Nutrient limitation and saline–alkaline stress primarily drive community and function shifts in protists inhabiting saline–sodic soils

Understanding the responses of protist communities and their functions to saline–alkaline stress is essential for predicting and managing soil biota-mediated ecosystem functions under global salinization scenarios. However, our knowledge of protists in salt-affected soils remains largely insufficient. To address this limitation, we examined the composition and functional traits of protist communities across a salinity gradient in China’s Songnen Plain and compared natural saline–sodic soils (solonetz) with low-salinity soils from adjacent farmlands. Protistan diversity was significantly greater in farmland soils, whereas absolute abundance was greater in solonetz soils. Long-term agricultural reclamation notably reshaped the community composition: consumers were enriched in solonetz soils, whereas phototrophs were more prevalent in farmland soils. Protist diversity was positively correlated with total carbon (TC), total nitrogen (TN), and total phosphorus (TP), but negatively correlated with the exchangeable sodium percentage (ESP), exchangeable Na⁺ (ENa), and pH. Conversely, protist abundance indicated the opposite correlation. TN and prokaryotic abundance were the most effective predictors of protistan abundance, whereas the ESP, followed by TN, most notably influenced community composition. Predatory nematodes also accounted for some variation in protist abundance and structure while climate variables played minor roles. Our findings provide novel insights into soil microbial ecology, demonstrate that both nutrient limitation and saline–alkaline stress strongly shape protist communities, and offer new perspectives on microbial food webs in salt-affected terrestrial ecosystems.