Seasonal responses of soil bacteria, fungi and protists to nitrogen and phosphorus addition in a subtropical forest

Protists are ubiquitous in soils and play vital roles in biogeochemical cycling in forest ecosystems. While their sensitivity to nitrogen (N) addition is well-documented, the responses to phosphorus (P) enrichment remain less understood. In this study, we examined the seasonal (i.e., spring and autumn) variations in six-year simulated N and P addition, including control, N, P and NP treatments, on the diversity and community composition of soil bacteria, fungi and protists in a subtropical forest. Results showed a significant increase in the α-diversity of soil protists with N or P addition across both seasons, while bacterial and fungal α-diversity remained unchanged compared to the control. Principal coordinate analysis revealed significant differences in microbial community structure across treatments and seasons. Consumer protists dominated the community (90% of total abundance), with their relative abundance showing seasonally consistently positive responses to both N and P additions. Phylum Cercozoa dominated the protistan community in both seasons, with its relative abundance showing a seasonally stable increase in the N treatment compared to the control. The relative abundances of the classes Sarcomonadea and Imbricatea were significantly higher in the samples with N addition. Random forest and structural equation modelling analyses identified soil nutrients (i.e., NH₄⁺-N, available P, and total P) as significant drivers of protist community compositions in both seasons. These findings revealed seasonal sensitivity of soil protists to nutrient addition, indicating their divergent resource-use strategies and adaptive plasticity in response to environmental disturbance.