Objectives Soil protists refer to all the eukaryotes excluding plants, fungi and animals. Soil protists have distinct phylogenetic and functional diversity characteristics, and are one of the key components serving underground ecosystem. The roles of protists in soil food web, compared with bacteria and fungi, are often overlooked. In this paper, we reviewed the diversity and ecological functions of soil protists, summarized the effects of soil nutrients, water, pH, temperature, heavy metals and microplastics on soil protists. The purpose of this study is to clarify the ecological effects of soil protists on nutrient transformation, soil food web and plant health, and their responses to the main regulatory factors.

Advances Soil protists are varied in number, sizes and morphologies, showing wide diversity of trophic groups. They are active players in soil food webs as phototrophic algae, microbiome predators, and plant and animal parasites. Photosynthetic protists can potentially increase soil oxygen concentrations and contribute significantly to carbon storage in terrestrial ecosystems. Consuming protists are found efficient to alter microbial communities, drive soil microbial loops, suppress plant diseases and improve plant performance by predation. Parasitic protists are generally considered to have negative effects on the host, such as changing animal diversity and causing plant diseases. However, parasitic protists also contribute to the promotion of nutrient turnover (e.g. phosphorus), as well as the biological and protective function of phyllosphere bacteria. On the whole, the protists drive the circulation of matter and the flow of energy towards higher trophic levels in terrestrial ecosystems and play a central role in soil food webs and key ecological functions.

Forecast Future researches should focus on the characteristics of protist diversity and their maintenance mechanism, the interactions and metabolism relationships between soil protists and other microorganisms and fauna (such as earthworms and nematodes), the contributions of protists to nutrient cycle and energy flow in the soil food web, and the related influencing factors. Additionally, the development and application of protistan biocontrol agents will be beneficial to the alleviation of pesticide residues in soil and plants, which is of great significance for ensuring soil health and its sustainable utilization and development.