Soil fauna is critical for maintaining ecosystem functioning, and its community could be significantly impacted by nitrogen (N) deposition. However, our knowledge of how soil-faunal community composition responds to N addition is still limited. In this study, we simulated N deposition (0, 50, 100, 150, and 300 kg N ha−1 year−1) to explore the effects of N addition on the total and the phytophagous soil fauna along the soil profile (0–10, 10–25, and 25–40 cm) in poplar plantations (Populus deltoids) on the east coast of China. Ammonium nitrate (NH4NO3) was dissolved in water and sprayed evenly under the canopy with a backpack sprayer to simulate N deposition. Our results showed that N addition either significantly increased or decreased the density (D) of both the total and the phytophagous soil fauna (Dtotal and Dp) at low or high N addition rates, respectively, indicating the existence of threshold effects over the range of N addition. However, N addition had no significant impacts on the number of groups (G) and diversity (H) of either the total or the phytophagous soil fauna (Gtotal, Gp and Htotal, Hp). With increasing soil depth, Dtotal, Dp, Gtotal, and Gp largely decreased, showing that the soil fauna have a propensity to aggregate at the soil surface. Htotal and Hp did not significantly vary along the soil profile. Importantly, the threshold effects of N addition on Dtotal and Dp increased from 50 and 100 to 150 kg N ha−1 year−1 along the soil profile. Fine root biomass was the dominant factor mediating variations in Dtotal and Dp. Our results suggested that N addition may drive changes in soil-faunal community composition by altering belowground food resources in poplar plantations.

Soil biodiversity and fuction have been altered by the increasing levels of nitrogen as a result of fertilization and atmospheric deposition. Although soil microarthropods are a crucial component of soil biodiversity and play a key role in a diverse range of soil functions, our understanding of the mechanisms by which N addition affects them remains limited. Using a long-term nitrogen addition experiment (2012–2016) in poplar plantations (Populus deltoides L. CL‘35′) located along the coast of Yellow Sea Forest Park in northern Jiangsu, eastern China (32°52′ N and 120°49′ E), where the soil was entisols, we examined the response of soil microarthropods across three soil depths (0–15 cm, 15–25 cm, 25–40 cm) to five N input levels (0, 5, 10, 15, 30 g N m−2 year−1) over four seasons. We found that the number of microarthropods per unit area initially grew and then dropped as more nitrogen was added to soils. Soil organic carbon (positive correlation, R2 = 0.53) and pH (negative correlation, R2 = 0.19) were the two dominant factors driving the effects of nitrogen addition on soil microarthropod densities at all soil depths. These results suggest that nitrogen input enhances the density of soil microarthropods via the increase in fresh organic matter input. However, the increase in organic matter may be offset by an indirect increase in acidity under high levels of N addition, providing one possible explanation for the reduced density of microarthropods in heavily fertilized soils.71