Changes In Diversity Pattern And Assembly Process Of Soil Multi-trophic Community During Forest Secondary Succession

Biodiversity pattern and community assembly are not only the core scientific issues of ecology,but also the key application issues in ecosystem restoration and protection.Plant secondary succession can represent community assembly in action.In the process of vegetation succession,soil microorganisms,protozoa and nematodes play a key role in different trophic levels of soil food web.The diversity of multi-trophic biological communities has an important impact on the structure and function of ecosystems.However,the evolution of the diversity pattern and assembly process of soil multi-trophic biological communities during vegetation succession is still unclear.Therefore,in this study,typical plant community plots at different succession stages(grassland,shrub,pioneer forest,mixed forest and climax community Quercus liaotungensis forest)were selected in the secondary forest area of Ziwuling in the Loess Plateau.The species composition of soil microorganisms,protozoa and nematode communities was investigated,and the α-diversity(OTU number,Shannon,Pielou and Simpson indices),β-diversity(Bray-Curtis dissimilarity index)and species coexistence patterns(represented by co-occurrence network characteristics)of different soil biological communities and the whole soil multi-trophic biological communities were analyzed.Then,combined with the geographical and environmental characteristics of the plots,the changes of soil multi-trophic biodiversity pattern and community assembly mechanism during the secondary succession of forests in the Loess Plateau were explored.The results provide a scientific basis for understanding the formation and maintenance mechanisms of soil biodiversity and guiding biodiversity conservation and vegetation restoration in arid and semi-arid areas.The main results are as follows:(1)The species composition,diversity and co-occurrence network characteristics of soil bacteria,fungi,protozoa and nematode communities changed significantly during forest secondary succession.The species composition of soil bacteria,fungi and protozoa changed greatly from shrub to pioneer forest,while the composition of nematode community changed more significantly from pioneer forest to mixed forest.The α diversity of bacteria and nematodes increased firstly and then decreased during the secondary succession of forest,and reached the highest in the pioneer forest stage.On the contrary,the α diversity of fungal community decreased to the lowest in the pioneer forest stage Theα diversity of protozoan community was the highest in grassland stage,and gradually decreased with vegetation succession.The β diversity of soil bacteria,fungi and protozoa communities increased with vegetation succession,while the β diversity of nematode communities decreased with vegetation succession.The co-occurrence networks of soil bacteria,fungi and nematodes had the highest complexity in the early or middle stage of succession(from grassland to pioneer forest),but all of them had high compactness and stability in the mixed forest stage.The connectivity and complexity of protozoan co-occurrence network increased significantly with vegetation succession.The negative correlation ratios of all soil biological co-occurrence networks reached the highest in the early stages(grassland or shrub)of succession(bacteria:40.63%;fungi:36.35%;protozoa:41.43%;nematodes:27.18%),but they decreased with the succession of vegetation,indicating that forest secondary succession gradually weakened the competition among soil organisms.(2)In the process of forest secondary succession,the factors driving the diversity evolution of different soil biological communities were different.In the process of forest secondary succession,the changes of plant and soil properties(aboveground plant carbon-nitrogen ratio,litter phosphorus,readily oxidizable organic carbon,soil available phosphorus and soil ammonium nitrate and nitrogen,etc.)jointly drove the changes of bacterial community α and β diversity,protozoa β diversity and nematode β diversity.The a and β diversity of fungi and the a diversity of soil nematode community were mainly affected by plant properties(aboveground plant and root carbon-nitrogen ratios,litter biomass and total phosphorus,root organic carbon,etc.),and the α diversity of soil protozoa community was mainly affected by soil properties(soil water content and ammonium nitrogen,etc.).The β diversity of soil bacteria,fungi,protozoa and nematode communities increased gradually with the increase of geographical distance between habitats,namely,the community similarity showed a certain distance decay trend,and the specific decline rate decreased according to the order of fungi>bacteria>protozoa>nematodes.(3)Compared with soil microorganisms,the deterministic process plays a more significant role in the assembly process of organisms at higher trophic levels of soil food web,such as protozoa and nematodes.The assembly of soil bacterial community was dominated by deterministic process in grassland and shrub stages(the relative contribution of homogeneous selection was 45.14%-57.27%),and changed to be dominated by random process in pioneer forest stage(the relative contributions of dispersal limitation and drift were 48.15%-47.28%).The assembly of soil fungal community was dominated by stochastic process(the relative contribution of dispersal limitation was 57.36%-60.19%),while the assembly of protozoa and nematode community was more strongly affected by deterministic process(the relative contributions of homogeneous selection were 70.68%-76.48%and 72.68%-80.16%).(4)In forest secondary succession,the diversity and co-occurrence network of multi-trophic communities composed of soil bacteria,fungi,protozoa and nematodes showed significant changes.The a diversity of soil multi-trophic community showed a decreasing trend,but increased in the mixed forest stage,while the β diversity gradually increased and reached the maximum in the climax community stage,indicating that vegetation succession had a more significant effect on the diversity of soil multi-trophic community.With the succession to the pioneer forest stage,the complexity and tightness of the soil multi-trophic biological co-occurrence network reached the highest,while in the climax community stage,the soil multi-trophic biological co-occurrence network had rich species composition and the lowest negative correlation ratio,but the network was more dispersed and the stability was worse.Forest secondary succession can regulate soil biological diversity through bottom-up effect(resource control).Plant community succession led to changes in plant resources(litter and roots,etc.)entering into soil and soil properties,and then regulated soil microorganisms,protozoa and nematode communities in the soil food network along the direction from low to high trophic levels,and finally affected the diversity pattern and co-occurrence network characteristics of soil multi-trophic community(α diversity:χ2=78.534;β diversity:χ2=87.804;co-occurrence network characteristics:χ2=35.261).Changes in plant and soil properties(environmental factors)had the most significant effects on the assembly of bacterial and protozoan communities.Fungal community assembly was dominated by plant properties and geographical distance,while nematode community assembly was mainly driven by biological interactions.