Protist communities as indicators of fertilization-induced changes in a species-rich grassland ecosystem

Anthropogenic activities such as long-term fertilizer application are known to lead to losses in above and belowground biodiversity, thereby negatively impacting ecosystem function. However, our understanding of the relative sensitivity of different soil organisms groups to increasing fertilizer application levels remains largely unknown. To address this knowledge gap, we investigated the impacts of different long-term fertilization regimes, and the associated changes in plant communities, on a broad range of soil organisms, including bacteria, fungi, protists, and nematodes, in the alpine meadow ecosystem of the Qinghai-Xizang Plateau. Results showed that the community composition of protists was the most sensitive to different fertilization regimes as compared to those of bacteria, fungi, and nematodes. Changes in the protist community were also most strongly linked to changes in plant richness and biomass under prolonged fertilization. Nitrogen fertilizer addition leads to more deterministic community assembly processes for protist communities, while phosphorus fertilizer application leads to more stochastic assembly processes. Together, our results suggest that protists may be among the most susceptible soil organisms concerning the impact of human disturbances like fertilizer application, highlighting their potential importance as sensitive ecological indicators of human-induced disturbances in terrestrial ecosystems.