Changes in diversity patterns and assembly processes of soil nematode communities during forest secondary succession on the Loess Plateau

The secondary succession of plant communities involves considerable changes in belowground communities of microbes and animals. Nematodes are the most abundant animals in soil and play a vital role in maintaining soil biodiversity and functions. Exploring the ecological processes that drive soil nematode diversity and community assembly during secondary succession can help to gain a predictive and mechanistic understanding of soil biodiversity maintenance and ecosystem recovery. In this study, based on the investigation of soil nematode community composition and structure, we examined the changes in nematode alpha-and beta-diversity along forest secondary succession and disentangled the contributions of different community assembly processes (deter-ministic and stochastic processes) in each successional stage on the Loess Plateau, a key region of ecological restoration in China. The results showed that: (1) Abundances of plant-parasitic and bacterial-feeding nematodes were lower in the pioneer and climax forest stages than in the grassland and shrub stages. Meanwhile, nematode community composition considerably changed, with the dominant trophic group changing from plant-parasitic (grassland stage: 35.48%) and bacterial-feeding (shrub stage: 35.63%) to fungal-feeding (pioneer and climax forest stages: 30.65-36.44%). These changes might be associated with higher carbon-to-nitrogen ratios of fine roots and litters, as well as higher litter phosphorus content (LTP), in pioneer and climax forests; (2) Nematode alpha-diversity and beta-diversity peaked in the shrub stage and decreased substantially in the climax forest stage, but beta-diversity differed more significantly between successional stages than alpha-diversity. Fine root biomass, root carbon to nitrogen ratio, and litter phosphorus content were the main predictors of nematode alpha-and beta-diversity, while geographic distance positively affected nematode beta-diversity; (3) In the grassland and climax forest stages, deterministic processes dominated nematode community assembly, probably due to the strong influence of environmental factors such as the quantity or quality of plant resources. Especially, in the climax forest stage, the low quality of plant resources might be responsible for the low nematode diversity and network complexity. In the shrub and pioneer forest stages, the contribution of stochastic processes gradually increased, and nematode networks became more complex. Taken together, our results emphasize the importance of litter and root quality in regulating nematode diversity and community assembly, especially in the late successional stage, which provides insights into forest restoration and sustainable management in the Loess Plateau and other similar arid and semiarid regions.