematode community composition.At the Betula platyphylla forest(BP)and Quercus liaotungensis forest(QL)stages,the increase of litter biomass greatly improved the survival environment and food resources of soil nematodes,leading to the high percentage of omnivorous-predatory nematodes.At the Bothriochloa ischaemum grassland(BI)stage,soil nematode abundance reached the peak,but Tylencholaimus genus had high dominance in nematode community(48.8%),therefore nematode diversity lowering to the bottom.At the Hippophae rhamnoides shrub(HR)stage,nematode abundance slightly decreased,and the percentages of different trophic groups were relatively equivalent.Therefore,soil nematode diversity,as well as the TNMF and FMF,reached the highest,indicating the metabolic function of soil food web was greatly improved.At arbor forest community stages(Populus davidiana forest,Betula platyphylla forest and Quercus liaotungensis forest),soil nematode abundance and diversity kept relatively stable.Especially,at the climax community stage(QL),the percentage of omnivorous-predatory nematodes,as well as nematode trophic diversity(TD),MI and WI index all reached the peak,while TNMF and FMF decreased to the lowest,which indicated that soil food web could kept mature and stable based on the relatively low level of energy metabolism.(3)During the restoration process of artificial Caragana korshinskii shrub forest(CK),the changes of understory herbaceous plant diversity and fine root biomass significantly influenced soil nematode abundance,community composition and metabolic footprint.With the increase of the planting years of C.korshinskii plantation,total nematode abundance,bacterial-feeding nematode abundance(BF),plant-feeding nematode abundance(PP),omnivorous-predatory nematode abundance(OP),as well as TNMF,enrichment footprint(Fe),structural footprint(Fs)and functional footprint(FMF)increased significantly,which all peaked after 25 years of C.korshinskii planting(CK25),implying that soil resource availability reached the highest,as well as the energy input and metabolic efficiency of soil food web,at this time.When the planting years of C.korshinskii plantation reached 35 years,soil nematode abundance,and metabolic footprint showed the decreasing tendency,and nematode channel index(NCR)lowered to the bottom,indicating that soil organic decomposition was dominated by the fungal-mediated pathway.However,the diversity index(?H'),maturity index(MI),Wasilewska index(MI)and the structure index(SI)reached the maximum after 35 years of C.korshinskii planting(CK35),indicating that soil food web kept mature and stable based on the relatively low level of energy metabolism at the later restoration stage of C.korshinskii plantation.(4)During the restoration process of artificial Medicago sativa grassland,the changes of litter biomass,root biomass and root C/N ratio were the main environmental factors influencing the abundance,diversity and metabolic footprint of soil nematodes.With the increase of M.sativa planting years,the abundance and metabolic footprint of total soil nematodes,as well as those of different trophic groups,gradually increased,which reached the peak in the 7th year after M.sativa planting(MS7).The percentage of omnivorous-predatory nematodes(32.11%),maturity index(MI)and functional footprint(FMF)reached the highest at the same time,which indicated that soil food web was the most mature and stable after the MS7 site,with the highest structural degree and metabolic function.However,when the planting years of M.sativa continued to increase,the abundance and metabolic footprint of soil nematodes gradually decreased,and the structure and function of soil food web tended to be degraded.(5)The distribution characteristics and evolution law of soil nematode communities significantly varied in different vegetation ecosystem on the Loess Plateau.In naturally restored grassland,artificial M.sativa grassland and C.korshinskii plantation,plant-feeding nematodes dominated soil nematode communities,while in secondary forest ecosystem,bacterial-feeding nematodes or fungal-feeding nematodes dominated soil nematode communities.The Shannon-Wiener index(?H')and Wasilewska index(WI)listed according to the order of secondary forest ecosystem(SFE)> C.korshinskii plantations ecosystem(CKP)> naturally restored grassland ecosystem(NRG)> M.sativa artificial grassland ecosystem(MSA),indicating that the soil food web in the two types of forest ecosystems had higher species diversity and structural stability than grassland ecosystem.Net-work analysis also showed that the interaction of different groups was more complex and diverse in SFE and CKP.Soil nematode community comprehensive index(NCI)was calculated based on the mean value measurement method of ecosystem multi-function index(EMF).The soil nematode community comprehensive index(NCI)also showed that secondary forest ecosystem(0.44)> C.korshinskii plantations ecosystem(0.10)> naturally restored grassland ecosystem(-0.12)> artificial M.sativa grassland ecosystem(-0.18).The NCI has a good correlation with the ecosystem multifunctional,which indicates that the NCI index can effectively indicate the ecological effect of vegetation restoration.(6)In the two types of grassland ecosystems,soil nematode abundance,metabolic footprint and maturity index(MI)all showed the decreasing tendency at the later stages of vegetation restoration,while the magnitude of plant energy channel increased to some extents.In the forest ecosystems(SFE and CKP),soil food webs had higher stability than those in grassland ecosystems.At t35 years of C.koragana(CK35),the function of fungal energy channel gradually increased.At the secondary forest ecosystem,the function of fungal channel and bacterial channel gradually increased,while that of plant channel decreased,at the arbor forest community stage,leading to the equivalent proportions of the three energy channels,which indicated that forest vegetation restoration promoted the energy flow pathways of soil food web to be more diversified and efficient.(7)In the typical vegetation ecosystems on the Loess Plateau,the bottom-up effect(resource control)is the main driving force of soil nematode community.In natural and artificial grassland ecosystems,the changes in litter and root biomass mainly affect the metabolic footprints of bacterial-feeding nematodes,fungal-feeding nematodes and omnivorous-predatory nematodes through bacterial and fungal energy channels.In addition,the abundant root resources in grassland ecosystems significantly promoted the metabolic footprint of herbivorous nematodes,leading to the high dominance of plant channels in the food web,which had no significant effect on the metabolic footprint of omnivorous nematodes,indicating that the energy utilization efficiency of the soil food web in the grasslands was low.In forest ecosystems(secondary forest and C.korshinskii plantation),the changes in fine root biomass affected the metabolic footprints of soil nematode communities through bacterial energy channel and plant channel,and the soil food webs had higher energy utilization efficiency.Besides,in secondary forest ecosystem,the changes of litter biomass greatly affected soil water content,which directly influenced the metabolic footprint of predator-omnivorous nematodes by changing their habitat conditions,leading to the possible impact of top-down effect(predation control)on the soil food web.Therefore,in secondary forest ecosystem,the bidirectional driving forces may lead to the complex impacts on soil food web.