Soil properties have more significant effects on the community composition of protists than the rhizosphere effect of rice plants in alkaline paddy field soils

Protists are among the most diverse and major microbial groups in the soil ecosystem and play versatile functional roles for soil fertility and agricultural productivity. However, protist community composition is poorly understood in paddy field soil, especially in alkaline paddy fields. Here we aimed to characterise protist communities of alkaline paddy field soils with a particular focus on the effects of physicochemical properties of the soils and the rhizosphere effect of rice. We selected several alkaline paddy fields across three regions that differed in their soil physicochemical properties along the Kizilirmak River, Turkey, as a model ecosystem. The soils were incubated under submerged conditions with and without rice plants (Oryza sativa L.) to understand the rhizosphere effect on protist community composition. The protist communities were analysed with a high throughput sequencing method. The results showed that Amoebozoa (29.5%) were the most abundant taxonomic group of protists in the paddy fields, followed by Stramenopiles (23.7%), Rhizaria (19.5%), and Alveolata (12.6%). Among the functional groups, consumers (decomposers and predators) were the most dominant protist group (67.7%), followed by autotrophs (21.0%) and pathogens (microbial/animal parasites and plant pathogens) (9.2%). The soil properties have more significant effects driving the community composition of protists than the rhizosphere effect of rice in the paddy field soils. Among the soil properties, pH, exchangeable Na and Ca, EC, and lime were significantly correlated with the shift in the protist community composition. The rhizosphere effect of rice mainly affected phagotrophs and plant pathogens, especially Pythium sp. A significant negative correlation was observed in the relative abundances between phagotrophic protists and plant pathogens, which indicates that the plant pathogens could be top-down controlled by the phagotrophs.