Soil protist functional composition shifts with atmospheric nitrogen deposition in subtropical forests

<p><span>1. </span><span>Soil protist plays a key role in ecological functions through predation and parasitism. However, little is known about how nitrogen (N) deposition and seasonal variations influence soil protist function in forest soils. </span></p> <p><span>2. </span><span>Here, we assessed firstly the impacts of N deposition (control, 50 kg N ha^-1 yr^-1, 100 kg </span><span>N ha^-1 yr^-1</span><span>, and 150 kg </span><span>N ha^-1 yr^-1</span><span>) on the functional composition of the soil protist community in summer and winter, using amplicon sequencing of environmental DNA from a subtropical natural forest. </span></p> <p><span>3. </span><span>We found that soil protists were dominated by consumers (42.6–51.6%), followed by parasites (32.9–40.9%) and phototrophs (3.2–13.1%), implying a predominant role of consumers and potential top-down effects on the other trophic groups in subtropical forest soils. The functional composition of soil protists was greatly influenced by N deposition, but these responses were dependent on seasonal variations. The diversity of phototrophs was lower in summer than in winter. Instead, an opposite pattern was observed for consumers, resulting in a significantly higher protist diversity in summer than in winter, which indicates a greater sensitivity of soil protists to seasonal variations. Furthermore, low and high N deposition simplified the structural complexity of soil protist communities, suggesting a nonlinear response of the protist structural stability to N deposition.</span></p> <p><span>4. <em>Synthesis and applications.</em></span><span><em> </em>This study provides unprecedented evidence that season variation plays an important role in regulating responses of soil protist functional composition to N deposition, and highlights the nonlinear effects of rising N deposition levels on the soil food web. </span></p>