Soil microfauna play a crucial role in maintaining multiple functions associated with soil phosphorous, nitrogen, and carbon cycling. Although both soil microfauna diversity and multifunctionality are strongly affected by climate warming, it remains unclear how their relationships respond to different levels of warming. We conducted a 3-year multilevel warming experiment with five warming treatments in a subtropical primary forest. Using infrared heating systems, the soil surface temperature in plots was maintained at 0.8 ℃, 1.5 ℃, 3.0 ℃, and 4.2 ℃ above ambient temperature (control, CK).

Our findings indicate that low-level (+0.8-1.5 ℃) warming increased soil multifunctionality, nematode, and protist diversity compared to control, while high-level (+4.2 ℃) warming significantly and negatively affected these variables. We also identified significant positive correlations between soil multifunctionality and nematode and protist diversity in the 0-10 cm soil layer. Importantly, we found that soil multifunctionality and protist diversity did not change significantly under temperatures 3.0 ℃ above ambient. Our results imply that an increase of ~3 ℃ may represent a critical threshold in subtropical forests, which is of great importance for identifying response measures to global warming from the perspective of microfauna in the surface soil. Our finding offers new evidence of the role of soil microfauna in regulating multifunctionality with the magnitude of warming in primary forests.